go.image/webp/nycbcra: new package.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/147500043
This commit is contained in:
Nigel Tao 2014-10-08 14:30:35 +11:00
parent 8d4fc1653b
commit d354b8dda9

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webp/nycbcra/nycbcra.go Normal file
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// 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,
}
}