image/draw: reduce drawPaletted allocations for special source cases
drawPaletted has to discover R,G,B,A color values of each source image
pixel in a given rectangle. Doing that by calling image.Image.At()
method returning color.Color interface is quite taxing allocation-wise
since interface values go through heap. Introduce special cases for some
concrete source types by fetching color values using type-specific
methods.
name old time/op new time/op delta
Paletted-4 7.62ms ± 4% 3.72ms ± 3% -51.20% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
Paletted-4 480kB ± 0% 0kB ± 0% -99.99% (p=0.000 n=4+5)
name old allocs/op new allocs/op delta
Paletted-4 120k ± 0% 0k ± 0% -100.00% (p=0.008 n=5+5)
Updates #15759.
Change-Id: I0ce1770ff600ac80599541aaad4c2c826855c8fb
Reviewed-on: https://go-review.googlesource.com/72370 Reviewed-by: Nigel Tao <nigeltao@golang.org>