jpeg: decode to a YCbCr image instead of an RGBA image.

R=r
CC=golang-dev
https://golang.org/cl/4436053

src/pkg/image/jpeg/reader.go

@@ -10,10 +10,14 @@ package jpeg
 import (
 	"bufio"
 	"image"
+	"image/ycbcr"
 	"io"
 	"os"
 )
 
+// TODO(nigeltao): fix up the doc comment style so that sentences start with
+// the name of the type or function that they annotate.
+
 // A FormatError reports that the input is not a valid JPEG.
 type FormatError string
 
@@ -26,9 +30,9 @@ func (e UnsupportedError) String() string { return "unsupported JPEG feature: "
 
 // Component specification, specified in section B.2.2.
 type component struct {
+	h  int   // Horizontal sampling factor.
+	v  int   // Vertical sampling factor.
 	c  uint8 // Component identifier.
-	h  uint8 // Horizontal sampling factor.
-	v  uint8 // Vertical sampling factor.
 	tq uint8 // Quantization table destination selector.
 }
 
@@ -86,7 +90,7 @@ type Reader interface {
 type decoder struct {
 	r             Reader
 	width, height int
-	image         *image.RGBA
+	img           *ycbcr.YCbCr
 	ri            int // Restart Interval.
 	comps         [nComponent]component
 	huff          [maxTc + 1][maxTh + 1]huffman
@@ -132,9 +136,9 @@ func (d *decoder) processSOF(n int) os.Error {
 	}
 	for i := 0; i < nComponent; i++ {
 		hv := d.tmp[7+3*i]
+		d.comps[i].h = int(hv >> 4)
+		d.comps[i].v = int(hv & 0x0f)
 		d.comps[i].c = d.tmp[6+3*i]
-		d.comps[i].h = hv >> 4
-		d.comps[i].v = hv & 0x0f
 		d.comps[i].tq = d.tmp[8+3*i]
 		// We only support YCbCr images, and 4:4:4, 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
@@ -178,71 +182,47 @@ func (d *decoder) processDQT(n int) os.Error {
 	return nil
 }
 
-// Set the Pixel (px, py)'s RGB value, based on its YCbCr value.
-func (d *decoder) calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex int) {
-	y, cb, cr := d.blocks[0][lumaBlock][lumaIndex], d.blocks[1][0][chromaIndex], d.blocks[2][0][chromaIndex]
-	// The JFIF specification (http://www.w3.org/Graphics/JPEG/jfif3.pdf, page 3) gives the formula
-	// for translating YCbCr to RGB as:
-	//   R = Y + 1.402 (Cr-128)
-	//   G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128)
-	//   B = Y + 1.772 (Cb-128)
-	yPlusHalf := 100000*y + 50000
-	cb -= 128
-	cr -= 128
-	r := (yPlusHalf + 140200*cr) / 100000
-	g := (yPlusHalf - 34414*cb - 71414*cr) / 100000
-	b := (yPlusHalf + 177200*cb) / 100000
-	if r < 0 {
-		r = 0
-	} else if r > 255 {
-		r = 255
-	}
-	if g < 0 {
-		g = 0
-	} else if g > 255 {
-		g = 255
+// Clip x to the range [0, 255] inclusive.
+func clip(x int) uint8 {
+	if x < 0 {
+		return 0
 	}
-	if b < 0 {
-		b = 0
-	} else if b > 255 {
-		b = 255
+	if x > 255 {
+		return 255
 	}
-	d.image.Pix[py*d.image.Stride+px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff}
+	return uint8(x)
 }
 
-// Convert the MCU from YCbCr to RGB.
-func (d *decoder) convertMCU(mx, my, h0, v0 int) {
-	lumaBlock := 0
+// Store the MCU to the image.
+func (d *decoder) storeMCU(mx, my int) {
+	h0, v0 := d.comps[0].h, d.comps[0].v
+	// Store the luma blocks.
 	for v := 0; v < v0; v++ {
 		for h := 0; h < h0; h++ {
-			chromaBase := 8*4*v + 4*h
-			py := 8 * (v0*my + v)
-			for y := 0; y < 8 && py < d.height; y++ {
-				px := 8 * (h0*mx + h)
-				lumaIndex := 8 * y
-				chromaIndex := chromaBase + 8*(y/v0)
-				for x := 0; x < 8 && px < d.width; x++ {
-					d.calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex)
-					if h0 == 1 {
-						chromaIndex += 1
-					} else {
-						chromaIndex += x % 2
-					}
-					lumaIndex++
-					px++
+			p := 8 * ((v0*my+v)*d.img.YStride + (h0*mx + h))
+			for y := 0; y < 8; y++ {
+				for x := 0; x < 8; x++ {
+					d.img.Y[p] = clip(d.blocks[0][h0*v+h][8*y+x])
+					p++
 				}
-				py++
+				p += d.img.YStride - 8
 			}
-			lumaBlock++
 		}
 	}
+	// Store the chroma blocks.
+	p := 8 * (my*d.img.CStride + mx)
+	for y := 0; y < 8; y++ {
+		for x := 0; x < 8; x++ {
+			d.img.Cb[p] = clip(d.blocks[1][0][8*y+x])
+			d.img.Cr[p] = clip(d.blocks[2][0][8*y+x])
+			p++
+		}
+		p += d.img.CStride - 8
+	}
 }
 
 // Specified in section B.2.3.
 func (d *decoder) processSOS(n int) os.Error {
-	if d.image == nil {
-		d.image = image.NewRGBA(d.width, d.height)
-	}
 	if n != 4+2*nComponent {
 		return UnsupportedError("SOS has wrong length")
 	}
@@ -257,7 +237,6 @@ func (d *decoder) processSOS(n int) os.Error {
 		td uint8 // DC table selector.
 		ta uint8 // AC table selector.
 	}
-	h0, v0 := int(d.comps[0].h), int(d.comps[0].v) // The h and v values from the Y components.
 	for i := 0; i < nComponent; i++ {
 		cs := d.tmp[1+2*i] // Component selector.
 		if cs != d.comps[i].c {
@@ -267,8 +246,33 @@ func (d *decoder) processSOS(n int) os.Error {
 		scanComps[i].ta = d.tmp[2+2*i] & 0x0f
 	}
 	// mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
-	mxx := (d.width + 8*int(h0) - 1) / (8 * int(h0))
-	myy := (d.height + 8*int(v0) - 1) / (8 * int(v0))
+	h0, v0 := d.comps[0].h, d.comps[0].v // The h and v values from the Y components.
+	mxx := (d.width + 8*h0 - 1) / (8 * h0)
+	myy := (d.height + 8*v0 - 1) / (8 * v0)
+	if d.img == nil {
+		var subsampleRatio ycbcr.SubsampleRatio
+		n := h0 * v0
+		switch n {
+		case 1:
+			subsampleRatio = ycbcr.SubsampleRatio444
+		case 2:
+			subsampleRatio = ycbcr.SubsampleRatio422
+		case 4:
+			subsampleRatio = ycbcr.SubsampleRatio420
+		default:
+			panic("unreachable")
+		}
+		b := make([]byte, mxx*myy*(1*8*8*n+2*8*8))
+		d.img = &ycbcr.YCbCr{
+			Y:              b[mxx*myy*(0*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+0*8*8)],
+			Cb:             b[mxx*myy*(1*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+1*8*8)],
+			Cr:             b[mxx*myy*(1*8*8*n+1*8*8) : mxx*myy*(1*8*8*n+2*8*8)],
+			SubsampleRatio: subsampleRatio,
+			YStride:        mxx * 8 * h0,
+			CStride:        mxx * 8,
+			Rect:           image.Rect(0, 0, d.width, d.height),
+		}
+	}
 
 	mcu, expectedRST := 0, uint8(rst0Marker)
 	var allZeroes block
@@ -277,7 +281,7 @@ func (d *decoder) processSOS(n int) os.Error {
 		for mx := 0; mx < mxx; mx++ {
 			for i := 0; i < nComponent; i++ {
 				qt := &d.quant[d.comps[i].tq]
-				for j := 0; j < int(d.comps[i].h*d.comps[i].v); j++ {
+				for j := 0; j < d.comps[i].h*d.comps[i].v; j++ {
 					d.blocks[i][j] = allZeroes
 
 					// Decode the DC coefficient, as specified in section F.2.2.1.
@@ -301,20 +305,20 @@ func (d *decoder) processSOS(n int) os.Error {
 						if err != nil {
 							return err
 						}
-						v0 := value >> 4
-						v1 := value & 0x0f
-						if v1 != 0 {
-							k += int(v0)
+						val0 := value >> 4
+						val1 := value & 0x0f
+						if val1 != 0 {
+							k += int(val0)
 							if k > blockSize {
 								return FormatError("bad DCT index")
 							}
-							ac, err := d.receiveExtend(v1)
+							ac, err := d.receiveExtend(val1)
 							if err != nil {
 								return err
 							}
 							d.blocks[i][j][unzig[k]] = ac * qt[k]
 						} else {
-							if v0 != 0x0f {
+							if val0 != 0x0f {
 								break
 							}
 							k += 0x0f
@@ -324,7 +328,7 @@ func (d *decoder) processSOS(n int) os.Error {
 					idct(&d.blocks[i][j])
 				} // for j
 			} // for i
-			d.convertMCU(mx, my, int(d.comps[0].h), int(d.comps[0].v))
+			d.storeMCU(mx, my)
 			mcu++
 			if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
 				// A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
@@ -433,7 +437,7 @@ func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, os.Error) {
 			return nil, err
 		}
 	}
-	return d.image, nil
+	return d.img, nil
 }
 
 // Decode reads a JPEG image from r and returns it as an image.Image.