We Keep Coding

Can't draw image to canvas in javascript

I'm attempting to put a watermark image on an input image before saving it to a database. I'm using a canvas to do this then convert the canvas back to a blob but when I reference the watermark image in the projects local file directory to draw onto the canvas, I get a "Tainted canvases may not be exported." error in the console.
// Read input file
const reader = new FileReader();
reader.readAsDataURL(file);
reader.onload = () => {
// Create canvas & calculate Stamp size and position
const canvas = document.createElement("canvas");
canvas.width = canvasSize.width;
canvas.height = canvasSize.height;
var calcW = canvasSize.width/2;
var calcH = calcW/2.16;
var calcX = (canvasSize.width - calcW)/2;
var calcY = (canvasSize.height - calcH)/2;
// Load input image into canvas
const img = document.createElement("img");
img.src = reader.result;
img.onload = () => {
const ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0);
//Load & draw the Stamp image
const mark = new Image();
mark.src = 'assets/img/compliant.png'; // <<TAINTED ASSET?
mark.onload = () => {
ctx.drawImage(mark, calcX,calcY,calcW,calcH);
// Convert to blob and get URL
canvas.toBlob((canvasBlob) => {
const newImg = document.createElement("img"),
url = URL.createObjectURL(canvasBlob);
newImg.onload = function () {
//Destroy objecturl after image loading
URL.revokeObjectURL(url);
};
// Show stamped image
newImg.src = url;
window.open(url);
});
};
};
};
});

You have to use
img.crossOrigin='anonymous';
On every image you want to manipulate (draw in a canvas, transform to an array buffer… this is to prevent you from manipulating potentially private user data)
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video#attr-crossorigin

Convert blob to image file

I am trying to convert an image file captured from an input type=file element without success. Below is my javascript code
var img = document.getElementById('myimage');
var file = document.querySelector('input[type=file]').files[0];
var reader = new FileReader();
reader.addEventListener("load", function () {
var theBlob = reader.result;
theBlob.lastModifiedDate = new Date();
theBlob.name = file;
img.src = theBlob;
var newfile = new File([theBlob], "c:files/myfile.jpg");
}, false);
if (file) {
reader.readAsDataURL(file);
}
The image is displayed properly in the img element but the File command does not create the myfile.jpg image file. I am trying to capture the image and then resizing it using javascript before I upload it to the server. Anyone know why the image file is not being created? Better yet, anyone have code on how to resize an image file on the client and then upload it to a server?

This is how you can get a resized jpeg file from your "myimage" element using Canvas.
I commented every line of code so you could understand what I was doing.
// Set the Width and Height you want your resized image to be
var width = 1920;
var height = 1080;
var canvas = document.createElement('canvas'); // Dynamically Create a Canvas Element
canvas.width = width; // Set the width of the Canvas
canvas.height = height; // Set the height of the Canvas
var ctx = canvas.getContext("2d"); // Get the "context" of the canvas
var img = document.getElementById("myimage"); // The id of your image container
ctx.drawImage(img,0,0,width,height); // Draw your image to the canvas
var jpegFile = canvas.toDataURL("image/jpeg"); // This will save your image as a
//jpeg file in the base64 format.
The javascript variable "jpegFile" now contains your image encoded into a URL://Base64 format. Which looks something like this:
// data:image/jpeg;base64,/9j/4AAQSkZJRgABAQ...9oADAMBAAIRAxEAPwD/AD/6AP/Z"
You can put that variable as an image source in HTML code and it will display your image in the browser, or you can upload the Base64 encoded string to the server.
Edit: How to convert the file to a blob (binary file) and upload it to the server
// This function is used to convert base64 encoding to mime type (blob)
function base64ToBlob(base64, mime)
{
mime = mime || '';
var sliceSize = 1024;
var byteChars = window.atob(base64);
var byteArrays = [];
for (var offset = 0, len = byteChars.length; offset < len; offset += sliceSize) {
var slice = byteChars.slice(offset, offset + sliceSize);
var byteNumbers = new Array(slice.length);
for (var i = 0; i < slice.length; i++) {
byteNumbers[i] = slice.charCodeAt(i);
}
var byteArray = new Uint8Array(byteNumbers);
byteArrays.push(byteArray);
}
return new Blob(byteArrays, {type: mime});
}
Now clean the base64 up and then pass it into the function above:
var jpegFile64 = jpegFile.replace(/^data:image\/(png|jpeg);base64,/, "");
var jpegBlob = base64ToBlob(jpegFile64, 'image/jpeg');
Now send the "jpegBlob" with ajax
var oReq = new XMLHttpRequest();
oReq.open("POST", url, true);
oReq.onload = function (oEvent) {
// Uploaded.
};
oReq.send(jpegBlob);

You can't manipulate hard drive directly from the browser. What you can do though is create a link that can be downloaded.
To do this change your var newfile = new File([theBlob], "c:files/myfile.jpg"); into:
const a = document.createElement('a');
a.setAttribute('download', "some image name");
a.setAttribute('href', theBlob);
a.click();

Use Javascript to upload anything instead of only images

Here is my code where it is accepting only images that can able to view it is uploading. But i want to upload an jpeg image that was renamed from image.gzip to image.jpeg & this image i cant able to view in system as it is renamed from gzip to jpeg.it is not accepting here to load that type of images.. please help
function readImage(form, file, imageName, size, width, height, i, total){
var flag = true;
var reader = new FileReader();
var image = new Image();
reader.readAsDataURL(file);
reader.onload = function(_file) {
image.src = _file.target.result;
alert("onload");
image.onload = function() {
var w = this.width,
h = this.height,
t = file.type,
n = file.name,
s = ~~(file.size/1024);

Canvas to img with src blob

I'm creating a PNG file from a canvas, but before I want to display the canvas as an img element, using a blob as the src of the img. What I tried so far:
canvas = document.getElementById("canvas");
//Get data from canvas
img_b64 = canvas.toDataURL('image/png');
//Create PNG
png = img_b64.split(',')[1];
//Create new img
img = document.createElement("img");
img.src = img_b64;
//Append img to document
//Save png
This works, but instead of img_b64 I want a blob to be the src of the img, like this:
//Create blob from new PNG
blob = new Blob([window.atob(png)], { type: 'image/png', encoding: 'utf-8' });
//Create new img with blob as src
img = document.createElement("img");
img.src = URL.createObjectURL(blob);
//Append img to document
//Save png
The above code only shows me an empty img. Is there a way to do this or do I have to create the png file first and afterwards create a blob for it?

I just found an solution using the answer to this question: Convert Data URI to File then append to FormData. Using the dataURItoBlob function my code is:
canvas = document.getElementById("canvas");
//Get data from canvas
img_b64 = canvas.toDataURL('image/png');
//Create blob from DataURL
blob = dataURItoBlob(img_b64)
img = document.createElement("img");
img.src = URL.createObjectURL(blob);
//Insert image
dataURItoBlob function:
function dataURItoBlob(dataURI) {
// convert base64/URLEncoded data component to raw binary data held in a string
var byteString;
if (dataURI.split(',')[0].indexOf('base64') >= 0)
byteString = atob(dataURI.split(',')[1]);
else
byteString = unescape(dataURI.split(',')[1]);
// separate out the mime component
var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0];
// write the bytes of the string to a typed array
var ia = new Uint8Array(byteString.length);
for (var i = 0; i < byteString.length; i++) {
ia[i] = byteString.charCodeAt(i);
}
return new Blob([ia], {type:mimeString});
}

With toBlob this should be easy to solve from IE>=10.
const img = document.createElement("img");
canvas.toBlob((blob) => {
img.src = URL.createObjectURL(blob);
});

Serialize canvas content to ArrayBuffer and deserialize again

I have two canvases, and I want to pass the content of canvas1, serialize it to an ArrayBuffer, and then load it in canvas2. In the future I will send the canvas1 content to the server, process it, and return it to canvas2, but right now I just want to serialize and deserialize it.
I found this way of getting the canvas info in bytes:
var img1 = context.getImageData(0, 0, 400, 320);
var binary = new Uint8Array(img1.data.length);
for (var i = 0; i < img1.data.length; i++) {
binary[i] = img1.data[i];
}
And also found this way of set the information to a Image object:
var blob = new Blob( [binary], { type: "image/png" } );
var urlCreator = window.URL || window.webkitURL;
var imageUrl = urlCreator.createObjectURL( blob );
var img = new Image();
img.src = imageUrl;
But unfortunately it doesn't seem to work.
Which would be the right way of doing this?

The ImageData you get from getImageData() is already using an ArrayBuffer (used by the Uint8ClampedArray view). Just grab it and send it:
var imageData = context.getImageData(x, y, w, h);
var buffer = imageData.data.buffer; // ArrayBuffer
To set it again:
var imageData = context.createImageData(w, h);
imageData.data.set(incomingBuffer);
You probably want to consider some form of byte encoding though (such as f.ex base-64) as any byte value above 127 (ASCII) is subject to character encoding used on a system. Or make sure all steps on the trip uses the same (f.ex. UTF8).

Create an ArrayBuffer and send it into to the Uint8Array constructor, then send the buffer using websockets:
var img1 = context.getImageData(0, 0, 400, 320);
var data=img1.data;
var buffer = new ArrayBuffer(data.length);
var binary = new Uint8Array(buffer);
for (var i=0; i<binary.length; i++) {
binary[i] = data[i];
}
websocket.send(buffer);
[ Previous answer using canvas.toDataURL removed ]

Consider using canvas.toBlob() instead of context.getImageData() if you want compact data rather than a raw ImageData object.
Example:
const imageIn = document.querySelector('#image-in');
const imageOut = document.querySelector('#image-out');
const canvas = document.querySelector('#canvas');
const imageDataByteLen = document.querySelector('#imagedata-byte-length');
const bufferByteLen = document.querySelector('#arraybuffer-byte-length');
const mimeType = 'image/png';
imageIn.addEventListener('load', () => {
// Draw image to canvas.
canvas.width = imageIn.width;
canvas.height = imageIn.height;
const ctx = canvas.getContext('2d');
ctx.drawImage(imageIn, 0, 0);
// Convert canvas to ImageData.
const imageData = ctx.getImageData(0, 0, imageIn.width, imageIn.height);
imageDataByteLen.textContent = imageData.data.byteLength + ' bytes.';
// Convert canvas to Blob, then Blob to ArrayBuffer.
canvas.toBlob((blob) => {
const reader = new FileReader();
reader.addEventListener('loadend', () => {
const arrayBuffer = reader.result;
bufferByteLen.textContent = arrayBuffer.byteLength + ' bytes.';
// Dispay Blob content in an Image.
const blob = new Blob([arrayBuffer], {type: mimeType});
imageOut.src = URL.createObjectURL(blob);
});
reader.readAsArrayBuffer(blob);
}, mimeType);
});
<h1>Canvas ↔ ArrayBuffer</h1>
<h2>1. Source <code><img></code></h2>
<img id="image-in" src="https://ucarecdn.com/a0338bfa-9f88-4ce7-b53f-e6b61000df89/" crossorigin="">
<h2>2. Canvas</h2>
<canvas id="canvas"></canvas>
<h2>3. ImageData</h2>
<p id="imagedata-byte-length"></p>
<h2>4. ArrayBuffer</h2>
<p id="arraybuffer-byte-length"></p>
<h2>5. Final <code><img></code></h2>
<img id="image-out">
JSFiddle: https://jsfiddle.net/donmccurdy/jugzk15b/
Also note that images must be hosted on a service that provides CORS headers, or you'll see errors like "The canvas has been tainted by cross-origin data."