JavaScript canvas drawImage getImageData
Question
I cannot reach an understanding of how 2 methods work with canvas: drawImage getImageData in fact, I would like to parse the bitmap image into an array of RGBA points, be able to convert it and display it using the putImageData method. But only one thing works: drawImage or getImageData. I ask for help - explain how these methods influence each other.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title> Canvas to array </title>
</head>
<body>
<canvas id="canvas" width="1200" height="600" />
<script>
"use strict"
let imageData;
let My_img = new Image();
My_img.src = 'test1.jpg';
let cnv = document.getElementById("canvas");
let ctx = cnv.getContext("2d");
My_img.onload = function() {
ctx.drawImage(My_img, 0, 0);
imageData = ctx.getImageData(0, 0, canvas.width, canvas.height);
for (let i=0; i<imageData.data.length; i+=4){
imageData.data[i]=0;
imageData.data[i+1]=255;
imageData.data[i+2]=0;
imageData.data[i+3]=222;
}
ctx.putImageData(imageData,300,300);
};
</script>
</body>
</html>
1 answers
solution1
0 ACCPTED 2020-02-11 18:59:55
You need to set the cross-origin flag on the image object to be "Anonymous".
My_img.crossOrigin = "Anonymous";
See also: How to fix getImageData() error The canvas has been tainted by cross-origin data?
Demo
let ctx = document.getElementById("canvas").getContext("2d"); let img = new Image(); img.crossOrigin = "Anonymous"; img.src = 'https://placekitten.com/200/138'; img.onload = function() { ctx.drawImage(img, 0, 0); setTimeout(() => filterImage(ctx), 1000); }; // Multiply the value of the red channel by 2 and the blue channel by 3. function filterImage(ctx) { let d = ctx.getImageData(0, 0, ctx.canvas.width, ctx.canvas.height); for (let i = 0; i < d.data.length; i += 4) { d.data[i + 0] = Math.min(Math.floor(d.data[i + 0] * 2.0), 0xFF); // R d.data[i + 1] = Math.min(Math.floor(d.data[i + 1] * 1.0), 0xFF); // G d.data[i + 2] = Math.min(Math.floor(d.data[i + 2] * 3.0), 0xFF); // B d.data[i + 3] = 0xFF; // A } ctx.putImageData(d, 0, 0); }
<canvas id="canvas" width="200" height="138" />
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