src/pkg/image/ycbcr.go

// 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
)

func (s YCbCrSubsampleRatio) String() string {
	switch s {
	case YCbCrSubsampleRatio444:
		return "YCbCrSubsampleRatio444"
	case YCbCrSubsampleRatio422:
		return "YCbCrSubsampleRatio422"
	case YCbCrSubsampleRatio420:
		return "YCbCrSubsampleRatio420"
	}
	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.
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)
	}
	// 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
	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,
	}
}