code review 1223044: Fast-ish path for drawing onto an image.RGBA destination.

Fast-ish path for drawing onto an image.RGBA destination.

Time to draw.Draw a 200x200 image fell from 18.4ms (and 1 malloc) to
5.6ms (and 0 mallocs). It's still relatively slow since it assumes
nothing about the src or mask images, but it does remove the malloc.
There are existing faster, more specialized paths for copies, fills
and image glyph masks.

Also added a "compare to a slow but obviously correct implementation"
check to draw_test.go.

[nigeltao, 2010-05-20 13:25:19]

Hello rsc (cc: golang-dev@googlegroups.com),

I'd like you to review this change.

[r, 2010-05-20 16:38:06]

LGTM

[rsc, 2010-05-20 16:44:42]

LGTM

But are you sure about the * 0x101?
I am surprised that you can get away moving
the replication inside the / and the +.

Russ

[nigeltao, 2010-05-20 20:58:08]

*** Submitted as http://code.google.com/p/go/source/detail?r=62d483b851f5 ***

Fast-ish path for drawing onto an image.RGBA destination.

Time to draw.Draw a 200x200 image fell from 18.4ms (and 1 malloc) to
5.6ms (and 0 mallocs). It's still relatively slow since it assumes
nothing about the src or mask images, but it does remove the malloc.
There are existing faster, more specialized paths for copies, fills
and image glyph masks.

Also added a "compare to a slow but obviously correct implementation"
check to draw_test.go.

R=rsc, r
CC=golang-dev
http://codereview.appspot.com/1223044