Moving to glide

3 files changed, 762 insertions, 0 deletions

diff --git a/vendor/golang.org/x/image/webp/decode.go b/vendor/golang.org/x/image/webp/decode.go
new file mode 100644
index 000000000..134307e41
--- /dev/null
+++ b/vendor/golang.org/x/image/webp/decode.go
@@ -0,0 +1,274 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package webp implements a decoder for WEBP images.
+//
+// WEBP is defined at:
+// https://developers.google.com/speed/webp/docs/riff_container
+package webp // import "golang.org/x/image/webp"
+
+import (
+	"bytes"
+	"errors"
+	"image"
+	"image/color"
+	"io"
+
+	"golang.org/x/image/riff"
+	"golang.org/x/image/vp8"
+	"golang.org/x/image/vp8l"
+)
+
+var errInvalidFormat = errors.New("webp: invalid format")
+
+var (
+	fccALPH = riff.FourCC{'A', 'L', 'P', 'H'}
+	fccVP8  = riff.FourCC{'V', 'P', '8', ' '}
+	fccVP8L = riff.FourCC{'V', 'P', '8', 'L'}
+	fccVP8X = riff.FourCC{'V', 'P', '8', 'X'}
+	fccWEBP = riff.FourCC{'W', 'E', 'B', 'P'}
+)
+
+func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
+	formType, riffReader, err := riff.NewReader(r)
+	if err != nil {
+		return nil, image.Config{}, err
+	}
+	if formType != fccWEBP {
+		return nil, image.Config{}, errInvalidFormat
+	}
+
+	var (
+		alpha          []byte
+		alphaStride    int
+		wantAlpha      bool
+		widthMinusOne  uint32
+		heightMinusOne uint32
+		buf            [10]byte
+	)
+	for {
+		chunkID, chunkLen, chunkData, err := riffReader.Next()
+		if err == io.EOF {
+			err = errInvalidFormat
+		}
+		if err != nil {
+			return nil, image.Config{}, err
+		}
+
+		switch chunkID {
+		case fccALPH:
+			if !wantAlpha {
+				return nil, image.Config{}, errInvalidFormat
+			}
+			wantAlpha = false
+			// Read the Pre-processing | Filter | Compression byte.
+			if _, err := io.ReadFull(chunkData, buf[:1]); err != nil {
+				if err == io.EOF {
+					err = errInvalidFormat
+				}
+				return nil, image.Config{}, err
+			}
+			alpha, alphaStride, err = readAlpha(chunkData, widthMinusOne, heightMinusOne, buf[0]&0x03)
+			if err != nil {
+				return nil, image.Config{}, err
+			}
+			unfilterAlpha(alpha, alphaStride, (buf[0]>>2)&0x03)
+
+		case fccVP8:
+			if wantAlpha || int32(chunkLen) < 0 {
+				return nil, image.Config{}, errInvalidFormat
+			}
+			d := vp8.NewDecoder()
+			d.Init(chunkData, int(chunkLen))
+			fh, err := d.DecodeFrameHeader()
+			if err != nil {
+				return nil, image.Config{}, err
+			}
+			if configOnly {
+				return nil, image.Config{
+					ColorModel: color.YCbCrModel,
+					Width:      fh.Width,
+					Height:     fh.Height,
+				}, nil
+			}
+			m, err := d.DecodeFrame()
+			if err != nil {
+				return nil, image.Config{}, err
+			}
+			if alpha != nil {
+				return &image.NYCbCrA{
+					YCbCr:   *m,
+					A:       alpha,
+					AStride: alphaStride,
+				}, image.Config{}, nil
+			}
+			return m, image.Config{}, nil
+
+		case fccVP8L:
+			if wantAlpha || alpha != nil {
+				return nil, image.Config{}, errInvalidFormat
+			}
+			if configOnly {
+				c, err := vp8l.DecodeConfig(chunkData)
+				return nil, c, err
+			}
+			m, err := vp8l.Decode(chunkData)
+			return m, image.Config{}, err
+
+		case fccVP8X:
+			if chunkLen != 10 {
+				return nil, image.Config{}, errInvalidFormat
+			}
+			if _, err := io.ReadFull(chunkData, buf[:10]); err != nil {
+				return nil, image.Config{}, err
+			}
+			const (
+				animationBit    = 1 << 1
+				xmpMetadataBit  = 1 << 2
+				exifMetadataBit = 1 << 3
+				alphaBit        = 1 << 4
+				iccProfileBit   = 1 << 5
+			)
+			if buf[0] != alphaBit {
+				return nil, image.Config{}, errors.New("webp: non-Alpha VP8X is not implemented")
+			}
+			widthMinusOne = uint32(buf[4]) | uint32(buf[5])<<8 | uint32(buf[6])<<16
+			heightMinusOne = uint32(buf[7]) | uint32(buf[8])<<8 | uint32(buf[9])<<16
+			if configOnly {
+				return nil, image.Config{
+					ColorModel: color.NYCbCrAModel,
+					Width:      int(widthMinusOne) + 1,
+					Height:     int(heightMinusOne) + 1,
+				}, nil
+			}
+			wantAlpha = true
+
+		default:
+			return nil, image.Config{}, errInvalidFormat
+		}
+	}
+}
+
+func readAlpha(chunkData io.Reader, widthMinusOne, heightMinusOne uint32, compression byte) (
+	alpha []byte, alphaStride int, err error) {
+
+	switch compression {
+	case 0:
+		w := int(widthMinusOne) + 1
+		h := int(heightMinusOne) + 1
+		alpha = make([]byte, w*h)
+		if _, err := io.ReadFull(chunkData, alpha); err != nil {
+			return nil, 0, err
+		}
+		return alpha, w, nil
+
+	case 1:
+		// Read the VP8L-compressed alpha values. First, synthesize a 5-byte VP8L header:
+		// a 1-byte magic number, a 14-bit widthMinusOne, a 14-bit heightMinusOne,
+		// a 1-bit (ignored, zero) alphaIsUsed and a 3-bit (zero) version.
+		// TODO(nigeltao): be more efficient than decoding an *image.NRGBA just to
+		// extract the green values to a separately allocated []byte. Fixing this
+		// will require changes to the vp8l package's API.
+		if widthMinusOne > 0x3fff || heightMinusOne > 0x3fff {
+			return nil, 0, errors.New("webp: invalid format")
+		}
+		alphaImage, err := vp8l.Decode(io.MultiReader(
+			bytes.NewReader([]byte{
+				0x2f, // VP8L magic number.
+				uint8(widthMinusOne),
+				uint8(widthMinusOne>>8) | uint8(heightMinusOne<<6),
+				uint8(heightMinusOne >> 2),
+				uint8(heightMinusOne >> 10),
+			}),
+			chunkData,
+		))
+		if err != nil {
+			return nil, 0, err
+		}
+		// The green values of the inner NRGBA image are the alpha values of the
+		// outer NYCbCrA image.
+		pix := alphaImage.(*image.NRGBA).Pix
+		alpha = make([]byte, len(pix)/4)
+		for i := range alpha {
+			alpha[i] = pix[4*i+1]
+		}
+		return alpha, int(widthMinusOne) + 1, nil
+	}
+	return nil, 0, errInvalidFormat
+}
+
+func unfilterAlpha(alpha []byte, alphaStride int, filter byte) {
+	if len(alpha) == 0 || alphaStride == 0 {
+		return
+	}
+	switch filter {
+	case 1: // Horizontal filter.
+		for i := 1; i < alphaStride; i++ {
+			alpha[i] += alpha[i-1]
+		}
+		for i := alphaStride; i < len(alpha); i += alphaStride {
+			// The first column is equivalent to the vertical filter.
+			alpha[i] += alpha[i-alphaStride]
+
+			for j := 1; j < alphaStride; j++ {
+				alpha[i+j] += alpha[i+j-1]
+			}
+		}
+
+	case 2: // Vertical filter.
+		// The first row is equivalent to the horizontal filter.
+		for i := 1; i < alphaStride; i++ {
+			alpha[i] += alpha[i-1]
+		}
+
+		for i := alphaStride; i < len(alpha); i++ {
+			alpha[i] += alpha[i-alphaStride]
+		}
+
+	case 3: // Gradient filter.
+		// The first row is equivalent to the horizontal filter.
+		for i := 1; i < alphaStride; i++ {
+			alpha[i] += alpha[i-1]
+		}
+
+		for i := alphaStride; i < len(alpha); i += alphaStride {
+			// The first column is equivalent to the vertical filter.
+			alpha[i] += alpha[i-alphaStride]
+
+			// The interior is predicted on the three top/left pixels.
+			for j := 1; j < alphaStride; j++ {
+				c := int(alpha[i+j-alphaStride-1])
+				b := int(alpha[i+j-alphaStride])
+				a := int(alpha[i+j-1])
+				x := a + b - c
+				if x < 0 {
+					x = 0
+				} else if x > 255 {
+					x = 255
+				}
+				alpha[i+j] += uint8(x)
+			}
+		}
+	}
+}
+
+// Decode reads a WEBP image from r and returns it as an image.Image.
+func Decode(r io.Reader) (image.Image, error) {
+	m, _, err := decode(r, false)
+	if err != nil {
+		return nil, err
+	}
+	return m, err
+}
+
+// DecodeConfig returns the color model and dimensions of a WEBP image without
+// decoding the entire image.
+func DecodeConfig(r io.Reader) (image.Config, error) {
+	_, c, err := decode(r, true)
+	return c, err
+}
+
+func init() {
+	image.RegisterFormat("webp", "RIFF????WEBPVP8", Decode, DecodeConfig)
+}
diff --git a/vendor/golang.org/x/image/webp/decode_test.go b/vendor/golang.org/x/image/webp/decode_test.go
new file mode 100644
index 000000000..ad65b1088
--- /dev/null
+++ b/vendor/golang.org/x/image/webp/decode_test.go
@@ -0,0 +1,294 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package webp
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/png"
+	"io/ioutil"
+	"os"
+	"strings"
+	"testing"
+)
+
+// hex is like fmt.Sprintf("% x", x) but also inserts dots every 16 bytes, to
+// delineate VP8 macroblock boundaries.
+func hex(x []byte) string {
+	buf := new(bytes.Buffer)
+	for len(x) > 0 {
+		n := len(x)
+		if n > 16 {
+			n = 16
+		}
+		fmt.Fprintf(buf, " . % x", x[:n])
+		x = x[n:]
+	}
+	return buf.String()
+}
+
+func testDecodeLossy(t *testing.T, tc string, withAlpha bool) {
+	webpFilename := "../testdata/" + tc + ".lossy.webp"
+	pngFilename := webpFilename + ".ycbcr.png"
+	if withAlpha {
+		webpFilename = "../testdata/" + tc + ".lossy-with-alpha.webp"
+		pngFilename = webpFilename + ".nycbcra.png"
+	}
+
+	f0, err := os.Open(webpFilename)
+	if err != nil {
+		t.Errorf("%s: Open WEBP: %v", tc, err)
+		return
+	}
+	defer f0.Close()
+	img0, err := Decode(f0)
+	if err != nil {
+		t.Errorf("%s: Decode WEBP: %v", tc, err)
+		return
+	}
+
+	var (
+		m0 *image.YCbCr
+		a0 *image.NYCbCrA
+		ok bool
+	)
+	if withAlpha {
+		a0, ok = img0.(*image.NYCbCrA)
+		if ok {
+			m0 = &a0.YCbCr
+		}
+	} else {
+		m0, ok = img0.(*image.YCbCr)
+	}
+	if !ok || m0.SubsampleRatio != image.YCbCrSubsampleRatio420 {
+		t.Errorf("%s: decoded WEBP image is not a 4:2:0 YCbCr or 4:2:0 NYCbCrA", tc)
+		return
+	}
+	// w2 and h2 are the half-width and half-height, rounded up.
+	w, h := m0.Bounds().Dx(), m0.Bounds().Dy()
+	w2, h2 := int((w+1)/2), int((h+1)/2)
+
+	f1, err := os.Open(pngFilename)
+	if err != nil {
+		t.Errorf("%s: Open PNG: %v", tc, err)
+		return
+	}
+	defer f1.Close()
+	img1, err := png.Decode(f1)
+	if err != nil {
+		t.Errorf("%s: Open PNG: %v", tc, err)
+		return
+	}
+
+	// The split-into-YCbCr-planes golden image is a 2*w2 wide and h+h2 high
+	// (or 2*h+h2 high, if with Alpha) gray image arranged in IMC4 format:
+	//   YYYY
+	//   YYYY
+	//   BBRR
+	//   AAAA
+	// See http://www.fourcc.org/yuv.php#IMC4
+	pngW, pngH := 2*w2, h+h2
+	if withAlpha {
+		pngH += h
+	}
+	if got, want := img1.Bounds(), image.Rect(0, 0, pngW, pngH); got != want {
+		t.Errorf("%s: bounds0: got %v, want %v", tc, got, want)
+		return
+	}
+	m1, ok := img1.(*image.Gray)
+	if !ok {
+		t.Errorf("%s: decoded PNG image is not a Gray", tc)
+		return
+	}
+
+	type plane struct {
+		name     string
+		m0Pix    []uint8
+		m0Stride int
+		m1Rect   image.Rectangle
+	}
+	planes := []plane{
+		{"Y", m0.Y, m0.YStride, image.Rect(0, 0, w, h)},
+		{"Cb", m0.Cb, m0.CStride, image.Rect(0*w2, h, 1*w2, h+h2)},
+		{"Cr", m0.Cr, m0.CStride, image.Rect(1*w2, h, 2*w2, h+h2)},
+	}
+	if withAlpha {
+		planes = append(planes, plane{
+			"A", a0.A, a0.AStride, image.Rect(0, h+h2, w, 2*h+h2),
+		})
+	}
+
+	for _, plane := range planes {
+		dx := plane.m1Rect.Dx()
+		nDiff, diff := 0, make([]byte, dx)
+		for j, y := 0, plane.m1Rect.Min.Y; y < plane.m1Rect.Max.Y; j, y = j+1, y+1 {
+			got := plane.m0Pix[j*plane.m0Stride:][:dx]
+			want := m1.Pix[y*m1.Stride+plane.m1Rect.Min.X:][:dx]
+			if bytes.Equal(got, want) {
+				continue
+			}
+			nDiff++
+			if nDiff > 10 {
+				t.Errorf("%s: %s plane: more rows differ", tc, plane.name)
+				break
+			}
+			for i := range got {
+				diff[i] = got[i] - want[i]
+			}
+			t.Errorf("%s: %s plane: m0 row %d, m1 row %d\ngot %s\nwant%s\ndiff%s",
+				tc, plane.name, j, y, hex(got), hex(want), hex(diff))
+		}
+	}
+}
+
+func TestDecodeVP8(t *testing.T) {
+	testCases := []string{
+		"blue-purple-pink",
+		"blue-purple-pink-large.no-filter",
+		"blue-purple-pink-large.simple-filter",
+		"blue-purple-pink-large.normal-filter",
+		"video-001",
+		"yellow_rose",
+	}
+
+	for _, tc := range testCases {
+		testDecodeLossy(t, tc, false)
+	}
+}
+
+func TestDecodeVP8XAlpha(t *testing.T) {
+	testCases := []string{
+		"yellow_rose",
+	}
+
+	for _, tc := range testCases {
+		testDecodeLossy(t, tc, true)
+	}
+}
+
+func TestDecodeVP8L(t *testing.T) {
+	testCases := []string{
+		"blue-purple-pink",
+		"blue-purple-pink-large",
+		"gopher-doc.1bpp",
+		"gopher-doc.2bpp",
+		"gopher-doc.4bpp",
+		"gopher-doc.8bpp",
+		"tux",
+		"yellow_rose",
+	}
+
+loop:
+	for _, tc := range testCases {
+		f0, err := os.Open("../testdata/" + tc + ".lossless.webp")
+		if err != nil {
+			t.Errorf("%s: Open WEBP: %v", tc, err)
+			continue
+		}
+		defer f0.Close()
+		img0, err := Decode(f0)
+		if err != nil {
+			t.Errorf("%s: Decode WEBP: %v", tc, err)
+			continue
+		}
+		m0, ok := img0.(*image.NRGBA)
+		if !ok {
+			t.Errorf("%s: WEBP image is %T, want *image.NRGBA", tc, img0)
+			continue
+		}
+
+		f1, err := os.Open("../testdata/" + tc + ".png")
+		if err != nil {
+			t.Errorf("%s: Open PNG: %v", tc, err)
+			continue
+		}
+		defer f1.Close()
+		img1, err := png.Decode(f1)
+		if err != nil {
+			t.Errorf("%s: Decode PNG: %v", tc, err)
+			continue
+		}
+		m1, ok := img1.(*image.NRGBA)
+		if !ok {
+			rgba1, ok := img1.(*image.RGBA)
+			if !ok {
+				t.Fatalf("%s: PNG image is %T, want *image.NRGBA", tc, img1)
+				continue
+			}
+			if !rgba1.Opaque() {
+				t.Fatalf("%s: PNG image is non-opaque *image.RGBA, want *image.NRGBA", tc)
+				continue
+			}
+			// The image is fully opaque, so we can re-interpret the RGBA pixels
+			// as NRGBA pixels.
+			m1 = &image.NRGBA{
+				Pix:    rgba1.Pix,
+				Stride: rgba1.Stride,
+				Rect:   rgba1.Rect,
+			}
+		}
+
+		b0, b1 := m0.Bounds(), m1.Bounds()
+		if b0 != b1 {
+			t.Errorf("%s: bounds: got %v, want %v", tc, b0, b1)
+			continue
+		}
+		for i := range m0.Pix {
+			if m0.Pix[i] != m1.Pix[i] {
+				y := i / m0.Stride
+				x := (i - y*m0.Stride) / 4
+				i = 4 * (y*m0.Stride + x)
+				t.Errorf("%s: at (%d, %d):\ngot  %02x %02x %02x %02x\nwant %02x %02x %02x %02x",
+					tc, x, y,
+					m0.Pix[i+0], m0.Pix[i+1], m0.Pix[i+2], m0.Pix[i+3],
+					m1.Pix[i+0], m1.Pix[i+1], m1.Pix[i+2], m1.Pix[i+3],
+				)
+				continue loop
+			}
+		}
+	}
+}
+
+// TestDecodePartitionTooLarge tests that decoding a malformed WEBP image
+// doesn't try to allocate an unreasonable amount of memory. This WEBP image
+// claims a RIFF chunk length of 0x12345678 bytes (291 MiB) compressed,
+// independent of the actual image size (0 pixels wide * 0 pixels high).
+//
+// This is based on golang.org/issue/10790.
+func TestDecodePartitionTooLarge(t *testing.T) {
+	data := "RIFF\xff\xff\xff\x7fWEBPVP8 " +
+		"\x78\x56\x34\x12" + // RIFF chunk length.
+		"\xbd\x01\x00\x14\x00\x00\xb2\x34\x0a\x9d\x01\x2a\x96\x00\x67\x00"
+	_, err := Decode(strings.NewReader(data))
+	if err == nil {
+		t.Fatal("got nil error, want non-nil")
+	}
+	if got, want := err.Error(), "too much data"; !strings.Contains(got, want) {
+		t.Fatalf("got error %q, want something containing %q", got, want)
+	}
+}
+
+func benchmarkDecode(b *testing.B, filename string) {
+	data, err := ioutil.ReadFile("../testdata/blue-purple-pink-large." + filename + ".webp")
+	if err != nil {
+		b.Fatal(err)
+	}
+	s := string(data)
+	cfg, err := DecodeConfig(strings.NewReader(s))
+	if err != nil {
+		b.Fatal(err)
+	}
+	b.SetBytes(int64(cfg.Width * cfg.Height * 4))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Decode(strings.NewReader(s))
+	}
+}
+
+func BenchmarkDecodeVP8NoFilter(b *testing.B)     { benchmarkDecode(b, "no-filter.lossy") }
+func BenchmarkDecodeVP8SimpleFilter(b *testing.B) { benchmarkDecode(b, "simple-filter.lossy") }
+func BenchmarkDecodeVP8NormalFilter(b *testing.B) { benchmarkDecode(b, "normal-filter.lossy") }
+func BenchmarkDecodeVP8L(b *testing.B)            { benchmarkDecode(b, "lossless") }
diff --git a/vendor/golang.org/x/image/webp/nycbcra/nycbcra.go b/vendor/golang.org/x/image/webp/nycbcra/nycbcra.go
new file mode 100644
index 000000000..101c41fcf
--- /dev/null
+++ b/vendor/golang.org/x/image/webp/nycbcra/nycbcra.go
@@ -0,0 +1,194 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package nycbcra provides non-alpha-premultiplied Y'CbCr-with-alpha image and
+// color types.
+//
+// Deprecated: as of Go 1.6. Use the standard image and image/color packages
+// instead.
+package nycbcra // import "golang.org/x/image/webp/nycbcra"
+
+import (
+	"image"
+	"image/color"
+)
+
+func init() {
+	println("The golang.org/x/image/webp/nycbcra package is deprecated, as of Go 1.6. " +
+		"Use the standard image and image/color packages instead.")
+}
+
+// TODO: move this to the standard image and image/color packages, so that the
+// image/draw package can have fast-path code. Moving would rename:
+//	nycbcra.Color      to color.NYCbCrA
+//	nycbcra.ColorModel to color.NYCbCrAModel
+//	nycbcra.Image      to image.NYCbCrA
+
+// Color represents a non-alpha-premultiplied Y'CbCr-with-alpha color, having
+// 8 bits each for one luma, two chroma and one alpha component.
+type Color struct {
+	color.YCbCr
+	A uint8
+}
+
+func (c Color) RGBA() (r, g, b, a uint32) {
+	r8, g8, b8 := color.YCbCrToRGB(c.Y, c.Cb, c.Cr)
+	a = uint32(c.A) * 0x101
+	r = uint32(r8) * 0x101 * a / 0xffff
+	g = uint32(g8) * 0x101 * a / 0xffff
+	b = uint32(b8) * 0x101 * a / 0xffff
+	return
+}
+
+// ColorModel is the Model for non-alpha-premultiplied Y'CbCr-with-alpha colors.
+var ColorModel color.Model = color.ModelFunc(nYCbCrAModel)
+
+func nYCbCrAModel(c color.Color) color.Color {
+	switch c := c.(type) {
+	case Color:
+		return c
+	case color.YCbCr:
+		return Color{c, 0xff}
+	}
+	r, g, b, a := c.RGBA()
+
+	// Convert from alpha-premultiplied to non-alpha-premultiplied.
+	if a != 0 {
+		r = (r * 0xffff) / a
+		g = (g * 0xffff) / a
+		b = (b * 0xffff) / a
+	}
+
+	y, u, v := color.RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+	return Color{color.YCbCr{Y: y, Cb: u, Cr: v}, uint8(a >> 8)}
+}
+
+// Image is an in-memory image of non-alpha-premultiplied Y'CbCr-with-alpha
+// colors. A and AStride are analogous to the Y and YStride fields of the
+// embedded YCbCr.
+type Image struct {
+	image.YCbCr
+	A       []uint8
+	AStride int
+}
+
+func (p *Image) ColorModel() color.Model {
+	return ColorModel
+}
+
+func (p *Image) At(x, y int) color.Color {
+	return p.NYCbCrAAt(x, y)
+}
+
+func (p *Image) NYCbCrAAt(x, y int) Color {
+	if !(image.Point{X: x, Y: y}.In(p.Rect)) {
+		return Color{}
+	}
+	yi := p.YOffset(x, y)
+	ci := p.COffset(x, y)
+	ai := p.AOffset(x, y)
+	return Color{
+		color.YCbCr{
+			Y:  p.Y[yi],
+			Cb: p.Cb[ci],
+			Cr: p.Cr[ci],
+		},
+		p.A[ai],
+	}
+}
+
+// AOffset returns the index of the first element of A that corresponds to
+// the pixel at (x, y).
+func (p *Image) AOffset(x, y int) int {
+	return (y-p.Rect.Min.Y)*p.AStride + (x - p.Rect.Min.X)
+}
+
+// SubImage returns an image representing the portion of the image p visible
+// through r. The returned value shares pixels with the original image.
+func (p *Image) SubImage(r image.Rectangle) image.Image {
+	// TODO: share code with image.NewYCbCr when this type moves into the
+	// standard image package.
+	r = r.Intersect(p.Rect)
+	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
+	// either r1 or r2 if the intersection is empty. Without explicitly checking for
+	// this, the Pix[i:] expression below can panic.
+	if r.Empty() {
+		return &Image{
+			YCbCr: image.YCbCr{
+				SubsampleRatio: p.SubsampleRatio,
+			},
+		}
+	}
+	yi := p.YOffset(r.Min.X, r.Min.Y)
+	ci := p.COffset(r.Min.X, r.Min.Y)
+	ai := p.AOffset(r.Min.X, r.Min.Y)
+	return &Image{
+		YCbCr: image.YCbCr{
+			Y:              p.Y[yi:],
+			Cb:             p.Cb[ci:],
+			Cr:             p.Cr[ci:],
+			SubsampleRatio: p.SubsampleRatio,
+			YStride:        p.YStride,
+			CStride:        p.CStride,
+			Rect:           r,
+		},
+		A:       p.A[ai:],
+		AStride: p.AStride,
+	}
+}
+
+// Opaque scans the entire image and reports whether it is fully opaque.
+func (p *Image) Opaque() bool {
+	if p.Rect.Empty() {
+		return true
+	}
+	i0, i1 := 0, p.Rect.Dx()
+	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
+		for _, a := range p.A[i0:i1] {
+			if a != 0xff {
+				return false
+			}
+		}
+		i0 += p.AStride
+		i1 += p.AStride
+	}
+	return true
+}
+
+// New returns a new Image with the given bounds and subsample ratio.
+func New(r image.Rectangle, subsampleRatio image.YCbCrSubsampleRatio) *Image {
+	// TODO: share code with image.NewYCbCr when this type moves into the
+	// standard image package.
+	w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
+	switch subsampleRatio {
+	case image.YCbCrSubsampleRatio422:
+		cw = (r.Max.X+1)/2 - r.Min.X/2
+		ch = h
+	case image.YCbCrSubsampleRatio420:
+		cw = (r.Max.X+1)/2 - r.Min.X/2
+		ch = (r.Max.Y+1)/2 - r.Min.Y/2
+	case image.YCbCrSubsampleRatio440:
+		cw = w
+		ch = (r.Max.Y+1)/2 - r.Min.Y/2
+	default:
+		// Default to 4:4:4 subsampling.
+		cw = w
+		ch = h
+	}
+	b := make([]byte, 2*w*h+2*cw*ch)
+	// TODO: use s[i:j:k] notation to set the cap.
+	return &Image{
+		YCbCr: image.YCbCr{
+			Y:              b[:w*h],
+			Cb:             b[w*h+0*cw*ch : w*h+1*cw*ch],
+			Cr:             b[w*h+1*cw*ch : w*h+2*cw*ch],
+			SubsampleRatio: subsampleRatio,
+			YStride:        w,
+			CStride:        cw,
+			Rect:           r,
+		},
+		A:       b[w*h+2*cw*ch:],
+		AStride: w,
+	}
+}