Canvas PutImageData color loss with no/low alpha

There's a 3x3 image. The CanvasPixelArray is:

[12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255,12,24,48,255]

I change 开发者_如何转开发the alpha of all pixels to 0 and back by:

bobs = this.gfx.getImageData(0,0,3,3).data
for (a=3;a<bobs.data.length;a+=4)
bobs.data[a] = 0
this.gfx.putImageData(bobs,0,0)
bobs = this.gfx.getImageData(0,0,3,3).data
for (a=3;a<bobs.data.length;a+=4)
bobs.data[a] = 255
this.gfx.putImageData(bobs,0,0)

All pixels became black. The browser changed the colors to black to save memory. Is there a way to prevent this or should I save a duplicate?

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I think the reason for this is that Canvas uses premultiplied alpha, meaning all rgb values are multiplied by the alpha value for those pixels. It's done to speed up alpha blending with backgrounds, etc.

There's a section on premultiplied alpha in this article: wikipedia:Alpha_compositing

You'll probably have to either, as you say keep a copy of the unmodified values, or perhaps store an alpha value for the image and set the globalAlpha property before drawing the image. (When I say image, you can equally create a 3x3 canvas, store the pixels there and use drawImage() to draw it to the main canvas).