[ccgus]

One of the reasons it's slow is because CG bitmap contexts are pre-multiplied, but the Canvas spec requires things to be not-premultiplied, so there's a bit of extra math that needs to happen w/ Canvas vs. straight CG/Quartz.

[daeken]

> the Canvas spec requires things to be not-premultiplied

It does, but unfortunately neither Webkit nor Gecko respect this. Take a PNG with an alpha channel and draw it to a canvas over a white div then read back the pixels; you get the pixels premultiplied by the white behind it. Same with other backing colors. There are cases where it won't premultiply, but unfortunately they're a minority.

(The reason I ran across this particular case is that I use PNGs to compress data, and using the alpha channel to store data is impossible because of the premultiplication.)