image/jpeg: encoding an *image.YCbCr is sub-optimal

[tbonfort]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

master

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/tbonfort/dev/go"
GORACE=""
GOROOT="/usr/lib/go-1.6"
GOTOOLDIR="/usr/lib/go-1.6/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT="1"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

What did you do?

func BenchmarkEncodeYCbCr(b *testing.B) {
	b.StopTimer()
	img := image.NewYCbCr(image.Rect(0, 0, 640, 480), image.YCbCrSubsampleRatio420)
	bo := img.Bounds()
	rnd := rand.New(rand.NewSource(123))
	for y := bo.Min.Y; y < bo.Max.Y; y++ {
		for x := bo.Min.X; x < bo.Max.X; x++ {
			cy := img.YOffset(x, y)
			ci := img.COffset(x, y)
			img.Y[cy] = uint8(rnd.Intn(256))
			img.Cb[ci] = uint8(rnd.Intn(256))
			img.Cr[ci] = uint8(rnd.Intn(256))
		}
	}
	b.SetBytes(640 * 480 * 4)
	b.StartTimer()
	options := &Options{Quality: 90}
	for i := 0; i < b.N; i++ {
		Encode(ioutil.Discard, img, options)
	}
}

What did you expect to see?

jpeg.Encode() of *image.YCbCr should be faster than for *image.RGBA

What did you see instead?

This is slow as jpeg.Encode() uses the generic image.At() which involves unnecessary ycbr->rgba->ycbcr conversions, and more importantly image.color allocations

[gopherbot]

CL https://golang.org/cl/34773 mentions this issue.

[tbonfort]

quick benchmark:

benchmark                  old ns/op     new ns/op     delta
BenchmarkEncodeYCbCr-4     43990846      24201148      -44.99%

benchmark                  old MB/s     new MB/s     speedup
BenchmarkEncodeYCbCr-4     20.95        38.08        1.82x