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How can I get screenshot of image using html2canvas?

I am trying to save screenshot of image using html2canvas. Below is my code:
var a = document.getElementById('a2')
html2canvas(document.getElementById('a1'), {
onrendered: function(canvas) {
var img = canvas.toDataURL("image/jpeg");
window.open(img);
}
}
However, the code is saving the screenshot for local image. But for the image from external source like :
http://europa.promaticstechnologies.com/QuotePic/img/QuotePro/bg1.jpg
giving me black screenshot. Please assist.

To use image from external domain you may want to use crossOrigin attribute and current Date as a parameter to src attribute.
e.g.
var image = document.createElement('img');
image.onload = function () {
// onload callback
}
image.setAttribute('crossOrigin', '');
image.src = url + '?' + new Date().getTime();
Check this for details.

Execute Function After Canvas toDataURL() Function is Complete

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...

javascript- unable to convert canvas to image data

guys!
I'm trying to make a demo using which a user will be able to capture an image from the web camera enabled through his browser and this image will be saved in the backend by passing it to a PHP service using AJAX call.
I'm able to capture as canvas but not being to convert it into image data.
I'm getting canvas is undefined error
Setup is pretty simple.
HTML
<video id="videoElement" autoplay></video>
<button id="snap">Snap Photo</button>
<canvas id="canvas" style="display:none" ></canvas>
JAVASCRIPT
// Grab elements, create settings, etc.
var video = document.getElementById('videoElement');
// Get access to the camera!
if(navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
// Not adding `{ audio: true }` since we only want video now
navigator.mediaDevices.getUserMedia({ video: true }).then(function(stream) {
video.src = window.URL.createObjectURL(stream);
video.play();
});
}
// Elements for taking the snapshot
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
var video = document.getElementById('videoElement');
// Trigger photo take
document.getElementById("snap").addEventListener("click", function() {
var cann= context.drawImage(video, 0, 0, 640, 480); //draw canvas
//then convert canvas to image so i can obtain dataurl
//and pass it to another function using AJAX
var img= convertCanvasToImage(cann);
console.log(img);
});
// Converts canvas to an image
function convertCanvasToImage(canvas)
{
var image = new Image();
image.src = canvas.toDataURL("image/png");
return image;
}
I'm pretty new to this field. I have one more question. Will this work on latest browsers?
Can anyone help me figure out the mistake I made?
FIDDLE
Reference:
https://davidwalsh.name/browser-camera
https://davidwalsh.name/convert-canvas-image

Just call this
var img= convertCanvasToImage();
function convertCanvasToImage()
{
var image = new Image();
image.src = canvas.toDataURL("image/png");
return image;
}
No need to pass canvas as you defined canvas above. But if you have convertCanvasToImage function in another file then you can use
var img= convertCanvasToImage(canvas);

This may help.
function demoFromHTML() {
html2canvas(document.getElementById("talltweets"), {
onrendered: function(canvas) {
var imageData = canvas.toDataURL('image/png',1.0);
}
});
}

Pass canvas object instead of cann
var img= convertCanvasToImage(canvas);

If you can try to avoid dataURL and use blob instead - it will save you more memory and load images faster
$canvas.toBlob(function(blob){
var url = URL.createObjectURL(blob)
var img = new Image
img.onload = function() {
URL.revokeObjecURL(this.src)
}
img.src = url
console.log(blob)
console.log(url)
})
<canvas id="$canvas">
there is polyfills you can use to support canvas#toBlob

Convert Canvas to image then download

I am trying to convert a section of my site into a downloadable image.
Firstly I convert the html to a canvas using:
$(function() {
$("#download").click(function() {
html2canvas($("#the-grid"), {
onrendered: function(canvas) {
theCanvas = canvas;
document.body.appendChild(canvas);
$("#saved").append(canvas);
$("#saved canvas").attr('id', 'scan');
}
});
Which works fine the canvas get generated and all look's good.
I then want to turn that into an image which I can use for thumbnails later but also initiate a download of the image.
To do so I complete the function like this.
$(function() {
$("#download").click(function() {
html2canvas($("#the-grid"), {
onrendered: function(canvas) {
theCanvas = canvas;
document.body.appendChild(canvas);
$("#saved").append(canvas);
$("#saved canvas").attr('id', 'scan');
var c=document.getElementById("scan");
var d=c.toDataURL("image/png");
var w=window.open('about:blank','Download Mix');
w.document.write("<img src='"+d+"' alt='Custom Blend'/>");
}
});
But it doesn't work.
The error's I get are totally irrelevant.
I am an experienced developer but I'm pretty new to Jquery so any help would be appreciated.
UPDATE
Got it to work.
Create image like this
$(function () {
$("#download").click(function () {
html2canvas($("#the-grid"), {
onrendered: function (canvas) {
theCanvas = canvas;
document.body.appendChild(canvas);
$("#saved").append(canvas);
$("#saved canvas").attr('id', 'scan');
var image = canvas.toDataURL("image/png");
$("#saved").append("<img src='"+image+"' alt='Custom Blend'/>");
}
});
image html ends up looking like
<img src="data:image/png;base64,iVBORw0KGgoAAAANS..." alt="Custom Blend">

Maybe this can help you :
var image = canvas.toDataURL("image/png"); // build url of the image
window.location.href=image; //activate download
image = "<img src='"+image+"'/>"; // set canvas image as source

Here's how I did this (note, there's no way to download a file in Safari and set the filename without pinging a server):
First, you need to get the canvas as a png: var img = canvas.toDataUrl('image/png');
Then, you'll want to convert that dataURL to a blob. For a good way to do that, see this function.
Now you want to download that blob. This will work in all browsers but Safari:
if (window.navigator.msSaveBlob) {
window.navigator.msSaveBlob(blob, 'image.png');
} else {
var url = window.URL || window.webkitURL;
var objectURL = url.createObjectURL(blob);
var ele = $('<a target="_blank"></a>')
.hide()
.attr('download', 'image.png')
.attr('href', objectURL);
$('body').append(ele);
var clickEvent = new MouseEvent('click', {
'view': window,
'bubbles': true,
'cancelable': false,
});
ele[0].dispatchEvent(clickEvent);
window.setTimeout(function() {
url.revokeObjectURL(objectURL);
ele.remove();
}, 1000);
}
This creates an invisible link and simulates a click on it. The click() function won't work in Firefox, so you have to create an event and dispatch it by hand. Finally, it does a bit of clean up by removing the invisible link after one second. In IE, it uses the method provided by Microsoft. This will download the image with the filename "image.png". It also has the benefit of being able to download any blob, if you need to be able to do more with your code. Hopefully this helps!

Display Image using canvas in JavaScript/jQuery

I have the following code :
function createImage(source) {
var pastedImage = new Image();
pastedImage.onload = function() {
document.write('<br><br><br>Image: <img src="'+pastedImage.src+'" height="700" width="700"/>');
}
pastedImage.src = source;
}
Here I am displaying the image through html image tag which I wrote in document.write and provide appropriate height and width to image.
My question is can it possible to displaying image into the canvas instead of html img tag? So that I can drag and crop that image as I want?
But how can I display it in canvas?
Further I want to implement save that image using PHP but for now let me know about previous issue.

Try This
var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image(); // Create new img element
img.onload = function(){
// execute drawImage statements here This is essential as it waits till image is loaded before drawing it.
ctx.drawImage(img , 0, 0);
};
img.src = 'myImage.png'; // Set source path
Make sure the image is hosted in same domain as your site. Read this for Javascript Security Restrictions Same Origin Policy.
E.g. If your site is http://example.com/
then the Image should be hosted on http://example.com/../myImage.png
if you try http://facebook.com/..image/ or something then it will throw security error.

Use
CanvasRenderingContext2D.drawImage.
function createImage(source) {
var ctx = document.getElementById('canvas').getContext('2d');
var pastedImage = new Image();
pastedImage.onload = function(){
ctx.drawImage(pastedImage, 0, 0);
};
pastedImage = source;
}
Also MDN seems to be have nice examples.