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How to convert Canvas element to string

In general we can convert the HTML elements to string and then we can insert it into DOM later when needed. Similarly, I want to convert the "CANVAS" element to string along with its context properties.
In the following example, I am getting the string value of span tag with outerHTML property. Likewise I want to get the "CANVAS"element along with context properties.
Is there any method or property for this support?
Example code snippets:
var sp=document.createElement("span");
sp.innerHTML = "E2"
var e2 = sp.outerHTML;
$("#test1").append(e2);
var c=document.createElement("CANVAS");
var ctx=c.getContext("2d");
ctx.beginPath();
ctx.moveTo(20,20);
ctx.lineTo(100,20);
ctx.arcTo(150,20,150,70,50);
ctx.lineTo(150,120);
ctx.stroke();
var cn = c.outerHTML;
$("#test2").append(cn);
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id="test1">
<span>E1</span>
</div>
<div id="test2">
</div>

Seems like you already know how to get dom properties of the canvas object.
Now you only need "context" infos (image data as I understand it)
You can get the image data as a base64 string like this:
function CreateDrawing(canvasId) {
let canvas = document.getElementById(canvasId);
let ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(20,20);
ctx.lineTo(100,20);
ctx.arcTo(150,20,150,70,50);
ctx.lineTo(150,120);
ctx.stroke();
}
function GetDrawingAsString(canvasId) {
let canvas = document.getElementById(canvasId);
let pngUrl = canvas.toDataURL(); // PNG is the default
// or as jpeg for eg
// var jpegUrl = canvas.toDataURL("image/jpeg");
return pngUrl;
}
function ReuseCanvasString(canvasId, url) {
let img = new Image();
img.onload = () => {
// Note: here img.naturalHeight & img.naturalWidth will be your original canvas size
let canvas = document.getElementById(canvasId);
let ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0);
}
img.src = url;
}
// Create something
CreateDrawing("mycanvas");
// save the image data somewhere
var url = GetDrawingAsString("mycanvas");
// re use it later
ReuseCanvasString("replicate", url);
<canvas id="mycanvas"></canvas>
<canvas id="replicate"></canvas>

In short no!
You should realize the difference between a standard DOM-element and a canvas-element:
A created DOM-element is part of the mark-up language that can be viewed and changed.
In the canvas a vector image is drawn based upon the rules created in script. These rules are not stored in the element as text but as the image and can't be subtracted from the canvas element.
However there are other possibilities. We can get the variables from the ctx-object. But no info about coordinates:
var c=document.createElement("CANVAS");
var ctx=c.getContext("2d");
ctx.beginPath();
ctx.moveTo(20,20);
ctx.lineTo(100,20);
ctx.arcTo(150,20,150,70,50);
ctx.lineTo(150,120);
ctx.stroke();
var ctxInfo = [];
for (ctxKey in ctx)
{
if (Object.prototype.toString.call(ctx[ctxKey]) !== "[object Function]" )
{
ctxInfo.push(ctxKey + " : " + ctx[ctxKey]);
}
}
console.log(ctxInfo);
To transfer from one canvas to the other I would keep a list (array or object) of instructions and write a generic function that applies them.
canvasObject = [["beginPath"], ["moveTo", 20, 20], ["lineTo", 100, 20], ["arcTo", 150, 20, 150, 70, 50], ["lineTo", 150, 120], ["stroke"]];
function createCanvas(cnvsObj)
{
var c = document.createElement("canvas");
var ctx = c.getContext("2d");
cnvsObj.forEach(function(element){
//loop through instructions
ctx[element[0]].apply(ctx, element.slice(1));
});
return c;
}
var a = createCanvas(canvasObject);
document.body.appendChild(a);

Compress and upload multiple images [duplicate]

TL;DR;
Is there a way to compress an image (mostly jpeg, png and gif) directly browser-side, before uploading it ? I'm pretty sure JavaScript can do this, but I can't find a way to achieve it.
Here's the full scenario I would like to implement:
the user goes to my website, and choose an image via an input type="file" element,
this image is retrieved via JavaScript, we do some verification such as correct file format, maximum file size etc,
if every thing is OK, a preview of the image is displayed on the page,
the user can do some basic operations such as rotate the image by 90°/-90°, crop it following a pre-defined ratio, etc, or the user can upload another image and return to step 1,
when the user is satisfied, the edited image is then compressed and "saved" locally (not saved to a file, but in the browser memory/page),-
the user fill a form with data like name, age etc,
the user click on the "Finish" button, then the form containing datas + compressed image is sent to the server (without AJAX),
The full process up to the last step should be done client side, and should be compatible on latest Chrome and Firefox, Safari 5+ and IE 8+. If possible, only JavaScript should be used (but I'm pretty sure this is not possible).
I've not code anything right now, but I've thought about it already. File reading locally is possible via File API, image previewing and editing could be done using Canvas element, but I can't find a way to do the image compression part.
According to html5please.com and caniuse.com, supporting those browser is quite hard (thanks to IE), but could be done using polyfill such as FlashCanvas and FileReader.
Actually, the goal is to reduce file size, so I see image compression as a solution. But, I know that uploaded images are going to be displayed on my website, every time at the same place, and I know the dimension of this display area (eg. 200x400). So, I could resize the image to fit those dimensions, thus reducing file size. I have no idea what would be the compression ratio for this technique.
What do you think ? Do you have any advice to tell me ? Do you know any way to compress an image browser-side in JavaScript ? Thanks for your replies.

In short:
Read the files using the HTML5 FileReader API with .readAsArrayBuffer
Create a Blob with the file data and get its url with window.URL.createObjectURL(blob)
Create new Image element and set it's src to the file blob url
Send the image to the canvas. The canvas size is set to desired output size
Get the scaled-down data back from canvas via canvas.toDataURL("image/jpeg",0.7) (set your own output format and quality)
Attach new hidden inputs to the original form and transfer the dataURI images basically as normal text
On backend, read the dataURI, decode from Base64, and save it
Source: code.

I see two things missing from the other answers:
canvas.toBlob (when available) is more performant than canvas.toDataURL, and also async.
the file -> image -> canvas -> file conversion loses EXIF data; in particular, data about image rotation commonly set by modern phones/tablets.
The following script deals with both points:
// From https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob, needed for Safari:
if (!HTMLCanvasElement.prototype.toBlob) {
Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
value: function(callback, type, quality) {
var binStr = atob(this.toDataURL(type, quality).split(',')[1]),
len = binStr.length,
arr = new Uint8Array(len);
for (var i = 0; i < len; i++) {
arr[i] = binStr.charCodeAt(i);
}
callback(new Blob([arr], {type: type || 'image/png'}));
}
});
}
window.URL = window.URL || window.webkitURL;
// Modified from https://stackoverflow.com/a/32490603, cc by-sa 3.0
// -2 = not jpeg, -1 = no data, 1..8 = orientations
function getExifOrientation(file, callback) {
// Suggestion from http://code.flickr.net/2012/06/01/parsing-exif-client-side-using-javascript-2/:
if (file.slice) {
file = file.slice(0, 131072);
} else if (file.webkitSlice) {
file = file.webkitSlice(0, 131072);
}
var reader = new FileReader();
reader.onload = function(e) {
var view = new DataView(e.target.result);
if (view.getUint16(0, false) != 0xFFD8) {
callback(-2);
return;
}
var length = view.byteLength, offset = 2;
while (offset < length) {
var marker = view.getUint16(offset, false);
offset += 2;
if (marker == 0xFFE1) {
if (view.getUint32(offset += 2, false) != 0x45786966) {
callback(-1);
return;
}
var little = view.getUint16(offset += 6, false) == 0x4949;
offset += view.getUint32(offset + 4, little);
var tags = view.getUint16(offset, little);
offset += 2;
for (var i = 0; i < tags; i++)
if (view.getUint16(offset + (i * 12), little) == 0x0112) {
callback(view.getUint16(offset + (i * 12) + 8, little));
return;
}
}
else if ((marker & 0xFF00) != 0xFF00) break;
else offset += view.getUint16(offset, false);
}
callback(-1);
};
reader.readAsArrayBuffer(file);
}
// Derived from https://stackoverflow.com/a/40867559, cc by-sa
function imgToCanvasWithOrientation(img, rawWidth, rawHeight, orientation) {
var canvas = document.createElement('canvas');
if (orientation > 4) {
canvas.width = rawHeight;
canvas.height = rawWidth;
} else {
canvas.width = rawWidth;
canvas.height = rawHeight;
}
if (orientation > 1) {
console.log("EXIF orientation = " + orientation + ", rotating picture");
}
var ctx = canvas.getContext('2d');
switch (orientation) {
case 2: ctx.transform(-1, 0, 0, 1, rawWidth, 0); break;
case 3: ctx.transform(-1, 0, 0, -1, rawWidth, rawHeight); break;
case 4: ctx.transform(1, 0, 0, -1, 0, rawHeight); break;
case 5: ctx.transform(0, 1, 1, 0, 0, 0); break;
case 6: ctx.transform(0, 1, -1, 0, rawHeight, 0); break;
case 7: ctx.transform(0, -1, -1, 0, rawHeight, rawWidth); break;
case 8: ctx.transform(0, -1, 1, 0, 0, rawWidth); break;
}
ctx.drawImage(img, 0, 0, rawWidth, rawHeight);
return canvas;
}
function reduceFileSize(file, acceptFileSize, maxWidth, maxHeight, quality, callback) {
if (file.size <= acceptFileSize) {
callback(file);
return;
}
var img = new Image();
img.onerror = function() {
URL.revokeObjectURL(this.src);
callback(file);
};
img.onload = function() {
URL.revokeObjectURL(this.src);
getExifOrientation(file, function(orientation) {
var w = img.width, h = img.height;
var scale = (orientation > 4 ?
Math.min(maxHeight / w, maxWidth / h, 1) :
Math.min(maxWidth / w, maxHeight / h, 1));
h = Math.round(h * scale);
w = Math.round(w * scale);
var canvas = imgToCanvasWithOrientation(img, w, h, orientation);
canvas.toBlob(function(blob) {
console.log("Resized image to " + w + "x" + h + ", " + (blob.size >> 10) + "kB");
callback(blob);
}, 'image/jpeg', quality);
});
};
img.src = URL.createObjectURL(file);
}
Example usage:
inputfile.onchange = function() {
// If file size > 500kB, resize such that width <= 1000, quality = 0.9
reduceFileSize(this.files[0], 500*1024, 1000, Infinity, 0.9, blob => {
let body = new FormData();
body.set('file', blob, blob.name || "file.jpg");
fetch('/upload-image', {method: 'POST', body}).then(...);
});
};

#PsychoWoods' answer is good. I would like to offer my own solution. This Javascript function takes an image data URL and a width, scales it to the new width, and returns a new data URL.
// Take an image URL, downscale it to the given width, and return a new image URL.
function downscaleImage(dataUrl, newWidth, imageType, imageArguments) {
"use strict";
var image, oldWidth, oldHeight, newHeight, canvas, ctx, newDataUrl;
// Provide default values
imageType = imageType || "image/jpeg";
imageArguments = imageArguments || 0.7;
// Create a temporary image so that we can compute the height of the downscaled image.
image = new Image();
image.src = dataUrl;
oldWidth = image.width;
oldHeight = image.height;
newHeight = Math.floor(oldHeight / oldWidth * newWidth)
// Create a temporary canvas to draw the downscaled image on.
canvas = document.createElement("canvas");
canvas.width = newWidth;
canvas.height = newHeight;
// Draw the downscaled image on the canvas and return the new data URL.
ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0, newWidth, newHeight);
newDataUrl = canvas.toDataURL(imageType, imageArguments);
return newDataUrl;
}
This code can be used anywhere you have a data URL and want a data URL for a downscaled image.

You can take a look at image-conversion,Try it here --> demo page

Try this Customizable Pure JS Sample - Compress over 90% :
<div id="root">
<p>Upload an image and see the result</p>
<input id="img-input" type="file" accept="image/*" style="display:block" />
</div>
<script>
const MAX_WIDTH = 320;
const MAX_HEIGHT = 180;
const MIME_TYPE = "image/jpeg";
const QUALITY = 0.7;
const input = document.getElementById("img-input");
input.onchange = function (ev) {
const file = ev.target.files[0]; // get the file
const blobURL = URL.createObjectURL(file);
const img = new Image();
img.src = blobURL;
img.onerror = function () {
URL.revokeObjectURL(this.src);
// Handle the failure properly
console.log("Cannot load image");
};
img.onload = function () {
URL.revokeObjectURL(this.src);
const [newWidth, newHeight] = calculateSize(img, MAX_WIDTH, MAX_HEIGHT);
const canvas = document.createElement("canvas");
canvas.width = newWidth;
canvas.height = newHeight;
const ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, newWidth, newHeight);
canvas.toBlob(
(blob) => {
// Handle the compressed image. es. upload or save in local state
displayInfo('Original file', file);
displayInfo('Compressed file', blob);
},
MIME_TYPE,
QUALITY
);
document.getElementById("root").append(canvas);
};
};
function calculateSize(img, maxWidth, maxHeight) {
let width = img.width;
let height = img.height;
// calculate the width and height, constraining the proportions
if (width > height) {
if (width > maxWidth) {
height = Math.round((height * maxWidth) / width);
width = maxWidth;
}
} else {
if (height > maxHeight) {
width = Math.round((width * maxHeight) / height);
height = maxHeight;
}
}
return [width, height];
}
// Utility functions for demo purpose
function displayInfo(label, file) {
const p = document.createElement('p');
p.innerText = `${label} - ${readableBytes(file.size)}`;
document.getElementById('root').append(p);
}
function readableBytes(bytes) {
const i = Math.floor(Math.log(bytes) / Math.log(1024)),
sizes = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
return (bytes / Math.pow(1024, i)).toFixed(2) + ' ' + sizes[i];
}
</script>

I find that there's simpler solution compared to the accepted answer.
Read the files using the HTML5 FileReader API with .readAsArrayBuffer
Create a Blob with the file data and get its url with window.URL.createObjectURL(blob)
Create new Image element and set it's src to the file blob url
Send the image to the canvas. The canvas size is set to desired output size
Get the scaled-down data back from canvas via canvas.toDataURL("image/jpeg",0.7) (set your own output format and quality)
Attach new hidden inputs to the original form and transfer the dataURI images basically as normal text
On backend, read the dataURI, decode from Base64, and save it
As per your question:
Is there a way to compress an image (mostly jpeg, png and gif)
directly browser-side, before uploading it
My solution:
Create a blob with the file directly with URL.createObjectURL(inputFileElement.files[0]).
Same as accepted answer.
Same as accepted answer. Worth mentioning that, canvas size is necessary and use img.width and img.height to set canvas.width and canvas.height. Not img.clientWidth.
Get the scale-down image by canvas.toBlob(callbackfunction(blob){}, 'image/jpeg', 0.5). Setting 'image/jpg' has no effect. image/png is also supported. Make a new File object inside the callbackfunction body with let compressedImageBlob = new File([blob]).
Add new hidden inputs or send via javascript . Server doesn't have to decode anything.
Check https://javascript.info/binary for all information. I came up the solution after reading this chapter.
Code:
<!DOCTYPE html>
<html>
<body>
<form action="upload.php" method="post" enctype="multipart/form-data">
Select image to upload:
<input type="file" name="fileToUpload" id="fileToUpload" multiple>
<input type="submit" value="Upload Image" name="submit">
</form>
</body>
</html>
This code looks far less scary than the other answers..
Update:
One has to put everything inside img.onload. Otherwise canvas will not be able to get the image's width and height correctly as the time canvas is assigned.
function upload(){
var f = fileToUpload.files[0];
var fileName = f.name.split('.')[0];
var img = new Image();
img.src = URL.createObjectURL(f);
img.onload = function(){
var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0);
canvas.toBlob(function(blob){
console.info(blob.size);
var f2 = new File([blob], fileName + ".jpeg");
var xhr = new XMLHttpRequest();
var form = new FormData();
form.append("fileToUpload", f2);
xhr.open("POST", "upload.php");
xhr.send(form);
}, 'image/jpeg', 0.5);
}
}
3.4MB .png file compression test with image/jpeg argument set.
|0.9| 777KB |
|0.8| 383KB |
|0.7| 301KB |
|0.6| 251KB |
|0.5| 219kB |

I had an issue with the downscaleImage() function posted above by #daniel-allen-langdon in that the image.width and image.height properties are not available immediately because the image load is asynchronous.
Please see updated TypeScript example below that takes this into account, uses async functions, and resizes the image based on the longest dimension rather than just the width
function getImage(dataUrl: string): Promise<HTMLImageElement>
{
return new Promise((resolve, reject) => {
const image = new Image();
image.src = dataUrl;
image.onload = () => {
resolve(image);
};
image.onerror = (el: any, err: ErrorEvent) => {
reject(err.error);
};
});
}
export async function downscaleImage(
dataUrl: string,
imageType: string, // e.g. 'image/jpeg'
resolution: number, // max width/height in pixels
quality: number // e.g. 0.9 = 90% quality
): Promise<string> {
// Create a temporary image so that we can compute the height of the image.
const image = await getImage(dataUrl);
const oldWidth = image.naturalWidth;
const oldHeight = image.naturalHeight;
console.log('dims', oldWidth, oldHeight);
const longestDimension = oldWidth > oldHeight ? 'width' : 'height';
const currentRes = longestDimension == 'width' ? oldWidth : oldHeight;
console.log('longest dim', longestDimension, currentRes);
if (currentRes > resolution) {
console.log('need to resize...');
// Calculate new dimensions
const newSize = longestDimension == 'width'
? Math.floor(oldHeight / oldWidth * resolution)
: Math.floor(oldWidth / oldHeight * resolution);
const newWidth = longestDimension == 'width' ? resolution : newSize;
const newHeight = longestDimension == 'height' ? resolution : newSize;
console.log('new width / height', newWidth, newHeight);
// Create a temporary canvas to draw the downscaled image on.
const canvas = document.createElement('canvas');
canvas.width = newWidth;
canvas.height = newHeight;
// Draw the downscaled image on the canvas and return the new data URL.
const ctx = canvas.getContext('2d')!;
ctx.drawImage(image, 0, 0, newWidth, newHeight);
const newDataUrl = canvas.toDataURL(imageType, quality);
return newDataUrl;
}
else {
return dataUrl;
}
}

For Moderm browser use createImageBitmap() instead of img.onload
async function compressImage(blobImg, percent) {
let bitmap = await createImageBitmap(blobImg);
let canvas = document.createElement("canvas");
let ctx = canvas.getContext("2d");
canvas.width = bitmap.width;
canvas.height = bitmap.height;
ctx.drawImage(bitmap, 0, 0);
let dataUrl = canvas.toDataURL("image/jpeg", percent/100);
return dataUrl;
}
inputImg.addEventListener('change', async(e) => {
let img = e.target.files[0];
console.log('File Name: ', img.name)
console.log('Original Size: ', img.size.toLocaleString())
let imgCompressed = await compressImage(img, 75) // set to 75%
let compSize = atob(imgCompressed.split(",")[1]).length;
console.log('Compressed Size: ', compSize.toLocaleString())
//console.log(imgCompressed)
})
<input type="file" id="inputImg">

Edit: As per the Mr Me comment on this answer, it looks like compression is now available for JPG/WebP formats ( see https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL ).
As far as I know, you cannot compress images using canvas, instead, you can resize it. Using canvas.toDataURL will not let you choose the compression ratio to use. You can take a look at canimage that does exactly what you want : https://github.com/nfroidure/CanImage/blob/master/chrome/canimage/content/canimage.js
In fact, it's often sufficient to just resize the image to decrease it's size but if you want to go further, you'll have to use newly introduced method file.readAsArrayBuffer to get a buffer containing the image data.
Then, just use a DataView to read it's content according to the image format specification (http://en.wikipedia.org/wiki/JPEG or http://en.wikipedia.org/wiki/Portable_Network_Graphics).
It'll be hard to deal with image data compression, but it is worse a try. On the other hand, you can try to delete the PNG headers or the JPEG exif data to make your image smaller, it should be easier to do so.
You'll have to create another DataWiew on another buffer and fill it with the filtered image content. Then, you'll just have to encode you're image content to DataURI using window.btoa.
Let me know if you implement something similar, will be interesting to go through the code.

Compressor.js
https://github.com/fengyuanchen/compressorjs
import axios from 'axios';
import Compressor from 'compressorjs';
document.getElementById('file').addEventListener('change', (e) => {
const file = e.target.files[0];
if (!file) {
return;
}
new Compressor(file, {
quality: 0.6,
// The compression process is asynchronous,
// which means you have to access the `result` in the `success` hook function.
success(result) {
const formData = new FormData();
// The third parameter is required for server
formData.append('file', result, result.name);
// Send the compressed image file to server with XMLHttpRequest.
axios.post('/path/to/upload', formData).then(() => {
console.log('Upload success');
});
},
error(err) {
console.log(err.message);
},
});
});

I used the following package:
https://www.npmjs.com/package/browser-image-compression
npm install browser-image-compression
or
yarn add browser-image-compression
Then just following the docs:
import imageCompression from 'browser-image-compression';
const options = {
maxSizeMB: 0.5, // pretty much self-explanatory
maxWidthOrHeight: 500, // apparently px
}
imageCompression(file, options)
.then(function(compressedFile) {
console.log(
"compressedFile instanceof Blob",
compressedFile instanceof Blob
); // true
console.log(
`compressedFile size ${compressedFile.size /
1024 /
1024} MB`
); // smaller than maxSizeMB
return uploader(compressedFile); // code to actual upload, in my case uploader() is a function to upload to Firebase storage.
})
Just in case if you were curios about the uploader(), here's the code of it:
import { initializeApp } from "firebase/app";
const firebaseConfig = {
// your config
};
initializeApp(firebaseConfig);
import { getStorage, ref, uploadBytes, getDownloadURL } from "firebase/storage";
const storage = getStorage();
const sRef = ref(storage);
const uploader = async (file) => {
/* uploads to root */
// const imageRef = ref(sRef, file.name);
// console.log(imageRef);
// await uploadBytes(imageRef, file).then((snapshot) => {
// console.log("Uploaded a blob or file!", snapshot);
// });
/* upload to folder 'techs/' */
const folderRef = ref(sRef, "techs/" + file.name);
await uploadBytes(folderRef, file);
// get URL
const url = await getDownloadURL(ref(storage, folderRef));
console.log("url: ", url);
return url;
};

You can compress an image using the HTML <canvas> element:
function compressImage(imgToCompress, resizingFactor, quality) {
// resizing the image
const canvas = document.createElement("canvas");
const context = canvas.getContext("2d");
const originalWidth = imgToCompress.width;
const originalHeight = imgToCompress.height;
const canvasWidth = originalWidth * resizingFactor;
const canvasHeight = originalHeight * resizingFactor;
canvas.width = canvasWidth;
canvas.height = canvasHeight;
context.drawImage(
imgToCompress,
0,
0,
originalWidth * resizingFactor,
originalHeight * resizingFactor
);
// reducing the quality of the image
canvas.toBlob(
(blob) => {
if (blob) {
// showing the compressed image
resizedImage.src = URL.createObjectURL(resizedImageBlob);
}
},
"image/jpeg",
quality
);
}
See this blog post for an in depth explanation: https://img.ly/blog/how-to-compress-an-image-before-uploading-it-in-javascript/

i improved the function a head to be this :
var minifyImg = function(dataUrl,newWidth,imageType="image/jpeg",resolve,imageArguments=0.7){
var image, oldWidth, oldHeight, newHeight, canvas, ctx, newDataUrl;
(new Promise(function(resolve){
image = new Image(); image.src = dataUrl;
log(image);
resolve('Done : ');
})).then((d)=>{
oldWidth = image.width; oldHeight = image.height;
log([oldWidth,oldHeight]);
newHeight = Math.floor(oldHeight / oldWidth * newWidth);
log(d+' '+newHeight);
canvas = document.createElement("canvas");
canvas.width = newWidth; canvas.height = newHeight;
log(canvas);
ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0, newWidth, newHeight);
//log(ctx);
newDataUrl = canvas.toDataURL(imageType, imageArguments);
resolve(newDataUrl);
});
};
the use of it :
minifyImg(<--DATAURL_HERE-->,<--new width-->,<--type like image/jpeg-->,(data)=>{
console.log(data); // the new DATAURL
});
enjoy ;)

For JPG Image compression you can use the best compression technique called JIC
(Javascript Image Compression)This will definitely help you -->https://github.com/brunobar79/J-I-C

saving user generated image using HTML/JS [duplicate]

Is it possible to capture or print what's displayed in an HTML canvas as an image or PDF?
I'd like to generate an image via canvas and be able to generate a PNG from that image.

Original answer was specific to a similar question. This has been revised:
const canvas = document.getElementById('mycanvas')
const img = canvas.toDataURL('image/png')
With the value in img you can write it out as a new image like so:
document.getElementById('existing-image-id').src = img
or
document.write('<img src="'+img+'"/>');

HTML5 provides Canvas.toDataURL(mimetype) which is implemented in Opera, Firefox, and Safari 4 beta. There are a number of security restrictions, however (mostly to do with drawing content from another origin onto the canvas).
So you don't need an additional library.
e.g.
<canvas id=canvas width=200 height=200></canvas>
<script>
window.onload = function() {
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
context.fillStyle = "green";
context.fillRect(50, 50, 100, 100);
// no argument defaults to image/png; image/jpeg, etc also work on some
// implementations -- image/png is the only one that must be supported per spec.
window.location = canvas.toDataURL("image/png");
}
</script>
Theoretically this should create and then navigate to an image with a green square in the middle of it, but I haven't tested.

I thought I'd extend the scope of this question a bit, with some useful tidbits on the matter.
In order to get the canvas as an image, you should do the following:
var canvas = document.getElementById("mycanvas");
var image = canvas.toDataURL("image/png");
You can use this to write the image to the page:
document.write('<img src="'+image+'"/>');
Where "image/png" is a mime type (png is the only one that must be supported). If you would like an array of the supported types you can do something along the lines of this:
var imageMimes = ['image/png', 'image/bmp', 'image/gif', 'image/jpeg', 'image/tiff']; //Extend as necessary
var acceptedMimes = new Array();
for(i = 0; i < imageMimes.length; i++) {
if(canvas.toDataURL(imageMimes[i]).search(imageMimes[i])>=0) {
acceptedMimes[acceptedMimes.length] = imageMimes[i];
}
}
You only need to run this once per page - it should never change through a page's lifecycle.
If you wish to make the user download the file as it is saved you can do the following:
var canvas = document.getElementById("mycanvas");
var image = canvas.toDataURL("image/png").replace("image/png", "image/octet-stream"); //Convert image to 'octet-stream' (Just a download, really)
window.location.href = image;
If you're using that with different mime types, be sure to change both instances of image/png, but not the image/octet-stream.
It is also worth mentioning that if you use any cross-domain resources in rendering your canvas, you will encounter a security error when you try to use the toDataUrl method.

function exportCanvasAsPNG(id, fileName) {
var canvasElement = document.getElementById(id);
var MIME_TYPE = "image/png";
var imgURL = canvasElement.toDataURL(MIME_TYPE);
var dlLink = document.createElement('a');
dlLink.download = fileName;
dlLink.href = imgURL;
dlLink.dataset.downloadurl = [MIME_TYPE, dlLink.download, dlLink.href].join(':');
document.body.appendChild(dlLink);
dlLink.click();
document.body.removeChild(dlLink);
}

I would use "wkhtmltopdf". It just work great. It uses webkit engine (used in Chrome, Safari, etc.), and it is very easy to use:
wkhtmltopdf stackoverflow.com/questions/923885/ this_question.pdf
That's it!
Try it

Here is some help if you do the download through a server (this way you can name/convert/post-process/etc your file):
-Post data using toDataURL
-Set the headers
$filename = "test.jpg"; //or png
header('Content-Description: File Transfer');
if($msie = !strstr($_SERVER["HTTP_USER_AGENT"],"MSIE")==false)
header("Content-type: application/force-download");else
header("Content-type: application/octet-stream");
header("Content-Disposition: attachment; filename=\"$filename\"");
header("Content-Transfer-Encoding: binary");
header("Expires: 0"); header("Cache-Control: must-revalidate");
header("Pragma: public");
-create image
$data = $_POST['data'];
$img = imagecreatefromstring(base64_decode(substr($data,strpos($data,',')+1)));
-export image as JPEG
$width = imagesx($img);
$height = imagesy($img);
$output = imagecreatetruecolor($width, $height);
$white = imagecolorallocate($output, 255, 255, 255);
imagefilledrectangle($output, 0, 0, $width, $height, $white);
imagecopy($output, $img, 0, 0, 0, 0, $width, $height);
imagejpeg($output);
exit();
-or as transparent PNG
imagesavealpha($img, true);
imagepng($img);
die($img);

This is the other way, without strings although I don't really know if it's faster or not. Instead of toDataURL (as all questions here propose). In my case want to prevent dataUrl/base64 since I need a Array buffer or view. So the other method in HTMLCanvasElement is toBlob. (TypeScript function):
export function canvasToArrayBuffer(canvas: HTMLCanvasElement, mime: string): Promise<ArrayBuffer> {
return new Promise((resolve, reject) => canvas.toBlob(async (d) => {
if (d) {
const r = new FileReader();
r.addEventListener('loadend', e => {
const ab = r.result;
if (ab) {
resolve(ab as ArrayBuffer);
}
else {
reject(new Error('Expected FileReader result'));
}
}); r.addEventListener('error', e => {
reject(e)
});
r.readAsArrayBuffer(d);
}
else {
reject(new Error('Expected toBlob() to be defined'));
}
}, mime));
}
Another advantage of blobs is you can create ObjectUrls to represent data as files, similar to HTMLInputFile's 'files' member. More info:
https://developer.mozilla.org/en/docs/Web/API/HTMLCanvasElement/toBlob

Another interesting solution is PhantomJS.
It's a headless WebKit scriptable with JavaScript or CoffeeScript.
One of the use case is screen capture : you can programmatically capture web contents, including SVG and Canvas and/or Create web site screenshots with thumbnail preview.
The best entry point is the screen capture wiki page.
Here is a good example for polar clock (from RaphaelJS):
>phantomjs rasterize.js http://raphaeljs.com/polar-clock.html clock.png
Do you want to render a page to a PDF ?
> phantomjs rasterize.js 'http://en.wikipedia.org/w/index.php?title=Jakarta&printable=yes' jakarta.pdf

If you are using jQuery, which quite a lot of people do, then you would implement the accepted answer like so:
var canvas = $("#mycanvas")[0];
var img = canvas.toDataURL("image/png");
$("#elememt-to-write-to").html('<img src="'+img+'"/>');

The key point is
canvas.toDataURL(type, quality)
And I want to provide an example for someone like me who wants to save SVG to PNG(also can add some text if you wish), which may be from an Online source or font-awesome icon, etc.
Example
100% javascript and no other 3-rd library.
<script>
(() => {
window.onload = () => {
// Test 1: SVG from Online
const canvas = new Canvas(650, 500)
// canvas.DrawGrid() // If you want to show grid, you can use it.
const svg2img = new SVG2IMG(canvas.canvas, "https://upload.wikimedia.org/wikipedia/commons/b/bd/Test.svg")
svg2img.AddText("Hello", 100, 250, {mode: "fill", color: "yellow", alpha: 0.8})
svg2img.AddText("world", 200, 250, {mode: "stroke", color: "red"})
svg2img.AddText("!", 280, 250, {color: "#f700ff", size: "72px"})
svg2img.Build("Test.png")
// Test 2: URI.data
const canvas2 = new Canvas(180, 180)
const uriData = "data:image/svg+xml;base64,PHN2ZyBjbGFzcz0ic3ZnLWlubGluZS0tZmEgZmEtc21pbGUtd2luayBmYS13LTE2IiBhcmlhLWhpZGRlbj0idHJ1ZSIgZm9jdXNhYmxlPSJmYWxzZSIgZGF0YS1wcmVmaXg9ImZhciIgZGF0YS1pY29uPSJzbWlsZS13aW5rIiByb2xlPSJpbWciIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyIgdmlld0JveD0iMCAwIDQ5NiA1MTIiIGRhdGEtZmEtaTJzdmc9IiI+PHBhdGggZmlsbD0iY3VycmVudENvbG9yIiBkPSJNMjQ4IDhDMTExIDggMCAxMTkgMCAyNTZzMTExIDI0OCAyNDggMjQ4IDI0OC0xMTEgMjQ4LTI0OFMzODUgOCAyNDggOHptMCA0NDhjLTExMC4zIDAtMjAwLTg5LjctMjAwLTIwMFMxMzcuNyA1NiAyNDggNTZzMjAwIDg5LjcgMjAwIDIwMC04OS43IDIwMC0yMDAgMjAwem0xMTcuOC0xNDYuNGMtMTAuMi04LjUtMjUuMy03LjEtMzMuOCAzLjEtMjAuOCAyNS01MS41IDM5LjQtODQgMzkuNHMtNjMuMi0xNC4zLTg0LTM5LjRjLTguNS0xMC4yLTIzLjctMTEuNS0zMy44LTMuMS0xMC4yIDguNS0xMS41IDIzLjYtMy4xIDMzLjggMzAgMzYgNzQuMSA1Ni42IDEyMC45IDU2LjZzOTAuOS0yMC42IDEyMC45LTU2LjZjOC41LTEwLjIgNy4xLTI1LjMtMy4xLTMzLjh6TTE2OCAyNDBjMTcuNyAwIDMyLTE0LjMgMzItMzJzLTE0LjMtMzItMzItMzItMzIgMTQuMy0zMiAzMiAxNC4zIDMyIDMyIDMyem0xNjAtNjBjLTI1LjcgMC01NS45IDE2LjktNTkuOSA0Mi4xLTEuNyAxMS4yIDExLjUgMTguMiAxOS44IDEwLjhsOS41LTguNWMxNC44LTEzLjIgNDYuMi0xMy4yIDYxIDBsOS41IDguNWM4LjUgNy40IDIxLjYuMyAxOS44LTEwLjgtMy44LTI1LjItMzQtNDIuMS01OS43LTQyLjF6Ij48L3BhdGg+PC9zdmc+"
const svg2img2 = new SVG2IMG(canvas2.canvas, uriData)
svg2img2.Build("SmileWink.png")
// Test 3: Exists SVG
ImportFontAwesome()
const range = document.createRange()
const fragSmile = range.createContextualFragment(`<i class="far fa-smile" style="background-color:black;color:yellow"></i>`)
document.querySelector(`body`).append(fragSmile)
// use MutationObserver wait the fontawesome convert ``<i class="far fa-smile"></i>`` to SVG. If you write the element in the HTML, then you can skip this hassle way.
const observer = new MutationObserver((mutationRecordList, observer) => {
for (const mutation of mutationRecordList) {
switch (mutation.type) {
case "childList":
const targetSVG = mutation.target.querySelector(`svg`)
if (targetSVG !== null) {
const canvas3 = new Canvas(64, 64) // 👈 Focus here. The part of the observer is not important.
const svg2img3 = new SVG2IMG(canvas3.canvas, SVG2IMG.Convert2URIData(targetSVG))
svg2img3.Build("Smile.png")
targetSVG.remove() // This SVG is created by font-awesome, and it's an extra element. I don't want to see it.
observer.disconnect()
return
}
}
}
})
observer.observe(document.querySelector(`body`), {childList: true})
}
})()
class SVG2IMG {
/**
* #param {HTMLCanvasElement} canvas
* #param {string} src "http://.../xxx.svg" or "data:image/svg+xml;base64,${base64}"
* */
constructor(canvas, src) {
this.canvas = canvas;
this.context = this.canvas.getContext("2d")
this.src = src
this.addTextList = []
}
/**
* #param {HTMLElement} node
* #param {string} mediaType: https://en.wikipedia.org/wiki/Media_type#Common_examples_%5B10%5D
* #see https://en.wikipedia.org/wiki/List_of_URI_schemes
* */
static Convert2URIData(node, mediaType = 'data:image/svg+xml') {
const base64 = btoa(node.outerHTML)
return `${mediaType};base64,${base64}`
}
/**
* #param {string} text
* #param {int} x
* #param {int} y
* #param {"stroke"|"fill"} mode
* #param {string} size, "30px"
* #param {string} font, example: "Arial"
* #param {string} color, example: "#3ae016" or "yellow"
* #param {int} alpha, 0.0 (fully transparent) to 1.0 (fully opaque) // https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Applying_styles_and_colors#transparency
* */
AddText(text, x, y, {mode = "fill", size = "32px", font = "Arial", color = "black", alpha = 1.0}) {
const drawFunc = (text, x, y, mode, font) => {
return () => {
// https://www.w3schools.com/graphics/canvas_text.asp
// https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/fillText
const context = this.context
const originAlpha = context.globalAlpha
context.globalAlpha = alpha
context.font = `${size} ${font}`
switch (mode) {
case "fill":
context.fillStyle = color
context.fillText(text, x, y)
break
case "stroke":
context.strokeStyle = color
context.strokeText(text, x, y)
break
default:
throw Error(`Unknown mode:${mode}`)
}
context.globalAlpha = originAlpha
}
}
this.addTextList.push(drawFunc(text, x, y, mode, font))
}
/**
* #description When the build is finished, you can click the filename to download the PNG or mouse enters to copy PNG to the clipboard.
* */
Build(filename = "download.png") {
const img = new Image()
img.src = this.src
img.crossOrigin = "anonymous" // Fixes: Tainted canvases may not be exported
img.onload = (event) => {
this.context.drawImage(event.target, 0, 0)
for (const drawTextFunc of this.addTextList) {
drawTextFunc()
}
// create a "a" node for download
const a = document.createElement('a')
document.querySelector('body').append(a)
a.innerText = filename
a.download = filename
const quality = 1.0
// a.target = "_blank"
a.href = this.canvas.toDataURL("image/png", quality)
a.append(this.canvas)
}
this.canvas.onmouseenter = (event) => {
// set background to white. Otherwise, background-color is black.
this.context.globalCompositeOperation = "destination-over" // https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/globalCompositeOperation // https://www.w3schools.com/tags/canvas_globalcompositeoperation.asp
this.context.fillStyle = "rgb(255,255,255)"
this.context.fillRect(0, 0, this.canvas.width, this.canvas.height)
this.canvas.toBlob(blob => navigator.clipboard.write([new ClipboardItem({'image/png': blob})])) // copy to clipboard
}
}
}
class Canvas {
/**
* #description for do something like that: ``<canvas width="" height=""></>canvas>``
**/
constructor(w, h) {
const canvas = document.createElement("canvas")
document.querySelector(`body`).append(canvas)
this.canvas = canvas;
[this.canvas.width, this.canvas.height] = [w, h]
}
/**
* #description If your SVG is large, you may want to know which part is what you wanted.
* */
DrawGrid(step = 100) {
const ctx = this.canvas.getContext('2d')
const w = this.canvas.width
const h = this.canvas.height
// Draw the vertical line.
ctx.beginPath();
for (let x = 0; x <= w; x += step) {
ctx.moveTo(x, 0);
ctx.lineTo(x, h);
}
// set the color of the line
ctx.strokeStyle = 'rgba(255,0,0, 0.5)'
ctx.lineWidth = 1
ctx.stroke();
// Draw the horizontal line.
ctx.beginPath();
for (let y = 0; y <= h; y += step) {
ctx.moveTo(0, y)
ctx.lineTo(w, y)
}
ctx.strokeStyle = 'rgba(128, 128, 128, 0.5)'
ctx.lineWidth = 5
ctx.stroke()
}
}
function ImportFontAwesome() {
const range = document.createRange()
const frag = range.createContextualFragment(`
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.2/css/all.min.css" integrity="sha512-HK5fgLBL+xu6dm/Ii3z4xhlSUyZgTT9tuc/hSrtw6uzJOvgRr2a9jyxxT1ely+B+xFAmJKVSTbpM/CuL7qxO8w==" crossorigin="anonymous" />
<script src="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.15.2/js/all.min.js" integrity="sha512-UwcC/iaz5ziHX7V6LjSKaXgCuRRqbTp1QHpbOJ4l1nw2/boCfZ2KlFIqBUA/uRVF0onbREnY9do8rM/uT/ilqw==" crossorigin="anonymous"/>
`)
document.querySelector("head").append(frag)
}
</script>
if you want to run on stackoverflow and move your mouse on the picture may get error
DOMException: The Clipboard API has been blocked because of a permissions policy applied to the current document
You can copy the code on your local machine and run it again, will be fine.

upload image from <canvas />:
async function canvasToBlob(canvas) {
if (canvas.toBlob) {
return new Promise(function (resolve) {
canvas.toBlob(resolve)
})
} else {
throw new Error('canvas.toBlob Invalid')
}
}
await canvasToBlob(yourCanvasEl)

On some versions of Chrome, you can:
Use the draw image function ctx.drawImage(image1, 0, 0, w, h);Right-click on the canvas

You can use jspdf to capture a canvas into an image or pdf like this:
var imgData = canvas.toDataURL('image/png');
var doc = new jsPDF('p', 'mm');
doc.addImage(imgData, 'PNG', 10, 10);
doc.save('sample-file.pdf');
More info: https://github.com/MrRio/jsPDF

The simple answer is just to take the blob of it and set the img src to a new object URL of that blob, then add that image to a PDF using some library, like
var ok = document.createElement("canvas")
ok.width = 400
ok.height = 140
var ctx = ok.getContext("2d");
for(let k = 0; k < ok.height; k++)
(
k
%
Math.floor(
(
Math.random()
) *
10
)
==
0
) && (y => {
for(var i = 0; i < ok.width; i++) {
if(i % 25 == 0) {
ctx.globalAlpha = Math.random()
ctx.fillStyle = (
"rgb(" +
Math.random() * 255 + "," +
Math.random() * 255 + "," +
Math.random() * 255 + ")"
);
(wdth =>
ctx.fillRect(
Math.sin(
i * Math.PI / 180
) *
Math.random() *
ok.width,
Math.cos(
i * Math.PI / 180,
) * wdth + y,
wdth,
wdth
)
)(15)
}
}
})(k)
ok.toBlob(blob => {
k.src = URL.createObjectURL(blob)
})
<img id=k>
Alternatively, if you wanted to work with low-level byte data, you can get the raw bytes of the canvas, then, depending on the file spec, write the raw image data into the necessary bytes of the data. you just need to call ctx.getImageData(0, 0, ctx.canvas.widht, ctx.canvas.height) to get the raw image data, then based on the file specification, write it to that

if you want to emebed the canvas you can use this snippet
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
</head>
<body>
<canvas id=canvas width=200 height=200></canvas>
<iframe id='img' width=200 height=200></iframe>
<script>
window.onload = function() {
var canvas = document.getElementById("canvas");
var context = canvas.getContext("2d");
context.fillStyle = "green";
context.fillRect(50, 50, 100, 100);
document.getElementById('img').src = canvas.toDataURL("image/jpeg");
console.log(canvas.toDataURL("image/jpeg"));
}
</script>
</body>
</html>

Resizing Image on client side and save to server [duplicate]

TL;DR;
Is there a way to compress an image (mostly jpeg, png and gif) directly browser-side, before uploading it ? I'm pretty sure JavaScript can do this, but I can't find a way to achieve it.
Here's the full scenario I would like to implement:
the user goes to my website, and choose an image via an input type="file" element,
this image is retrieved via JavaScript, we do some verification such as correct file format, maximum file size etc,
if every thing is OK, a preview of the image is displayed on the page,
the user can do some basic operations such as rotate the image by 90°/-90°, crop it following a pre-defined ratio, etc, or the user can upload another image and return to step 1,
when the user is satisfied, the edited image is then compressed and "saved" locally (not saved to a file, but in the browser memory/page),-
the user fill a form with data like name, age etc,
the user click on the "Finish" button, then the form containing datas + compressed image is sent to the server (without AJAX),
The full process up to the last step should be done client side, and should be compatible on latest Chrome and Firefox, Safari 5+ and IE 8+. If possible, only JavaScript should be used (but I'm pretty sure this is not possible).
I've not code anything right now, but I've thought about it already. File reading locally is possible via File API, image previewing and editing could be done using Canvas element, but I can't find a way to do the image compression part.
According to html5please.com and caniuse.com, supporting those browser is quite hard (thanks to IE), but could be done using polyfill such as FlashCanvas and FileReader.
Actually, the goal is to reduce file size, so I see image compression as a solution. But, I know that uploaded images are going to be displayed on my website, every time at the same place, and I know the dimension of this display area (eg. 200x400). So, I could resize the image to fit those dimensions, thus reducing file size. I have no idea what would be the compression ratio for this technique.
What do you think ? Do you have any advice to tell me ? Do you know any way to compress an image browser-side in JavaScript ? Thanks for your replies.

In short:
Read the files using the HTML5 FileReader API with .readAsArrayBuffer
Create a Blob with the file data and get its url with window.URL.createObjectURL(blob)
Create new Image element and set it's src to the file blob url
Send the image to the canvas. The canvas size is set to desired output size
Get the scaled-down data back from canvas via canvas.toDataURL("image/jpeg",0.7) (set your own output format and quality)
Attach new hidden inputs to the original form and transfer the dataURI images basically as normal text
On backend, read the dataURI, decode from Base64, and save it
Source: code.

I see two things missing from the other answers:
canvas.toBlob (when available) is more performant than canvas.toDataURL, and also async.
the file -> image -> canvas -> file conversion loses EXIF data; in particular, data about image rotation commonly set by modern phones/tablets.
The following script deals with both points:
// From https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob, needed for Safari:
if (!HTMLCanvasElement.prototype.toBlob) {
Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
value: function(callback, type, quality) {
var binStr = atob(this.toDataURL(type, quality).split(',')[1]),
len = binStr.length,
arr = new Uint8Array(len);
for (var i = 0; i < len; i++) {
arr[i] = binStr.charCodeAt(i);
}
callback(new Blob([arr], {type: type || 'image/png'}));
}
});
}
window.URL = window.URL || window.webkitURL;
// Modified from https://stackoverflow.com/a/32490603, cc by-sa 3.0
// -2 = not jpeg, -1 = no data, 1..8 = orientations
function getExifOrientation(file, callback) {
// Suggestion from http://code.flickr.net/2012/06/01/parsing-exif-client-side-using-javascript-2/:
if (file.slice) {
file = file.slice(0, 131072);
} else if (file.webkitSlice) {
file = file.webkitSlice(0, 131072);
}
var reader = new FileReader();
reader.onload = function(e) {
var view = new DataView(e.target.result);
if (view.getUint16(0, false) != 0xFFD8) {
callback(-2);
return;
}
var length = view.byteLength, offset = 2;
while (offset < length) {
var marker = view.getUint16(offset, false);
offset += 2;
if (marker == 0xFFE1) {
if (view.getUint32(offset += 2, false) != 0x45786966) {
callback(-1);
return;
}
var little = view.getUint16(offset += 6, false) == 0x4949;
offset += view.getUint32(offset + 4, little);
var tags = view.getUint16(offset, little);
offset += 2;
for (var i = 0; i < tags; i++)
if (view.getUint16(offset + (i * 12), little) == 0x0112) {
callback(view.getUint16(offset + (i * 12) + 8, little));
return;
}
}
else if ((marker & 0xFF00) != 0xFF00) break;
else offset += view.getUint16(offset, false);
}
callback(-1);
};
reader.readAsArrayBuffer(file);
}
// Derived from https://stackoverflow.com/a/40867559, cc by-sa
function imgToCanvasWithOrientation(img, rawWidth, rawHeight, orientation) {
var canvas = document.createElement('canvas');
if (orientation > 4) {
canvas.width = rawHeight;
canvas.height = rawWidth;
} else {
canvas.width = rawWidth;
canvas.height = rawHeight;
}
if (orientation > 1) {
console.log("EXIF orientation = " + orientation + ", rotating picture");
}
var ctx = canvas.getContext('2d');
switch (orientation) {
case 2: ctx.transform(-1, 0, 0, 1, rawWidth, 0); break;
case 3: ctx.transform(-1, 0, 0, -1, rawWidth, rawHeight); break;
case 4: ctx.transform(1, 0, 0, -1, 0, rawHeight); break;
case 5: ctx.transform(0, 1, 1, 0, 0, 0); break;
case 6: ctx.transform(0, 1, -1, 0, rawHeight, 0); break;
case 7: ctx.transform(0, -1, -1, 0, rawHeight, rawWidth); break;
case 8: ctx.transform(0, -1, 1, 0, 0, rawWidth); break;
}
ctx.drawImage(img, 0, 0, rawWidth, rawHeight);
return canvas;
}
function reduceFileSize(file, acceptFileSize, maxWidth, maxHeight, quality, callback) {
if (file.size <= acceptFileSize) {
callback(file);
return;
}
var img = new Image();
img.onerror = function() {
URL.revokeObjectURL(this.src);
callback(file);
};
img.onload = function() {
URL.revokeObjectURL(this.src);
getExifOrientation(file, function(orientation) {
var w = img.width, h = img.height;
var scale = (orientation > 4 ?
Math.min(maxHeight / w, maxWidth / h, 1) :
Math.min(maxWidth / w, maxHeight / h, 1));
h = Math.round(h * scale);
w = Math.round(w * scale);
var canvas = imgToCanvasWithOrientation(img, w, h, orientation);
canvas.toBlob(function(blob) {
console.log("Resized image to " + w + "x" + h + ", " + (blob.size >> 10) + "kB");
callback(blob);
}, 'image/jpeg', quality);
});
};
img.src = URL.createObjectURL(file);
}
Example usage:
inputfile.onchange = function() {
// If file size > 500kB, resize such that width <= 1000, quality = 0.9
reduceFileSize(this.files[0], 500*1024, 1000, Infinity, 0.9, blob => {
let body = new FormData();
body.set('file', blob, blob.name || "file.jpg");
fetch('/upload-image', {method: 'POST', body}).then(...);
});
};

#PsychoWoods' answer is good. I would like to offer my own solution. This Javascript function takes an image data URL and a width, scales it to the new width, and returns a new data URL.
// Take an image URL, downscale it to the given width, and return a new image URL.
function downscaleImage(dataUrl, newWidth, imageType, imageArguments) {
"use strict";
var image, oldWidth, oldHeight, newHeight, canvas, ctx, newDataUrl;
// Provide default values
imageType = imageType || "image/jpeg";
imageArguments = imageArguments || 0.7;
// Create a temporary image so that we can compute the height of the downscaled image.
image = new Image();
image.src = dataUrl;
oldWidth = image.width;
oldHeight = image.height;
newHeight = Math.floor(oldHeight / oldWidth * newWidth)
// Create a temporary canvas to draw the downscaled image on.
canvas = document.createElement("canvas");
canvas.width = newWidth;
canvas.height = newHeight;
// Draw the downscaled image on the canvas and return the new data URL.
ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0, newWidth, newHeight);
newDataUrl = canvas.toDataURL(imageType, imageArguments);
return newDataUrl;
}
This code can be used anywhere you have a data URL and want a data URL for a downscaled image.

You can take a look at image-conversion,Try it here --> demo page

Try this Customizable Pure JS Sample - Compress over 90% :
<div id="root">
<p>Upload an image and see the result</p>
<input id="img-input" type="file" accept="image/*" style="display:block" />
</div>
<script>
const MAX_WIDTH = 320;
const MAX_HEIGHT = 180;
const MIME_TYPE = "image/jpeg";
const QUALITY = 0.7;
const input = document.getElementById("img-input");
input.onchange = function (ev) {
const file = ev.target.files[0]; // get the file
const blobURL = URL.createObjectURL(file);
const img = new Image();
img.src = blobURL;
img.onerror = function () {
URL.revokeObjectURL(this.src);
// Handle the failure properly
console.log("Cannot load image");
};
img.onload = function () {
URL.revokeObjectURL(this.src);
const [newWidth, newHeight] = calculateSize(img, MAX_WIDTH, MAX_HEIGHT);
const canvas = document.createElement("canvas");
canvas.width = newWidth;
canvas.height = newHeight;
const ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0, newWidth, newHeight);
canvas.toBlob(
(blob) => {
// Handle the compressed image. es. upload or save in local state
displayInfo('Original file', file);
displayInfo('Compressed file', blob);
},
MIME_TYPE,
QUALITY
);
document.getElementById("root").append(canvas);
};
};
function calculateSize(img, maxWidth, maxHeight) {
let width = img.width;
let height = img.height;
// calculate the width and height, constraining the proportions
if (width > height) {
if (width > maxWidth) {
height = Math.round((height * maxWidth) / width);
width = maxWidth;
}
} else {
if (height > maxHeight) {
width = Math.round((width * maxHeight) / height);
height = maxHeight;
}
}
return [width, height];
}
// Utility functions for demo purpose
function displayInfo(label, file) {
const p = document.createElement('p');
p.innerText = `${label} - ${readableBytes(file.size)}`;
document.getElementById('root').append(p);
}
function readableBytes(bytes) {
const i = Math.floor(Math.log(bytes) / Math.log(1024)),
sizes = ['B', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
return (bytes / Math.pow(1024, i)).toFixed(2) + ' ' + sizes[i];
}
</script>

I find that there's simpler solution compared to the accepted answer.
Read the files using the HTML5 FileReader API with .readAsArrayBuffer
Create a Blob with the file data and get its url with window.URL.createObjectURL(blob)
Create new Image element and set it's src to the file blob url
Send the image to the canvas. The canvas size is set to desired output size
Get the scaled-down data back from canvas via canvas.toDataURL("image/jpeg",0.7) (set your own output format and quality)
Attach new hidden inputs to the original form and transfer the dataURI images basically as normal text
On backend, read the dataURI, decode from Base64, and save it
As per your question:
Is there a way to compress an image (mostly jpeg, png and gif)
directly browser-side, before uploading it
My solution:
Create a blob with the file directly with URL.createObjectURL(inputFileElement.files[0]).
Same as accepted answer.
Same as accepted answer. Worth mentioning that, canvas size is necessary and use img.width and img.height to set canvas.width and canvas.height. Not img.clientWidth.
Get the scale-down image by canvas.toBlob(callbackfunction(blob){}, 'image/jpeg', 0.5). Setting 'image/jpg' has no effect. image/png is also supported. Make a new File object inside the callbackfunction body with let compressedImageBlob = new File([blob]).
Add new hidden inputs or send via javascript . Server doesn't have to decode anything.
Check https://javascript.info/binary for all information. I came up the solution after reading this chapter.
Code:
<!DOCTYPE html>
<html>
<body>
<form action="upload.php" method="post" enctype="multipart/form-data">
Select image to upload:
<input type="file" name="fileToUpload" id="fileToUpload" multiple>
<input type="submit" value="Upload Image" name="submit">
</form>
</body>
</html>
This code looks far less scary than the other answers..
Update:
One has to put everything inside img.onload. Otherwise canvas will not be able to get the image's width and height correctly as the time canvas is assigned.
function upload(){
var f = fileToUpload.files[0];
var fileName = f.name.split('.')[0];
var img = new Image();
img.src = URL.createObjectURL(f);
img.onload = function(){
var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(img, 0, 0);
canvas.toBlob(function(blob){
console.info(blob.size);
var f2 = new File([blob], fileName + ".jpeg");
var xhr = new XMLHttpRequest();
var form = new FormData();
form.append("fileToUpload", f2);
xhr.open("POST", "upload.php");
xhr.send(form);
}, 'image/jpeg', 0.5);
}
}
3.4MB .png file compression test with image/jpeg argument set.
|0.9| 777KB |
|0.8| 383KB |
|0.7| 301KB |
|0.6| 251KB |
|0.5| 219kB |

I had an issue with the downscaleImage() function posted above by #daniel-allen-langdon in that the image.width and image.height properties are not available immediately because the image load is asynchronous.
Please see updated TypeScript example below that takes this into account, uses async functions, and resizes the image based on the longest dimension rather than just the width
function getImage(dataUrl: string): Promise<HTMLImageElement>
{
return new Promise((resolve, reject) => {
const image = new Image();
image.src = dataUrl;
image.onload = () => {
resolve(image);
};
image.onerror = (el: any, err: ErrorEvent) => {
reject(err.error);
};
});
}
export async function downscaleImage(
dataUrl: string,
imageType: string, // e.g. 'image/jpeg'
resolution: number, // max width/height in pixels
quality: number // e.g. 0.9 = 90% quality
): Promise<string> {
// Create a temporary image so that we can compute the height of the image.
const image = await getImage(dataUrl);
const oldWidth = image.naturalWidth;
const oldHeight = image.naturalHeight;
console.log('dims', oldWidth, oldHeight);
const longestDimension = oldWidth > oldHeight ? 'width' : 'height';
const currentRes = longestDimension == 'width' ? oldWidth : oldHeight;
console.log('longest dim', longestDimension, currentRes);
if (currentRes > resolution) {
console.log('need to resize...');
// Calculate new dimensions
const newSize = longestDimension == 'width'
? Math.floor(oldHeight / oldWidth * resolution)
: Math.floor(oldWidth / oldHeight * resolution);
const newWidth = longestDimension == 'width' ? resolution : newSize;
const newHeight = longestDimension == 'height' ? resolution : newSize;
console.log('new width / height', newWidth, newHeight);
// Create a temporary canvas to draw the downscaled image on.
const canvas = document.createElement('canvas');
canvas.width = newWidth;
canvas.height = newHeight;
// Draw the downscaled image on the canvas and return the new data URL.
const ctx = canvas.getContext('2d')!;
ctx.drawImage(image, 0, 0, newWidth, newHeight);
const newDataUrl = canvas.toDataURL(imageType, quality);
return newDataUrl;
}
else {
return dataUrl;
}
}

For Moderm browser use createImageBitmap() instead of img.onload
async function compressImage(blobImg, percent) {
let bitmap = await createImageBitmap(blobImg);
let canvas = document.createElement("canvas");
let ctx = canvas.getContext("2d");
canvas.width = bitmap.width;
canvas.height = bitmap.height;
ctx.drawImage(bitmap, 0, 0);
let dataUrl = canvas.toDataURL("image/jpeg", percent/100);
return dataUrl;
}
inputImg.addEventListener('change', async(e) => {
let img = e.target.files[0];
console.log('File Name: ', img.name)
console.log('Original Size: ', img.size.toLocaleString())
let imgCompressed = await compressImage(img, 75) // set to 75%
let compSize = atob(imgCompressed.split(",")[1]).length;
console.log('Compressed Size: ', compSize.toLocaleString())
//console.log(imgCompressed)
})
<input type="file" id="inputImg">

Edit: As per the Mr Me comment on this answer, it looks like compression is now available for JPG/WebP formats ( see https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toDataURL ).
As far as I know, you cannot compress images using canvas, instead, you can resize it. Using canvas.toDataURL will not let you choose the compression ratio to use. You can take a look at canimage that does exactly what you want : https://github.com/nfroidure/CanImage/blob/master/chrome/canimage/content/canimage.js
In fact, it's often sufficient to just resize the image to decrease it's size but if you want to go further, you'll have to use newly introduced method file.readAsArrayBuffer to get a buffer containing the image data.
Then, just use a DataView to read it's content according to the image format specification (http://en.wikipedia.org/wiki/JPEG or http://en.wikipedia.org/wiki/Portable_Network_Graphics).
It'll be hard to deal with image data compression, but it is worse a try. On the other hand, you can try to delete the PNG headers or the JPEG exif data to make your image smaller, it should be easier to do so.
You'll have to create another DataWiew on another buffer and fill it with the filtered image content. Then, you'll just have to encode you're image content to DataURI using window.btoa.
Let me know if you implement something similar, will be interesting to go through the code.

Compressor.js
https://github.com/fengyuanchen/compressorjs
import axios from 'axios';
import Compressor from 'compressorjs';
document.getElementById('file').addEventListener('change', (e) => {
const file = e.target.files[0];
if (!file) {
return;
}
new Compressor(file, {
quality: 0.6,
// The compression process is asynchronous,
// which means you have to access the `result` in the `success` hook function.
success(result) {
const formData = new FormData();
// The third parameter is required for server
formData.append('file', result, result.name);
// Send the compressed image file to server with XMLHttpRequest.
axios.post('/path/to/upload', formData).then(() => {
console.log('Upload success');
});
},
error(err) {
console.log(err.message);
},
});
});

I used the following package:
https://www.npmjs.com/package/browser-image-compression
npm install browser-image-compression
or
yarn add browser-image-compression
Then just following the docs:
import imageCompression from 'browser-image-compression';
const options = {
maxSizeMB: 0.5, // pretty much self-explanatory
maxWidthOrHeight: 500, // apparently px
}
imageCompression(file, options)
.then(function(compressedFile) {
console.log(
"compressedFile instanceof Blob",
compressedFile instanceof Blob
); // true
console.log(
`compressedFile size ${compressedFile.size /
1024 /
1024} MB`
); // smaller than maxSizeMB
return uploader(compressedFile); // code to actual upload, in my case uploader() is a function to upload to Firebase storage.
})
Just in case if you were curios about the uploader(), here's the code of it:
import { initializeApp } from "firebase/app";
const firebaseConfig = {
// your config
};
initializeApp(firebaseConfig);
import { getStorage, ref, uploadBytes, getDownloadURL } from "firebase/storage";
const storage = getStorage();
const sRef = ref(storage);
const uploader = async (file) => {
/* uploads to root */
// const imageRef = ref(sRef, file.name);
// console.log(imageRef);
// await uploadBytes(imageRef, file).then((snapshot) => {
// console.log("Uploaded a blob or file!", snapshot);
// });
/* upload to folder 'techs/' */
const folderRef = ref(sRef, "techs/" + file.name);
await uploadBytes(folderRef, file);
// get URL
const url = await getDownloadURL(ref(storage, folderRef));
console.log("url: ", url);
return url;
};

You can compress an image using the HTML <canvas> element:
function compressImage(imgToCompress, resizingFactor, quality) {
// resizing the image
const canvas = document.createElement("canvas");
const context = canvas.getContext("2d");
const originalWidth = imgToCompress.width;
const originalHeight = imgToCompress.height;
const canvasWidth = originalWidth * resizingFactor;
const canvasHeight = originalHeight * resizingFactor;
canvas.width = canvasWidth;
canvas.height = canvasHeight;
context.drawImage(
imgToCompress,
0,
0,
originalWidth * resizingFactor,
originalHeight * resizingFactor
);
// reducing the quality of the image
canvas.toBlob(
(blob) => {
if (blob) {
// showing the compressed image
resizedImage.src = URL.createObjectURL(resizedImageBlob);
}
},
"image/jpeg",
quality
);
}
See this blog post for an in depth explanation: https://img.ly/blog/how-to-compress-an-image-before-uploading-it-in-javascript/

i improved the function a head to be this :
var minifyImg = function(dataUrl,newWidth,imageType="image/jpeg",resolve,imageArguments=0.7){
var image, oldWidth, oldHeight, newHeight, canvas, ctx, newDataUrl;
(new Promise(function(resolve){
image = new Image(); image.src = dataUrl;
log(image);
resolve('Done : ');
})).then((d)=>{
oldWidth = image.width; oldHeight = image.height;
log([oldWidth,oldHeight]);
newHeight = Math.floor(oldHeight / oldWidth * newWidth);
log(d+' '+newHeight);
canvas = document.createElement("canvas");
canvas.width = newWidth; canvas.height = newHeight;
log(canvas);
ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0, newWidth, newHeight);
//log(ctx);
newDataUrl = canvas.toDataURL(imageType, imageArguments);
resolve(newDataUrl);
});
};
the use of it :
minifyImg(<--DATAURL_HERE-->,<--new width-->,<--type like image/jpeg-->,(data)=>{
console.log(data); // the new DATAURL
});
enjoy ;)

For JPG Image compression you can use the best compression technique called JIC
(Javascript Image Compression)This will definitely help you -->https://github.com/brunobar79/J-I-C

Create a waveform of the full track with Web Audio API

Realtime moving Waveform
I'm currently playing with Web Audio API and made a spectrum using canvas.
function animate(){
var a=new Uint8Array(analyser.frequencyBinCount),
y=new Uint8Array(analyser.frequencyBinCount),b,c,d;
analyser.getByteTimeDomainData(y);
analyser.getByteFrequencyData(a);
b=c=a.length;
d=w/c;
ctx.clearRect(0,0,w,h);
while(b--){
var bh=a[b]+1;
ctx.fillStyle='hsla('+(b/c*240)+','+(y[b]/255*100|0)+'%,50%,1)';
ctx.fillRect(1*b,h-bh,1,bh);
ctx.fillRect(1*b,y[b],1,1);
}
animation=webkitRequestAnimationFrame(animate);
}
Mini question: is there a way to not write 2 times new Uint8Array(analyser.frequencyBinCount)?
DEMO
add a MP3/MP4 file and wait. (tested in Chrome)
http://jsfiddle.net/pc76H/2/
But there are many problems. I can't find a proper documentation of the various audio filters.
Also, if you look at the spectrum you will notice that after 70% or the range there is no data. What does that mean? that maybe from 16k hz to 20k hz is no sound? I would apply a text to the canvas to show the various HZ. but where??
I found out that the returned data is a power of 32 in length with a max of 2048
and the height is always 256.
BUT the real question is ... I want to create a moving waveform like in traktor.
I already did that some time ago with PHP it converts the file to low bitrate than extracts the data and coverts that to a image. i found the script somewhere...but I don't remember where...
note: needs LAME
<?php
$a=$_GET["f"];
if(file_exists($a)){
if(file_exists($a.".png")){
header("Content-Type: image/png");
echo file_get_contents($a.".png");
}else{
$b=3000;$c=300;define("d",3);
ini_set("max_execution_time","30000");
function n($g,$h){
$g=hexdec(bin2hex($g));
$h=hexdec(bin2hex($h));
return($g+($h*256));
};
$k=substr(md5(time()),0,10);
copy(realpath($a),"/var/www/".$k."_o.mp3");
exec("lame /var/www/{$k}_o.mp3 -f -m m -b 16 --resample 8 /var/www/{$k}.mp3 && lame --decode /var/www/{$k}.mp3 /var/www/{$k}.wav");
//system("lame {$k}_o.mp3 -f -m m -b 16 --resample 8 {$k}.mp3 && lame --decode {$k}.mp3 {$k}.wav");
#unlink("/var/www/{$k}_o.mp3");
#unlink("/var/www/{$k}.mp3");
$l="/var/www/{$k}.wav";
$m=fopen($l,"r");
$n[]=fread($m,4);
$n[]=bin2hex(fread($m,4));
$n[]=fread($m,4);
$n[]=fread($m,4);
$n[]=bin2hex(fread($m,4));
$n[]=bin2hex(fread($m,2));
$n[]=bin2hex(fread($m,2));
$n[]=bin2hex(fread($m,4));
$n[]=bin2hex(fread($m,4));
$n[]=bin2hex(fread($m,2));
$n[]=bin2hex(fread($m,2));
$n[]=fread($m,4);
$n[]=bin2hex(fread($m,4));
$o=hexdec(substr($n[10],0,2));
$p=$o/8;
$q=hexdec(substr($n[6],0,2));
if($q==2){$r=40;}else{$r=80;};
while(!feof($m)){
$t=array();
for($i=0;$i<$p;$i++){
$t[$i]=fgetc($m);
};
switch($p){
case 1:$s[]=n($t[0],$t[1]);break;
case 2:if(ord($t[1])&128){$u=0;}else{$u=128;};$u=chr((ord($t[1])&127)+$u);$s[]= floor(n($t[0],$u)/256);break;
};
fread($m,$r);
};
fclose($m);
unlink("/var/www/{$k}.wav");
$x=imagecreatetruecolor(sizeof($s)/d,$c);
imagealphablending($x,false);
imagesavealpha($x,true);
$y=imagecolorallocatealpha($x,255,255,255,127);
imagefilledrectangle($x,0,0,sizeof($s)/d,$c,$y);
for($d=0;$d<sizeof($s);$d+=d){
$v=(int)($s[$d]/255*$c);
imageline($x,$d/d,0+($c-$v),$d/d,$c-($c-$v),imagecolorallocate($x,255,0,255));
};
$z=imagecreatetruecolor($b,$c);
imagealphablending($z,false);
imagesavealpha($z,true);
imagefilledrectangle($z,0,0,$b,$c,$y);
imagecopyresampled($z,$x,0,0,0,0,$b,$c,sizeof($s)/d,$c);
imagepng($z,realpath($a).".png");
header("Content-Type: image/png");
imagepng($z);
imagedestroy($z);
};
}else{
echo $a;
};
?>
The script works... but you are limited to a max image size of 4k pixels.
so you have not a nice waveform if it should rappresent only some milliseconds.
What do i need to store/create a realtime waveform like the traktors app or this php script? btw the traktor has also a colored waveform(the php script not).
EDIT
I rewrote your script that it fits my idea... it's relatively fast.
As you can see inside the function createArray i push the various lines into an object with the key as x coordinate.
I'm simply taking the the highest number.
here is where we could play with the colors.
var ajaxB,AC,B,LC,op,x,y,ARRAY={},W=1024,H=256;
var aMax=Math.max.apply.bind(Math.max, Math);
function error(a){
console.log(a);
};
function createDrawing(){
console.log('drawingArray');
var C=document.createElement('canvas');
C.width=W;
C.height=H;
document.body.appendChild(C);
var context=C.getContext('2d');
context.save();
context.strokeStyle='#121';
context.globalCompositeOperation='lighter';
L2=W*1;
while(L2--){
context.beginPath();
context.moveTo(L2,0);
context.lineTo(L2+1,ARRAY[L2]);
context.stroke();
}
context.restore();
};
function createArray(a){
console.log('creatingArray');
B=a;
LC=B.getChannelData(0);// Float32Array describing left channel
L=LC.length;
op=W/L;
for(var i=0;i<L;i++){
x=W*i/L|0;
y=LC[i]*H/2;
if(ARRAY[x]){
ARRAY[x].push(y)
}else{
!ARRAY[x-1]||(ARRAY[x-1]=aMax(ARRAY[x-1]));
// the above line contains an array of values
// which could be converted to a color
// or just simply create a gradient
// based on avg max min (frequency???) whatever
ARRAY[x]=[y]
}
};
createDrawing();
};
function decode(){
console.log('decodingMusic');
AC=new webkitAudioContext
AC.decodeAudioData(this.response,createArray,error);
};
function loadMusic(url){
console.log('loadingMusic');
ajaxB=new XMLHttpRequest;
ajaxB.open('GET',url);
ajaxB.responseType='arraybuffer';
ajaxB.onload=decode;
ajaxB.send();
}
loadMusic('AudioOrVideo.mp4');

Ok, so what i would do is to load the sound with an XMLHttpRequest, then decode it using webaudio, then display it 'carefully' to have the colors you are searching for.
I just made a quick version, copy-pasting from various of my projects, it is quite working, as you might see with this picture :
The issue is that it is slow as hell. To have (more) decent speed, you'll have to do some computation to reduce the number of lines to draw on the canvas, because at 441000 Hz, you very quickly get too many lines to draw.
// AUDIO CONTEXT
window.AudioContext = window.AudioContext || window.webkitAudioContext ;
if (!AudioContext) alert('This site cannot be run in your Browser. Try a recent Chrome or Firefox. ');
var audioContext = new AudioContext();
var currentBuffer = null;
// CANVAS
var canvasWidth = 512, canvasHeight = 120 ;
var newCanvas = createCanvas (canvasWidth, canvasHeight);
var context = null;
window.onload = appendCanvas;
function appendCanvas() { document.body.appendChild(newCanvas);
context = newCanvas.getContext('2d'); }
// MUSIC LOADER + DECODE
function loadMusic(url) {
var req = new XMLHttpRequest();
req.open( "GET", url, true );
req.responseType = "arraybuffer";
req.onreadystatechange = function (e) {
if (req.readyState == 4) {
if(req.status == 200)
audioContext.decodeAudioData(req.response,
function(buffer) {
currentBuffer = buffer;
displayBuffer(buffer);
}, onDecodeError);
else
alert('error during the load.Wrong url or cross origin issue');
}
} ;
req.send();
}
function onDecodeError() { alert('error while decoding your file.'); }
// MUSIC DISPLAY
function displayBuffer(buff /* is an AudioBuffer */) {
var leftChannel = buff.getChannelData(0); // Float32Array describing left channel
var lineOpacity = canvasWidth / leftChannel.length ;
context.save();
context.fillStyle = '#222' ;
context.fillRect(0,0,canvasWidth,canvasHeight );
context.strokeStyle = '#121';
context.globalCompositeOperation = 'lighter';
context.translate(0,canvasHeight / 2);
context.globalAlpha = 0.06 ; // lineOpacity ;
for (var i=0; i< leftChannel.length; i++) {
// on which line do we get ?
var x = Math.floor ( canvasWidth * i / leftChannel.length ) ;
var y = leftChannel[i] * canvasHeight / 2 ;
context.beginPath();
context.moveTo( x , 0 );
context.lineTo( x+1, y );
context.stroke();
}
context.restore();
console.log('done');
}
function createCanvas ( w, h ) {
var newCanvas = document.createElement('canvas');
newCanvas.width = w; newCanvas.height = h;
return newCanvas;
};
loadMusic('could_be_better.mp3');
Edit : The issue here is that we have too much data to draw. Take a 3 minutes mp3, you'll have 3*60*44100 = about 8.000.000 line to draw. On a display that has, say, 1024 px resolution, that makes 8.000 lines per pixel...
In the code above, the canvas is doing the 'resampling', by drawing lines with low-opacity and in 'ligther' composition mode (e.g. pixel's r,g,b will add-up).
To speed-up things, you have to re-sample by yourself, but to get some colors, it's not just a down-sampling, you'll have to handle a set (within a performance array most probably) of 'buckets', one for each horizontal pixel (so, say 1024), and in every bucket you compute the cumulated sound pressure, the variance, min, max and then, at display time, you decide how you will render that with colors.
For instance :
values between 0 positiveMin are very clear. (any sample is below that point).
values between positiveMin and positiveAverage - variance are darker,
values between positiveAverage - variance and positiveAverage + variance are darker,
and values between positiveAverage+variance and positiveMax lighter .
(same for negative values)
That makes 5 colors for each bucket, and it's still quite some work, for you to code and for the browser to compute.
I don't know if the performance could get decent with this, but i fear the statistics accuracy and the color coding of the software you mention can't be reached on a browser (obviously not in real-time), and that you'll have to make some compromises.
Edit 2 :
I tried to get some colors out of stats but it quite failed. My guess, now, is that the guys at tracktor also change color depending on frequency.... quite some work here....
Anyway, just for the record, the code for an average / mean variation follows.
(variance was too low, i had to use mean variation).
// MUSIC DISPLAY
function displayBuffer2(buff /* is an AudioBuffer */) {
var leftChannel = buff.getChannelData(0); // Float32Array describing left channel
// we 'resample' with cumul, count, variance
// Offset 0 : PositiveCumul 1: PositiveCount 2: PositiveVariance
// 3 : NegativeCumul 4: NegativeCount 5: NegativeVariance
// that makes 6 data per bucket
var resampled = new Float64Array(canvasWidth * 6 );
var i=0, j=0, buckIndex = 0;
var min=1e3, max=-1e3;
var thisValue=0, res=0;
var sampleCount = leftChannel.length;
// first pass for mean
for (i=0; i<sampleCount; i++) {
// in which bucket do we fall ?
buckIndex = 0 | ( canvasWidth * i / sampleCount );
buckIndex *= 6;
// positive or negative ?
thisValue = leftChannel[i];
if (thisValue>0) {
resampled[buckIndex ] += thisValue;
resampled[buckIndex + 1] +=1;
} else if (thisValue<0) {
resampled[buckIndex + 3] += thisValue;
resampled[buckIndex + 4] +=1;
}
if (thisValue<min) min=thisValue;
if (thisValue>max) max = thisValue;
}
// compute mean now
for (i=0, j=0; i<canvasWidth; i++, j+=6) {
if (resampled[j+1] != 0) {
resampled[j] /= resampled[j+1]; ;
}
if (resampled[j+4]!= 0) {
resampled[j+3] /= resampled[j+4];
}
}
// second pass for mean variation ( variance is too low)
for (i=0; i<leftChannel.length; i++) {
// in which bucket do we fall ?
buckIndex = 0 | (canvasWidth * i / leftChannel.length );
buckIndex *= 6;
// positive or negative ?
thisValue = leftChannel[i];
if (thisValue>0) {
resampled[buckIndex + 2] += Math.abs( resampled[buckIndex] - thisValue );
} else if (thisValue<0) {
resampled[buckIndex + 5] += Math.abs( resampled[buckIndex + 3] - thisValue );
}
}
// compute mean variation/variance now
for (i=0, j=0; i<canvasWidth; i++, j+=6) {
if (resampled[j+1]) resampled[j+2] /= resampled[j+1];
if (resampled[j+4]) resampled[j+5] /= resampled[j+4];
}
context.save();
context.fillStyle = '#000' ;
context.fillRect(0,0,canvasWidth,canvasHeight );
context.translate(0.5,canvasHeight / 2);
context.scale(1, 200);
for (var i=0; i< canvasWidth; i++) {
j=i*6;
// draw from positiveAvg - variance to negativeAvg - variance
context.strokeStyle = '#F00';
context.beginPath();
context.moveTo( i , (resampled[j] - resampled[j+2] ));
context.lineTo( i , (resampled[j +3] + resampled[j+5] ) );
context.stroke();
// draw from positiveAvg - variance to positiveAvg + variance
context.strokeStyle = '#FFF';
context.beginPath();
context.moveTo( i , (resampled[j] - resampled[j+2] ));
context.lineTo( i , (resampled[j] + resampled[j+2] ) );
context.stroke();
// draw from negativeAvg + variance to negativeAvg - variance
// context.strokeStyle = '#FFF';
context.beginPath();
context.moveTo( i , (resampled[j+3] + resampled[j+5] ));
context.lineTo( i , (resampled[j+3] - resampled[j+5] ) );
context.stroke();
}
context.restore();
console.log('done 231 iyi');
}

Based on the top answer, I have controlled that by reducing number of lines want to draw and little canvas function call placement. see following code for your reference.
// AUDIO CONTEXT
window.AudioContext = (window.AudioContext ||
window.webkitAudioContext ||
window.mozAudioContext ||
window.oAudioContext ||
window.msAudioContext);
if (!AudioContext) alert('This site cannot be run in your Browser. Try a recent Chrome or Firefox. ');
var audioContext = new AudioContext();
var currentBuffer = null;
// CANVAS
var canvasWidth = window.innerWidth, canvasHeight = 120 ;
var newCanvas = createCanvas (canvasWidth, canvasHeight);
var context = null;
window.onload = appendCanvas;
function appendCanvas() { document.body.appendChild(newCanvas);
context = newCanvas.getContext('2d'); }
// MUSIC LOADER + DECODE
function loadMusic(url) {
var req = new XMLHttpRequest();
req.open( "GET", url, true );
req.responseType = "arraybuffer";
req.onreadystatechange = function (e) {
if (req.readyState == 4) {
if(req.status == 200)
audioContext.decodeAudioData(req.response,
function(buffer) {
currentBuffer = buffer;
displayBuffer(buffer);
}, onDecodeError);
else
alert('error during the load.Wrong url or cross origin issue');
}
} ;
req.send();
}
function onDecodeError() { alert('error while decoding your file.'); }
// MUSIC DISPLAY
function displayBuffer(buff /* is an AudioBuffer */) {
var drawLines = 500;
var leftChannel = buff.getChannelData(0); // Float32Array describing left channel
var lineOpacity = canvasWidth / leftChannel.length ;
context.save();
context.fillStyle = '#080808' ;
context.fillRect(0,0,canvasWidth,canvasHeight );
context.strokeStyle = '#46a0ba';
context.globalCompositeOperation = 'lighter';
context.translate(0,canvasHeight / 2);
//context.globalAlpha = 0.6 ; // lineOpacity ;
context.lineWidth=1;
var totallength = leftChannel.length;
var eachBlock = Math.floor(totallength / drawLines);
var lineGap = (canvasWidth/drawLines);
context.beginPath();
for(var i=0;i<=drawLines;i++){
var audioBuffKey = Math.floor(eachBlock * i);
var x = i*lineGap;
var y = leftChannel[audioBuffKey] * canvasHeight / 2;
context.moveTo( x, y );
context.lineTo( x, (y*-1) );
}
context.stroke();
context.restore();
}
function createCanvas ( w, h ) {
var newCanvas = document.createElement('canvas');
newCanvas.width = w; newCanvas.height = h;
return newCanvas;
};
loadMusic('could_be_better.mp3');

this is a bit old, sorry to bump, but it's the only post about displaying a full waveform with the Web Audio Api and I'd like to share what method i used.
This method is not perfect but it only goes through the displayed audio and it only goes over it once. it also succeeds in displaying an actual waveform for short files or big zoom :
and a convincing loudness chart for bigger files dezoomed :
here is what it's like at middle zoom, kind of pleasant too:
notice that both zooms use the same algorythm.
I still struggle about scales (the zoomed waveform is bigger than the dezoomed one (though not so bigger than displayed on the images)
this algorythm i find is quite efficient (i can change zoom on 4mn music and it redraws flawlessly every 0.1s)
function drawWaveform (audioBuffer, canvas, pos = 0.5, zoom = 1) {
const canvasCtx = canvas.getContext("2d")
const width = canvas.clientWidth
const height = canvas.clientHeight
canvasCtx.clearRect(0, 0, width, height)
canvasCtx.fillStyle = "rgb(255, 0, 0)"
// calculate displayed part of audio
// and slice audio buffer to only process that part
const bufferLength = audioBuffer.length
const zoomLength = bufferLength / zoom
const start = Math.max(0, bufferLength * pos - zoomLength / 2)
const end = Math.min(bufferLength, start + zoomLength)
const rawAudioData = audioBuffer.getChannelData(0).slice(start, end)
// process chunks corresponding to 1 pixel width
const chunkSize = Math.max(1, Math.floor(rawAudioData.length / width))
const values = []
for (let x = 0; x < width; x++) {
const start = x*chunkSize
const end = start + chunkSize
const chunk = rawAudioData.slice(start, end)
// calculate the total positive and negative area
let positive = 0
let negative = 0
chunk.forEach(val =>
val > 0 && (positive += val) || val < 0 && (negative += val)
)
// make it mean (this part makes dezommed audio smaller, needs improvement)
negative /= chunk.length
positive /= chunk.length
// calculate amplitude of the wave
chunkAmp = -(negative - positive)
// draw the bar corresponding to this pixel
canvasCtx.fillRect(
x,
height / 2 - positive * height,
1,
Math.max(1, chunkAmp * height)
)
}
}
To use it :
async function decodeAndDisplayAudio (audioData) {
const source = audioCtx.createBufferSource()
source.buffer = await audioCtx.decodeAudioData(audioData)
drawWaveform(source.buffer, canvas, 0.5, 1)
// change position (0//start -> 0.5//middle -> 1//end)
// and zoom (0.5//full -> 400//zoomed) as you wish
}
// audioData comes raw from the file (server send it in my case)
decodeAndDisplayAudio(audioData)