image/jpeg: support for reading 4:4:0 subsampling.

Updates #2362.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6326057

diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go
index d9adf6e587627b89f7902c81fa00fe2ba5f4a663..bf67b7ee9a1ad400812ba6a59ba8bacbb6d15525 100644 (file)
--- a/src/pkg/image/jpeg/reader.go
+++ b/src/pkg/image/jpeg/reader.go
@@ -51,7 +51,7 @@ const (
        // A color JPEG image has Y, Cb and Cr components.
        nColorComponent = 3
 
-       // We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and therefore the
+       // We only support 4:4:4, 4:4:0, 4:2:2 and 4:2:0 downsampling, and therefore the
        // number of luma samples per chroma sample is at most 2 in the horizontal
        // and 2 in the vertical direction.
        maxH = 2
@@ -154,12 +154,12 @@ func (d *decoder) processSOF(n int) error {
                if d.nComp == nGrayComponent {
                        continue
                }
-               // For color images, we only support 4:4:4, 4:2:2 or 4:2:0 chroma
+               // For color images, we only support 4:4:4, 4:4:0, 4:2:2 or 4:2:0 chroma
                // downsampling ratios. This implies that the (h, v) values for the Y
-               // component are either (1, 1), (2, 1) or (2, 2), and the (h, v)
+               // component are either (1, 1), (1, 2), (2, 1) or (2, 2), and the (h, v)
                // values for the Cr and Cb components must be (1, 1).
                if i == 0 {
-                       if hv != 0x11 && hv != 0x21 && hv != 0x22 {
+                       if hv != 0x11 && hv != 0x21 && hv != 0x22 && hv != 0x12 {
                                return UnsupportedError("luma downsample ratio")
                        }
                } else if hv != 0x11 {
@@ -203,12 +203,14 @@ func (d *decoder) makeImg(h0, v0, mxx, myy int) {
                return
        }
        var subsampleRatio image.YCbCrSubsampleRatio
-       switch h0 * v0 {
-       case 1:
+       switch {
+       case h0 == 1 && v0 == 1:
                subsampleRatio = image.YCbCrSubsampleRatio444
-       case 2:
+       case h0 == 1 && v0 == 2:
+               subsampleRatio = image.YCbCrSubsampleRatio440
+       case h0 == 2 && v0 == 1:
                subsampleRatio = image.YCbCrSubsampleRatio422
-       case 4:
+       case h0 == 2 && v0 == 2:
                subsampleRatio = image.YCbCrSubsampleRatio420
        default:
                panic("unreachable")
@@ -313,8 +315,13 @@ func (d *decoder) processSOS(n int) error {
                                        } else {
                                                switch i {
                                                case 0:
-                                                       mx0 := h0*mx + (j % 2)
-                                                       my0 := v0*my + (j / 2)
+                                                       mx0, my0 := h0*mx, v0*my
+                                                       if h0 == 1 {
+                                                               my0 += j
+                                                       } else {
+                                                               mx0 += j % 2
+                                                               my0 += j / 2
+                                                       }
                                                        idct(d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride, &b)
                                                case 1:
                                                        idct(d.img3.Cb[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)

diff --git a/src/pkg/image/ycbcr.go b/src/pkg/image/ycbcr.go
index c1a0b666f8308935f061e398e49137330b4fda2b..5b73bef789581e21a684362dd8deec35ab0b3dfc 100644 (file)
--- a/src/pkg/image/ycbcr.go
+++ b/src/pkg/image/ycbcr.go
@@ -15,6 +15,7 @@ const (
        YCbCrSubsampleRatio444 YCbCrSubsampleRatio = iota
        YCbCrSubsampleRatio422
        YCbCrSubsampleRatio420
+       YCbCrSubsampleRatio440
 )
 
 func (s YCbCrSubsampleRatio) String() string {
@@ -25,6 +26,8 @@ func (s YCbCrSubsampleRatio) String() string {
                return "YCbCrSubsampleRatio422"
        case YCbCrSubsampleRatio420:
                return "YCbCrSubsampleRatio420"
+       case YCbCrSubsampleRatio440:
+               return "YCbCrSubsampleRatio440"
        }
        return "YCbCrSubsampleRatioUnknown"
 }
@@ -39,6 +42,7 @@ func (s YCbCrSubsampleRatio) String() string {
 //     For 4:4:4, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/1.
 //     For 4:2:2, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/2.
 //     For 4:2:0, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/4.
+//     For 4:4:0, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/2.
 type YCbCr struct {
        Y, Cb, Cr      []uint8
        YStride        int
@@ -82,6 +86,8 @@ func (p *YCbCr) COffset(x, y int) int {
                return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
        case YCbCrSubsampleRatio420:
                return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
+       case YCbCrSubsampleRatio440:
+               return (y/2-p.Rect.Min.Y/2)*p.CStride + (x - p.Rect.Min.X)
        }
        // Default to 4:4:4 subsampling.
        return (y-p.Rect.Min.Y)*p.CStride + (x - p.Rect.Min.X)
@@ -126,6 +132,9 @@ func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
        case YCbCrSubsampleRatio420:
                cw = (r.Max.X+1)/2 - r.Min.X/2
                ch = (r.Max.Y+1)/2 - r.Min.Y/2
+       case YCbCrSubsampleRatio440:
+               cw = w
+               ch = (r.Max.Y+1)/2 - r.Min.Y/2
        default:
                // Default to 4:4:4 subsampling.
                cw = w

diff --git a/src/pkg/image/ycbcr_test.go b/src/pkg/image/ycbcr_test.go
index 5fa95be3e07fd09bf4b964c724faaa1d755c05a4..a5f4482654feba6f29f246ef55d26b3eb0dc684b 100644 (file)
--- a/src/pkg/image/ycbcr_test.go
+++ b/src/pkg/image/ycbcr_test.go
@@ -36,6 +36,7 @@ func TestYCbCr(t *testing.T) {
                YCbCrSubsampleRatio444,
                YCbCrSubsampleRatio422,
                YCbCrSubsampleRatio420,
+               YCbCrSubsampleRatio440,
        }
        deltas := []Point{
                Pt(0, 0),