go.image/webp/nycbcra: new package.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/147500043

webp/nycbcra/nycbcra.go

@@ -0,0 +1,186 @@
+// 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.
+package nycbcra
+
+import (
+	"image"
+	"image/color"
+)
+
+// 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,
+	}
+}