I am currently using http://paperjs.org to create an HTML5 canvas drawing app. I want to let users upload images into the canvas. I know I need to make a login and signup but is there an easier way? I have seen the HTML5 drag and drop upload.
I assume you mean, to load an image into the canvas and not uploading the image from the canvas.
It'd probably be a good idea to read through all the canvas articles they have over here
https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Canvas_tutorial/Using_images
But basically what you want to do is create an image in javascript, and set the image.src = to whatever the file location is. In the case of loading images from the user on their end, you're going to want to use the File System API.
Threw together a brief example here: http://jsfiddle.net/influenztial/qy7h5/
function handleImage(e){
var reader = new FileReader();
reader.onload = function(event){
var img = new Image();
img.onload = function(){
canvas.width = img.width;
canvas.height = img.height;
ctx.drawImage(img,0,0);
}
img.src = event.target.result;
}
reader.readAsDataURL(e.target.files[0]);
}
One doesn't need a FileReader*, it is better to use the URL.createObjectURL method, which will create a symlink directly to the File on disk. This will incur less memory usage, and will have the added benefit to have only one async event to wait for (the one of the img.onload).
document.getElementById('inp').onchange = function(e) {
var img = new Image();
img.onload = draw;
img.onerror = failed;
img.src = URL.createObjectURL(this.files[0]);
};
function draw() {
var canvas = document.getElementById('canvas');
canvas.width = this.width;
canvas.height = this.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(this, 0,0);
}
function failed() {
console.error("The provided file couldn't be loaded as an Image media");
}
<input type="file" id="inp">
<canvas id="canvas"></canvas>
*IIRC only a few versions of Chrome did support FileReader while not yet supporting URL.createObejctURL, so if you target these very versions, you might need FileReader..
Modified answer by #kaiido to create a new canvas element each time and append it to a wrapper. Useful when you don't know how many canvases you may need.
Note: There is no new Canvas() constructor, therefore we must use createElement().
document.getElementById('inp').onchange = function(e) {
let img = new Image();
img.onload = draw;
img.onerror = failed;
img.src = URL.createObjectURL(this.files[0]);
};
function draw() {
let canvas = document.createElement('canvas'),
ctx = canvas.getContext('2d');
canvas.width = this.width;
canvas.height = this.height;
ctx.drawImage(this, 0, 0);
document.getElementById('gallery').append(canvas);
}
function failed() {
console.error("The provided file couldn't be loaded as an Image media");
}
/* entirely decorative */
#gallery {
display: flex;
gap: 1em;
margin: 1em;
}
#gallery canvas {
height: 100px;
border-radius: .5em;
aspect-ratio: 1/1;
object-fit: cover;
}
<input type="file" id="inp">
<div id='gallery'></div>
The most optimal way of creating an image consumable by the canvas is to create an ImageBitmap out of the File you get from the input.
This will use an optimized path to produce just what the browser needs to render that image, and will store the bitmap data in the GPU, allowing for fast drawing when asked to.
Given this is a quite recent feature (Safari added support only last year), you may want to use a polyfill like this one of mine.
document.querySelector("input").oninput = async (evt) => {
try {
const file = evt.target.files[0];
const bitmap = await createImageBitmap(file);
const canvas = document.querySelector("canvas");
canvas.width = bitmap.width;
canvas.height = bitmap.height;
const ctx = canvas.getContext("2d");
ctx.drawImage(bitmap, 0, 0);
}
catch(err) {
console.error(err);
}
};
<!-- createImageBitmap polyfill for old browsers --> <script src="https://cdn.jsdelivr.net/gh/Kaiido/createImageBitmap/dist/createImageBitmap.js"></script>
<input type="file">
<canvas></canvas>
<script>
window.onload = function() {
var canvas=document.getElementById(“drawing”); // grabs the canvas element
var context=canvas.getContext(“2d”); // returns the 2d context object
var img=new Image() //creates a variable for a new image
img.src= “images/vft.jpg” // specifies the location of the image
context.drawImage(img,20,20); // draws the image at the specified x and y location
}
</script>
Check Demo
Related
The question on a single line:
Is there a toDataURL method working on a <IMG> element?
Most of the people seem to be interested in how to get an IMG from a CANVAS, but I need the opposite.
Why? I need to use the toDataURL() method on the IMG.
So, this is the pseudo code that I wish existed in reality:
var img = document.getElementById('myImage');
var canvas = img.getCanvasFromImage();
var dataURL = canvas.toDataURL();
Is there already a method, or a workaround (e.g. creating an empty canvas, copying the IMG on top, etc.) to do the getCanvasFromImage? I couldn't find one.
Some more details WHY, but there is no need to read further. I am using the VIDEO tag to get the camera stream, and when the user clicks a button, I copy the CANVAS to the IMG.
But since I do not want to show the whole picture while taking a photo (I want it to be consistent over different devices, regardless of the camera resolution, keeping a 16:9 aspect ratio), I only show a portion of the image using object-fit: cover; .
So, now I have a "partial" image, but if I do a toDataURL on the canvas I have, it gives me the WHOLE picture, regardless of the "object-fit" value.
If this is not clear, no problem :) I only need a "toDataURL" method working on a <IMG> element :)
HTMLImageElement.prototype.getCanvasFromImage = function(){
const canvas = document.createElement('canvas');
canvas.width = this.width;
canvas.height = this.height;
const ctx = canvas.getContext('2d');
ctx.drawImage(this, 0, 0);
return canvas;
};
const img = document.getElementById('myImage');
img.onload = () => {
const canvas = img.getCanvasFromImage();
const dataURL = canvas.toDataURL();
console.log(dataURL);
}
<img id="myImage" crossorigin="anonymous" src="https://dl.dropbox.com/s/zpoxft30lzrr5mg/20201012_102150.jpg" />
Its work, but please READ THIS. Better way is create pure function and pass image as argument:
function getCanvasFromImage(image) {
const canvas = document.createElement('canvas');
canvas.width = image.width;
canvas.height = image.height;
const ctx = canvas.getContext('2d');
ctx.drawImage(image, 0, 0);
return canvas;
}
I'm converting images to base64 using canvas. What i need to do is convert those images and then show the result to the user (original image and base64 version). Everything works as expected with small images, but when i try to convert large images (>3MB) and the conversion time increases, the base64 version is empty.
This might be is caused because the result is shown before the toDataURL() function is completed.
I need to show the result after all the needed processing has ended, for testing purposes.
Here's my code:
var convertToBase64 = function(url, callback)
{
var image = new Image();
image.onload = function ()
{
//create canvas and draw image...
var imageData = canvas.toDataURL('image/png');
callback(imageData);
};
image.src = url;
};
convertToBase64('img/circle.png', function(imageData)
{
window.open(imageData);
});
Even though i'm using image.onload() with a callback, i'm unable to show the result after the toDataURL() has been processed.
What am i doing wrong?
UPDATE: I tried both the solutions below and they didn't work. I'm using AngularJS and Electron in this project. Any way i can force the code to be synchronous? Or maybe some solution using Promises?
UPDATE #2: #Kaiido pointed out that toDataURL() is in fact synchronous and this issue is more likely due to maximum URI length. Since i'm using Electron and the image preview was for testing purposes only, i'm going to save the file in a folder and analise it from there.
Your code seems absolutely fine. Not sure why isn't working you. Maybe, there are some issues with your browser. Perhaps try using a different one. Also you could use a custom event, which gets triggered when the image conversion is competed.
// using jQuery for custom event
function convertToBase64(url) {
var image = new Image();
image.src = url;
image.onload = function() {
var canvas = document.createElement('canvas');
canvas.width = image.width;
canvas.height = image.height;
var ctx = canvas.getContext('2d');
ctx.drawImage(image, 0, 0);
var imageData = canvas.toDataURL();
$(document).trigger('conversionCompleted', imageData);
};
};
convertToBase64('4mb.jpg');
$(document).on('conversionCompleted', function(e, d) {
window.open(d);
});
This approach might work for you. It shows the image onscreen using the native html element, then draws it to a canvas, then converts the canvas to Base64, then clears the canvas and draws the converted image onto the canvas. You can then scroll between the top image (original) and the bottom image (converted). I tried it on large images and it takes a second or two for the second image to draw but it seems to work...
Html is here:
<img id="imageID">
<canvas id="myCanvas" style="width:400;height:400;">
</canvas>
Script is here:
var ctx;
function convertToBase64(url, callback)
{
var image = document.getElementById("imageID");
image.onload = function() {
var canvas = document.getElementById("myCanvas");
canvas.width = image.naturalWidth;
canvas.height = image.naturalHeight;
ctx = canvas.getContext("2d");
ctx.drawImage(image,0,0);
var imageData = canvas.toDataURL('image/png');
ctx.fillStyle ="#FFFFFF";
ctx.fillRect(0,0,canvas.width,canvas.height);
callback(imageData);
};
image.src = url;
};
var imagename = 'images/bigfiletest.jpg';
window.onload = function () {
convertToBase64(imagename, function(imageData) {
var myImage = new Image();
myImage.src = imageData;
ctx.drawImage(myImage,0,0);
});
}
Note that I also tried it without the callback and it worked fine as well...
I have an html canvas that shows a preview of an image when the image is selected in an input. This works in Chrome, but I can't seem to get it to work in Safari. Specifically - in Safari - the onchange="previewFile()" does not seem to call the previewFile function.
<canvas id="canvas" width="0" height="0"></canvas>
<h2>Upload a photo </h2>
<input type="file" onchange="previewFile()"><br>
<script type="text/javascript">
// setup the canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
// grab the photo and display in canvas
var photo = new Image();
function previewFile() {
var file = document.querySelector('input[type=file]').files[0];
var reader = new FileReader();
reader.addEventListener("load", function () {
photo.src = reader.result;
canvas.height = photo.height;
canvas.width = photo.width;
ctx.drawImage(photo,0,0);
}, false);
if (file) {
reader.readAsDataURL(file);
}
}
</script>
Your problem is certainly due to the fact that you are not waiting for the image has loaded before you try to draw it on the canvas.
Even wen the source is a dataURI, the loading of the image is asynchronous, so you need to wrap your drawing operations in the image's onload event.
var photo = new Image();
photo.onload = function(){
canvas.height = photo.height;
...
}
...
reader.onload = function(){
// this will eventually trigger the image's onload event
photo.src = this.result;
}
But if all you need is to draw your image on the canvas, don't even use a FileReader, actually, the readAsDataURL() method should trigger a I'm doing something wrong error in your brain. Almost all you can do with a dataURI version of a Blob, you can also do it with the Blob itself, without computing it nor polluting the browser's memory.
For example, to display an image for user input, you can use URL.createObjectURL(blob) method.
// setup the canvas
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
// grab the photo and display in canvas
var photo = new Image();
// drawing operations should be in the mage's load event
photo.onload = function() {
// if you don't need to display this image elsewhere
URL.revokeObjectURL(this.src);
canvas.height = this.height;
canvas.width = this.width;
ctx.drawImage(this, 0, 0);
}
photo.onerror = function(e) {
console.warn('file format not recognised as a supported image');
}
file_input.onchange = function() {
// prefer 'this' over DOM selection
var file = this.files[0];
var url = URL.createObjectURL(file);
photo.src = url;
};
<canvas id="canvas" width="0" height="0"></canvas>
<h2>Upload a photo </h2>
<input type="file" id="file_input">
<br>
And for those who need to send this file to their server, use a FormData to send directly the Blob. If you really need a dataURI version, then convert it server-side.
Using JavaScript, I want to convert an img tag like this:
<img width="11" height="14" src="http://mysite/file.gif" alt="File Icon">
Into one with a dataurl like this:
<img width="11" height="14" src="data:image/gif;base64,R0lGODlhCwAOAMQfAP////7+/vj4+Hh4eHd3d/v7+/Dw8HV1dfLy8ubm5vX19e3t7fr6+nl5edra2nZ2dnx8fMHBwYODg/b29np6eujo6JGRkeHh4eTk5LCwsN3d3dfX13Jycp2dnevr6////yH5BAEAAB8ALAAAAAALAA4AAAVq4NFw1DNAX/o9imAsBtKpxKRd1+YEWUoIiUoiEWEAApIDMLGoRCyWiKThenkwDgeGMiggDLEXQkDoThCKNLpQDgjeAsY7MHgECgx8YR8oHwNHfwADBACGh4EDA4iGAYAEBAcQIg0Dk gcEIQA7" alt="File Icon">
Is this possible?
Here's how to get the data url of an image with fetch:
(async function() {
let blob = await fetch("https://example.com/image.png").then(r => r.blob());
let dataUrl = await new Promise(resolve => {
let reader = new FileReader();
reader.onload = () => resolve(reader.result);
reader.readAsDataURL(blob);
});
// now do something with `dataUrl`
})();
First, load the image into a canvas
var canvas = document.createElement("canvas");
context = canvas.getContext('2d');
make_base();
function make_base()
{
base_image = new Image();
base_image.src = 'img/base.png';
base_image.onload = function(){
context.drawImage(base_image, 100, 100);
}
}
Make sure to update the context.drawImage(base_image, 100, 100); to values appropriate for your application.
Source: https://stackoverflow.com/a/6011402/3969707
Then convert the canvas to data.
var jpegUrl = canvas.toDataURL("image/jpeg");
var pngUrl = canvas.toDataURL(); // PNG is the default
Source: https://stackoverflow.com/a/15685877/3969707
I'd solve this problem with a temporary canvas element with the same size of the image loaded:
function imageToUri(url, callback) {
const canvas = document.createElement('canvas');
const ctx = canvas.getContext('2d');
var base_image = new Image();
base_image.src = url;
base_image.onload = function() {
canvas.width = base_image.width;
canvas.height = base_image.height;
ctx.drawImage(base_image, 0, 0);
callback(canvas.toDataURL('image/png'));
canvas.remove();
}
}
imageToUri('./assets/some_image.png', function(uri) {
console.log(uri);
});
This really isn't something you may want to do in JavaScript as it requires the image be parsed in JavaScript which is very slow. You would be loading the image and then putting the output of the function into the img tag, which doesn't save bandwidth, decrease complexity, or improve security.
Your best bet is to do this server-side. I've used this to great success in the past for generating pages which need to have absolutely no linked/referenced external resources. You can, for jsPDF, then use raw JavaScript or jQuery to get the data of an image to pass into the PDF creation process.
This will work everywhere and not rely on resorting to canvas which doesn't have full mobile browser support.
I've tried all code variations that are online. I just want to display an image on a canvas. I've tried code from this site.
window.onLoad=function(){
function draw(){
var ctx = document.getElementById("canvas1").getContext("2d");
var img = new Image();
img.src = 'images/ball.png';
img.onload = function(){
ctx.drawImage(img,0,0);
};
};
};
It's not the file path that's a problem, that has been tested without the images folder. There are no errors in the console. Thanks.
One search in google and there you go with complete jsfiddle example:
// Grab the Canvas and Drawing Context
var canvas = document.getElementById('c');
var ctx = canvas.getContext('2d');
// Create an image element
var img = document.createElement('IMG');
// When the image is loaded, draw it
img.onload = function () {
ctx.drawImage(img, 0, 0);
}
// Specify the src to load the image
img.src = "http://i.imgur.com/gwlPu.jpg";
http://jsfiddle.net/jimrhoskins/Sv87G/