theclowerweb@:

I used OpenCV to exam these two images, and I found that for those pixels whose values are different, their alpha channel is 0. 

In that case, the behavior is expected. When drawing an image to canvas, the canvas will always generate an image buffer that is premultiplied by alpha. Since those pixels has alpha channel to be 0, the r, g, b value will be lost during the premultiplication.

Does that make sense?

Hi, and thanks for the explanation! Do you know of a workaround, or a way to extract the correct values? Is there a way to turn off the premultiplication, or tell my algorithm that if the alpha is 0, do something to get the correct rgb? I've tried getting the RGBA from PHP instead, but PHP has a weird 7-bit alpha value (0-127, where 127 is fully off and 0 is fully on), and I don't think there's a way to convert it to an accurate 8-bit value...

For example, a way to say: if the alpha is 0, get just the RGB without alpha?

Unfortunately I cannot think of anything with canvas. Maybe using some kind of pre-processing technique on the image, and change alpha value from 0-->255 before uploading the image to canvas?

The other possibility is to use WebGL's texture upload function:
https://developer.mozilla.org/en/docs/Web/API/WebGLRenderingContext/texImage2D

TexImage2D will preserve the alpha channel, but I am not sure if that works for your app or not.

If you used WebGL instead of 2D canvas, you would have all the knobs you need.

Thanks @xidac and @junov - I'll give the WebGL method a try!

closing this bug for now. Thanks.