Can't getImageData() in javascript with gif file; can with FillRect. What's wrong?
Question
Looks like getImageData works when I use FillRect to create a rect on the canvas, but it doesn't work when I draw a gif on the canvas despite it drawing correctly and everything. Have no idea why that is - is it a security risk? Help?
<html>
<body>
<p>Image to use:</p>
<img id="scream" width="230" height="60" src="http://www.thekitmaker.com/image/3.gif" alt="The Scream">
<p>Canvas:</p>
<canvas id="myCanvas" width="230" height="60" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
function getPixel(imgData, index) {
var i = index*4, d = imgData.data;
return [d[i],d[i+1],d[i+2],d[i+3]] // [R,G,B,A]
}
// AND/OR
function getPixelXY(imgData, x, y) {
return getPixel(imgData, y*imgData.width+x);
}
window.onload = function() {
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
//draw on canvas
var img = document.getElementById("scream");
ctx.drawImage(img,10,10,50,50);//,0,0);// c.width,c.height);
//ctx.fillStyle = "red";
//ctx.fillRect(10, 10, 50, 50);
//get image data
alert("h");
var imgData = ctx.getImageData(10, 10, 50, 50);
alert("h2");
// The magic®
//getPixel(idt, 852); // returns array [red, green, blue, alpha]
alert("p:"+getPixelXY(idt,1,1)[0]); // same pixel using x,y
}
</script>
</body>
</html>
1 answers
solution1
0 2015-06-15 00:08:48
You are correct that it is because of security reasons. If you draw an image from a different origin onto a canvas, the canvas will be marked as "tainted" and you can no longer get image data from it, among other things.
Reference: http://www.w3.org/TR/html51/semantics.html#security-with-canvas-elements
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