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

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Updates #2362.

R=nigeltao
CC=golang-dev
https://golang.org/cl/6326057
This commit is contained in:
Mathieu Lonjaret 2012-06-27 14:37:17 -07:00 committed by Nigel Tao
parent 71643b2fdb
commit b6cc39d0dd
3 changed files with 27 additions and 10 deletions
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27
src/pkg/image/jpeg/reader.go
View File

@ -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)

9
src/pkg/image/ycbcr.go
View File

@ -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

1
src/pkg/image/ycbcr_test.go
View File

@ -36,6 +36,7 @@ func TestYCbCr(t *testing.T) {
YCbCrSubsampleRatio444,
YCbCrSubsampleRatio422,
YCbCrSubsampleRatio420,
YCbCrSubsampleRatio440,
}
deltas := []Point{
Pt(0, 0),