From: Nigel Tao Date: Wed, 15 Jul 2015 04:55:02 +0000 (+1000) Subject: image/color: tweak the YCbCr to RGBA conversion formula. X-Git-Tag: go1.5beta2~31 X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=c2023a07915cccff1ffba9b96e160f444f77aeae;p=gostls13.git image/color: tweak the YCbCr to RGBA conversion formula. Before, calling the RGBA method of YCbCr color would return red values in the range [0x0080, 0xff80]. After, the range is [0x0000, 0xffff] and is consistent with what Gray colors' RGBA method returns. In particular, pure black, pure white and every Gray color in between are now exactly representable as a YCbCr color. This fixes a regression from Go 1.4 (where YCbCr{0x00, 0x80, 0x80} was no longer equivalent to pure black), introduced by golang.org/cl/8073 in the Go 1.5 development cycle. In Go 1.4, the +0x80 rounding was not noticable when Cb == 0x80 && Cr == 0x80, because the YCbCr to RGBA conversion truncated to 8 bits before multiplying by 0x101, so the output range was [0x0000, 0xffff]. The TestYCbCrRoundtrip fuzzy-match tolerance grows from 1 to 2 because the YCbCr to RGB conversion now maps to an ever-so-slightly larger range, along with the usual imprecision of accumulating rounding errors. Also s/int/int32/ in ycbcr.go. The conversion shouldn't overflow either way, as int is always at least 32 bits, but it does make it clearer that the computation doesn't depend on sizeof(int). Fixes #11691 Change-Id: I538ca0adf7e040fa96c5bc8b3aef4454535126b9 Reviewed-on: https://go-review.googlesource.com/12220 Reviewed-by: Rob Pike --- diff --git a/src/image/color/ycbcr.go b/src/image/color/ycbcr.go index f8c1326b3c..4bcb07dce2 100644 --- a/src/image/color/ycbcr.go +++ b/src/image/color/ycbcr.go @@ -11,9 +11,10 @@ func RGBToYCbCr(r, g, b uint8) (uint8, uint8, uint8) { // Cb = -0.1687*R - 0.3313*G + 0.5000*B + 128 // Cr = 0.5000*R - 0.4187*G - 0.0813*B + 128 // http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'. - r1 := int(r) - g1 := int(g) - b1 := int(b) + + r1 := int32(r) + g1 := int32(g) + b1 := int32(b) yy := (19595*r1 + 38470*g1 + 7471*b1 + 1<<15) >> 16 cb := (-11056*r1 - 21712*g1 + 32768*b1 + 257<<15) >> 16 cr := (32768*r1 - 27440*g1 - 5328*b1 + 257<<15) >> 16 @@ -42,9 +43,10 @@ func YCbCrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) { // G = Y' - 0.34414*(Cb-128) - 0.71414*(Cr-128) // B = Y' + 1.77200*(Cb-128) // http://www.w3.org/Graphics/JPEG/jfif3.pdf says Y but means Y'. - yy1 := int(y)<<16 + 1<<15 - cb1 := int(cb) - 128 - cr1 := int(cr) - 128 + + yy1 := int32(y) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(cb) - 128 + cr1 := int32(cr) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16 @@ -96,12 +98,12 @@ func (c YCbCr) RGBA() (uint32, uint32, uint32, uint32) { // fmt.Printf("0x%04x 0x%04x 0x%04x\n", r0, g0, b0) // fmt.Printf("0x%04x 0x%04x 0x%04x\n", r1, g1, b1) // prints: - // 0x7e19 0x808e 0x7dba + // 0x7e18 0x808e 0x7db9 // 0x7e7e 0x8080 0x7d7d - yy1 := int(c.Y)<<16 + 1<<15 - cb1 := int(c.Cb) - 128 - cr1 := int(c.Cr) - 128 + yy1 := int32(c.Y) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(c.Cb) - 128 + cr1 := int32(c.Cr) - 128 r := (yy1 + 91881*cr1) >> 8 g := (yy1 - 22554*cb1 - 46802*cr1) >> 8 b := (yy1 + 116130*cb1) >> 8 diff --git a/src/image/color/ycbcr_test.go b/src/image/color/ycbcr_test.go index 6bf53f1fd3..d64e38ef46 100644 --- a/src/image/color/ycbcr_test.go +++ b/src/image/color/ycbcr_test.go @@ -5,6 +5,7 @@ package color import ( + "fmt" "testing" ) @@ -15,8 +16,18 @@ func delta(x, y uint8) uint8 { return y - x } +func eq(c0, c1 Color) error { + r0, g0, b0, a0 := c0.RGBA() + r1, g1, b1, a1 := c1.RGBA() + if r0 != r1 || g0 != g1 || b0 != b1 || a0 != a1 { + return fmt.Errorf("got 0x%04x 0x%04x 0x%04x 0x%04x\nwant 0x%04x 0x%04x 0x%04x 0x%04x", + r0, g0, b0, a0, r1, g1, b1, a1) + } + return nil +} + // TestYCbCrRoundtrip tests that a subset of RGB space can be converted to YCbCr -// and back to within 1/256 tolerance. +// and back to within 2/256 tolerance. func TestYCbCrRoundtrip(t *testing.T) { for r := 0; r < 256; r += 7 { for g := 0; g < 256; g += 5 { @@ -24,8 +35,9 @@ func TestYCbCrRoundtrip(t *testing.T) { r0, g0, b0 := uint8(r), uint8(g), uint8(b) y, cb, cr := RGBToYCbCr(r0, g0, b0) r1, g1, b1 := YCbCrToRGB(y, cb, cr) - if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 { - t.Fatalf("\nr0, g0, b0 = %d, %d, %d\nr1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1) + if delta(r0, r1) > 2 || delta(g0, g1) > 2 || delta(b0, b1) > 2 { + t.Fatalf("\nr0, g0, b0 = %d, %d, %d\ny, cb, cr = %d, %d, %d\nr1, g1, b1 = %d, %d, %d", + r0, g0, b0, y, cb, cr, r1, g1, b1) } } } @@ -52,6 +64,15 @@ func TestYCbCrToRGBConsistency(t *testing.T) { } } +// TestYCbCrGray tests that YCbCr colors are a superset of Gray colors. +func TestYCbCrGray(t *testing.T) { + for i := 0; i < 256; i++ { + if err := eq(YCbCr{uint8(i), 0x80, 0x80}, Gray{uint8(i)}); err != nil { + t.Errorf("i=0x%02d:\n%v", i, err) + } + } +} + // TestCMYKRoundtrip tests that a subset of RGB space can be converted to CMYK // and back to within 1/256 tolerance. func TestCMYKRoundtrip(t *testing.T) { @@ -62,7 +83,8 @@ func TestCMYKRoundtrip(t *testing.T) { c, m, y, k := RGBToCMYK(r0, g0, b0) r1, g1, b1 := CMYKToRGB(c, m, y, k) if delta(r0, r1) > 1 || delta(g0, g1) > 1 || delta(b0, b1) > 1 { - t.Fatalf("\nr0, g0, b0 = %d, %d, %d\nr1, g1, b1 = %d, %d, %d", r0, g0, b0, r1, g1, b1) + t.Fatalf("\nr0, g0, b0 = %d, %d, %d\nc, m, y, k = %d, %d, %d, %d\nr1, g1, b1 = %d, %d, %d", + r0, g0, b0, c, m, y, k, r1, g1, b1) } } } @@ -91,6 +113,15 @@ func TestCMYKToRGBConsistency(t *testing.T) { } } +// TestCMYKGray tests that CMYK colors are a superset of Gray colors. +func TestCMYKGray(t *testing.T) { + for i := 0; i < 256; i++ { + if err := eq(CMYK{0x00, 0x00, 0x00, uint8(255 - i)}, Gray{uint8(i)}); err != nil { + t.Errorf("i=0x%02d:\n%v", i, err) + } + } +} + func TestPalette(t *testing.T) { p := Palette{ RGBA{0xff, 0xff, 0xff, 0xff}, diff --git a/src/image/decode_example_test.go b/src/image/decode_example_test.go index 21e90fea4f..81fa0378e1 100644 --- a/src/image/decode_example_test.go +++ b/src/image/decode_example_test.go @@ -61,22 +61,22 @@ func Example() { } // Output: // bin red green blue alpha - // 0x0000-0x0fff: 353 759 7228 0 - // 0x1000-0x1fff: 629 2944 1036 0 - // 0x2000-0x2fff: 1075 2319 984 0 - // 0x3000-0x3fff: 838 2291 988 0 - // 0x4000-0x4fff: 540 1302 542 0 - // 0x5000-0x5fff: 319 971 263 0 - // 0x6000-0x6fff: 316 377 178 0 - // 0x7000-0x7fff: 581 280 216 0 - // 0x8000-0x8fff: 3457 228 274 0 - // 0x9000-0x9fff: 2294 237 334 0 - // 0xa000-0xafff: 938 283 370 0 - // 0xb000-0xbfff: 322 338 401 0 - // 0xc000-0xcfff: 229 386 295 0 - // 0xd000-0xdfff: 263 416 281 0 - // 0xe000-0xefff: 538 433 312 0 - // 0xf000-0xffff: 2758 1886 1748 15450 + // 0x0000-0x0fff: 364 790 7242 0 + // 0x1000-0x1fff: 645 2967 1039 0 + // 0x2000-0x2fff: 1072 2299 979 0 + // 0x3000-0x3fff: 820 2266 980 0 + // 0x4000-0x4fff: 537 1305 541 0 + // 0x5000-0x5fff: 319 962 261 0 + // 0x6000-0x6fff: 322 375 177 0 + // 0x7000-0x7fff: 601 279 214 0 + // 0x8000-0x8fff: 3478 227 273 0 + // 0x9000-0x9fff: 2260 234 329 0 + // 0xa000-0xafff: 921 282 373 0 + // 0xb000-0xbfff: 321 335 397 0 + // 0xc000-0xcfff: 229 388 298 0 + // 0xd000-0xdfff: 260 414 277 0 + // 0xe000-0xefff: 516 428 298 0 + // 0xf000-0xffff: 2785 1899 1772 15450 } const data = ` diff --git a/src/image/draw/draw_test.go b/src/image/draw/draw_test.go index 29951bf05b..a58f0f4984 100644 --- a/src/image/draw/draw_test.go +++ b/src/image/draw/draw_test.go @@ -163,8 +163,8 @@ var drawTests = []drawTest{ // The source pixel is {0, 0, 136} in YCbCr-space, which is {11, 38, 0, 255} in RGB-space. {"ycbcr", vgradCr(), fillAlpha(255), Over, color.RGBA{11, 38, 0, 255}}, {"ycbcrSrc", vgradCr(), fillAlpha(255), Src, color.RGBA{11, 38, 0, 255}}, - {"ycbcrAlpha", vgradCr(), fillAlpha(192), Over, color.RGBA{42, 29, 0, 255}}, - {"ycbcrAlphaSrc", vgradCr(), fillAlpha(192), Src, color.RGBA{8, 29, 0, 192}}, + {"ycbcrAlpha", vgradCr(), fillAlpha(192), Over, color.RGBA{42, 28, 0, 255}}, + {"ycbcrAlphaSrc", vgradCr(), fillAlpha(192), Src, color.RGBA{8, 28, 0, 192}}, {"ycbcrNil", vgradCr(), nil, Over, color.RGBA{11, 38, 0, 255}}, {"ycbcrNilSrc", vgradCr(), nil, Src, color.RGBA{11, 38, 0, 255}}, // Uniform mask (100%, 75%, nil) and variable Gray source. diff --git a/src/image/internal/imageutil/gen.go b/src/image/internal/imageutil/gen.go index 6779b4959d..fc1e707f0f 100644 --- a/src/image/internal/imageutil/gen.go +++ b/src/image/internal/imageutil/gen.go @@ -95,9 +95,9 @@ const sratioCase = ` %s // This is an inline version of image/color/ycbcr.go's func YCbCrToRGB. - yy1 := int(src.Y[yi])<<16 + 1<<15 - cb1 := int(src.Cb[ci]) - 128 - cr1 := int(src.Cr[ci]) - 128 + yy1 := int32(src.Y[yi]) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(src.Cb[ci]) - 128 + cr1 := int32(src.Cr[ci]) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16 diff --git a/src/image/internal/imageutil/impl.go b/src/image/internal/imageutil/impl.go index d5dee468b3..fd7826d4a9 100644 --- a/src/image/internal/imageutil/impl.go +++ b/src/image/internal/imageutil/impl.go @@ -44,9 +44,9 @@ func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 { // This is an inline version of image/color/ycbcr.go's func YCbCrToRGB. - yy1 := int(src.Y[yi])<<16 + 1<<15 - cb1 := int(src.Cb[ci]) - 128 - cr1 := int(src.Cr[ci]) - 128 + yy1 := int32(src.Y[yi]) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(src.Cb[ci]) - 128 + cr1 := int32(src.Cr[ci]) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16 @@ -83,9 +83,9 @@ func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po ci := ciBase + sx/2 // This is an inline version of image/color/ycbcr.go's func YCbCrToRGB. - yy1 := int(src.Y[yi])<<16 + 1<<15 - cb1 := int(src.Cb[ci]) - 128 - cr1 := int(src.Cr[ci]) - 128 + yy1 := int32(src.Y[yi]) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(src.Cb[ci]) - 128 + cr1 := int32(src.Cr[ci]) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16 @@ -122,9 +122,9 @@ func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po ci := ciBase + sx/2 // This is an inline version of image/color/ycbcr.go's func YCbCrToRGB. - yy1 := int(src.Y[yi])<<16 + 1<<15 - cb1 := int(src.Cb[ci]) - 128 - cr1 := int(src.Cr[ci]) - 128 + yy1 := int32(src.Y[yi]) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(src.Cb[ci]) - 128 + cr1 := int32(src.Cr[ci]) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16 @@ -160,9 +160,9 @@ func DrawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Po for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 { // This is an inline version of image/color/ycbcr.go's func YCbCrToRGB. - yy1 := int(src.Y[yi])<<16 + 1<<15 - cb1 := int(src.Cb[ci]) - 128 - cr1 := int(src.Cr[ci]) - 128 + yy1 := int32(src.Y[yi]) * 0x10100 // Convert 0x12 to 0x121200. + cb1 := int32(src.Cb[ci]) - 128 + cr1 := int32(src.Cr[ci]) - 128 r := (yy1 + 91881*cr1) >> 16 g := (yy1 - 22554*cb1 - 46802*cr1) >> 16 b := (yy1 + 116130*cb1) >> 16