Is it possible to disable premultiplied alpha for the 2d canvas context?

[mechanic...@gmail.com]

I'm working on a small project that uses canvas to replace exact colors on video game sprites and I've run into a problem that I'm not sure I can fix. Essentially whenever I load an image onto the canvas by using putImageData, any pixels with alpha may end up slightly different when read out with getImageData. A quick example of this is:

var canvas = document.createElement('canvas');

var ctx = canvas.getContext('2d');

var px = ctx.getImageData(0, 0, 1, 1);

px.data[0] = 253;

px.data[1] = 168;

px.data[2] = 55;

px.data[3] = 165;

ctx.putImageData(px, 0, 0);

var px2 = ctx.getImageData(0, 0, 1, 1);

console.log(px2.data); // Gives: [253, 168, 56, 165] Expected: [253, 168, 55, 165]

In my research into the problem I believe I've narrowed it down to the fact that Chromium seems to use premultiplied alpha for this context, which is a problem for me since I need to work with the exact colors of the image, and as soon as alpha is introduced it makes it uncertain whether or not the color I am using is really the exact color from the source image. This appears in every version of Node-Webkit as a result. I've looked around and this issue seems to have very little attention. I'm not an expert with Chromium but I was wondering if anyone knew of a way to disable premultiplied alpha when using Canvas in a 2d context?

[Roger Wang]

Hello,

What's the result with Chrome browser?

Roger

[mechanic...@gmail.com]

Hi, it's the exact same issue in the Chrome browser.