Source: ycbcr.go in package image


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
	YCbCrSubsampleRatio411
	YCbCrSubsampleRatio410
)

func (s YCbCrSubsampleRatio) String() string {
	switch s {
	case YCbCrSubsampleRatio444:
		return "YCbCrSubsampleRatio444"
	case YCbCrSubsampleRatio422:
		return "YCbCrSubsampleRatio422"
	case YCbCrSubsampleRatio420:
		return "YCbCrSubsampleRatio420"
	case YCbCrSubsampleRatio440:
		return "YCbCrSubsampleRatio440"
	case YCbCrSubsampleRatio411:
		return "YCbCrSubsampleRatio411"
	case YCbCrSubsampleRatio410:
		return "YCbCrSubsampleRatio410"
	}
	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. For 4:1:1, CStride == YStride/4 && len(Cb) == len(Cr) == len(Y)/4. For 4:1:0, CStride == YStride/4 && len(Cb) == len(Cr) == len(Y)/8.

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 {
	return p.YCbCrAt(x, y)
}

func (p *YCbCr) YCbCrAt(x, y int) color.YCbCr {
	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)
	case YCbCrSubsampleRatio411:
		return (y-p.Rect.Min.Y)*p.CStride + (x/4 - p.Rect.Min.X/4)
	case YCbCrSubsampleRatio410:
		return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/4 - p.Rect.Min.X/4)

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 {

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
}

func yCbCrSize(r Rectangle, subsampleRatio YCbCrSubsampleRatio) (w, h, cw, ch int) {
	w, h = r.Dx(), r.Dy()
	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
	case YCbCrSubsampleRatio411:
		cw = (r.Max.X+3)/4 - r.Min.X/4
		ch = h
	case YCbCrSubsampleRatio410:
		cw = (r.Max.X+3)/4 - r.Min.X/4
		ch = (r.Max.Y+1)/2 - r.Min.Y/2

Default to 4:4:4 subsampling.

		cw = w
		ch = h
	}
	return
}

NewYCbCr returns a new YCbCr image with the given bounds and subsample ratio.

func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
	w, h, cw, ch := yCbCrSize(r, subsampleRatio)

totalLength should be the same as i2, below, for a valid Rectangle r.

	totalLength := add2NonNeg(
		mul3NonNeg(1, w, h),
		mul3NonNeg(2, cw, ch),
	)
	if totalLength < 0 {
		panic("image: NewYCbCr Rectangle has huge or negative dimensions")
	}

	i0 := w*h + 0*cw*ch
	i1 := w*h + 1*cw*ch
	i2 := w*h + 2*cw*ch
	b := make([]byte, i2)
	return &YCbCr{
		Y:              b[:i0:i0],
		Cb:             b[i0:i1:i1],
		Cr:             b[i1:i2:i2],
		SubsampleRatio: subsampleRatio,
		YStride:        w,
		CStride:        cw,
		Rect:           r,
	}
}

NYCbCrA 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 NYCbCrA struct {
	YCbCr
	A       []uint8
	AStride int
}

func (p *NYCbCrA) ColorModel() color.Model {
	return color.NYCbCrAModel
}

func (p *NYCbCrA) At(x, y int) color.Color {
	return p.NYCbCrAAt(x, y)
}

func (p *NYCbCrA) NYCbCrAAt(x, y int) color.NYCbCrA {
	if !(Point{X: x, Y: y}.In(p.Rect)) {
		return color.NYCbCrA{}
	}
	yi := p.YOffset(x, y)
	ci := p.COffset(x, y)
	ai := p.AOffset(x, y)
	return color.NYCbCrA{
		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 *NYCbCrA) 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 *NYCbCrA) SubImage(r Rectangle) Image {

	if r.Empty() {
		return &NYCbCrA{
			YCbCr: 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 &NYCbCrA{
		YCbCr: 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 *NYCbCrA) 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
}

NewNYCbCrA returns a new NYCbCrA image with the given bounds and subsample ratio.

func NewNYCbCrA(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *NYCbCrA {
	w, h, cw, ch := yCbCrSize(r, subsampleRatio)

totalLength should be the same as i3, below, for a valid Rectangle r.

	totalLength := add2NonNeg(
		mul3NonNeg(2, w, h),
		mul3NonNeg(2, cw, ch),
	)
	if totalLength < 0 {
		panic("image: NewNYCbCrA Rectangle has huge or negative dimension")
	}

	i0 := 1*w*h + 0*cw*ch
	i1 := 1*w*h + 1*cw*ch
	i2 := 1*w*h + 2*cw*ch
	i3 := 2*w*h + 2*cw*ch
	b := make([]byte, i3)
	return &NYCbCrA{
		YCbCr: YCbCr{
			Y:              b[:i0:i0],
			Cb:             b[i0:i1:i1],
			Cr:             b[i1:i2:i2],
			SubsampleRatio: subsampleRatio,
			YStride:        w,
			CStride:        cw,
			Rect:           r,
		},
		A:       b[i2:],
		AStride: w,
	}