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// 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 image
import (
"image/color"
)
// YCbCrSubsampleRatio is the chroma subsample ratio used in a YCbCr image.
type YCbCrSubsampleRatio int
const (
YCbCrSubsampleRatio444 YCbCrSubsampleRatio = iota
YCbCrSubsampleRatio422
YCbCrSubsampleRatio420
YCbCrSubsampleRatio440
)
func (s YCbCrSubsampleRatio) String() string {
switch s {
case YCbCrSubsampleRatio444:
return "YCbCrSubsampleRatio444"
case YCbCrSubsampleRatio422:
return "YCbCrSubsampleRatio422"
case YCbCrSubsampleRatio420:
return "YCbCrSubsampleRatio420"
case YCbCrSubsampleRatio440:
return "YCbCrSubsampleRatio440"
}
return "YCbCrSubsampleRatioUnknown"
}
// YCbCr is an in-memory image of Y'CbCr colors. There is one Y sample per
// pixel, but each Cb and Cr sample can span one or more pixels.
// YStride is the Y slice index delta between vertically adjacent pixels.
// CStride is the Cb and Cr slice index delta between vertically adjacent pixels
// that map to separate chroma samples.
// It is not an absolute requirement, but YStride and len(Y) are typically
// multiples of 8, and:
// 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
CStride int
SubsampleRatio YCbCrSubsampleRatio
Rect Rectangle
}
func (p *YCbCr) ColorModel() color.Model {
return color.YCbCrModel
}
func (p *YCbCr) Bounds() Rectangle {
return p.Rect
}
func (p *YCbCr) At(x, y int) color.Color {
if !(Point{x, y}.In(p.Rect)) {
return color.YCbCr{}
}
yi := p.YOffset(x, y)
ci := p.COffset(x, y)
return color.YCbCr{
p.Y[yi],
p.Cb[ci],
p.Cr[ci],
}
}
// YOffset returns the index of the first element of Y that corresponds to
// the pixel at (x, y).
func (p *YCbCr) YOffset(x, y int) int {
return (y-p.Rect.Min.Y)*p.YStride + (x - p.Rect.Min.X)
}
// COffset returns the index of the first element of Cb or Cr that corresponds
// to the pixel at (x, y).
func (p *YCbCr) COffset(x, y int) int {
switch p.SubsampleRatio {
case YCbCrSubsampleRatio422:
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)
}
// 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 *YCbCr) SubImage(r Rectangle) Image {
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 &YCbCr{
SubsampleRatio: p.SubsampleRatio,
}
}
yi := p.YOffset(r.Min.X, r.Min.Y)
ci := p.COffset(r.Min.X, r.Min.Y)
return &YCbCr{
Y: p.Y[yi:],
Cb: p.Cb[ci:],
Cr: p.Cr[ci:],
SubsampleRatio: p.SubsampleRatio,
YStride: p.YStride,
CStride: p.CStride,
Rect: r,
}
}
func (p *YCbCr) Opaque() bool {
return true
}
// NewYCbCr returns a new YCbCr with the given bounds and subsample ratio.
func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
switch subsampleRatio {
case YCbCrSubsampleRatio422:
cw = (r.Max.X+1)/2 - r.Min.X/2
ch = h
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
ch = h
}
b := make([]byte, w*h+2*cw*ch)
return &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,
}
}
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