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Source file src/pkg/image/ycbcr.go

     1	// Copyright 2011 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	package image
     6	
     7	import (
     8		"image/color"
     9	)
    10	
    11	// YCbCrSubsampleRatio is the chroma subsample ratio used in a YCbCr image.
    12	type YCbCrSubsampleRatio int
    13	
    14	const (
    15		YCbCrSubsampleRatio444 YCbCrSubsampleRatio = iota
    16		YCbCrSubsampleRatio422
    17		YCbCrSubsampleRatio420
    18		YCbCrSubsampleRatio440
    19	)
    20	
    21	func (s YCbCrSubsampleRatio) String() string {
    22		switch s {
    23		case YCbCrSubsampleRatio444:
    24			return "YCbCrSubsampleRatio444"
    25		case YCbCrSubsampleRatio422:
    26			return "YCbCrSubsampleRatio422"
    27		case YCbCrSubsampleRatio420:
    28			return "YCbCrSubsampleRatio420"
    29		case YCbCrSubsampleRatio440:
    30			return "YCbCrSubsampleRatio440"
    31		}
    32		return "YCbCrSubsampleRatioUnknown"
    33	}
    34	
    35	// YCbCr is an in-memory image of Y'CbCr colors. There is one Y sample per
    36	// pixel, but each Cb and Cr sample can span one or more pixels.
    37	// YStride is the Y slice index delta between vertically adjacent pixels.
    38	// CStride is the Cb and Cr slice index delta between vertically adjacent pixels
    39	// that map to separate chroma samples.
    40	// It is not an absolute requirement, but YStride and len(Y) are typically
    41	// multiples of 8, and:
    42	//	For 4:4:4, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/1.
    43	//	For 4:2:2, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/2.
    44	//	For 4:2:0, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/4.
    45	//	For 4:4:0, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/2.
    46	type YCbCr struct {
    47		Y, Cb, Cr      []uint8
    48		YStride        int
    49		CStride        int
    50		SubsampleRatio YCbCrSubsampleRatio
    51		Rect           Rectangle
    52	}
    53	
    54	func (p *YCbCr) ColorModel() color.Model {
    55		return color.YCbCrModel
    56	}
    57	
    58	func (p *YCbCr) Bounds() Rectangle {
    59		return p.Rect
    60	}
    61	
    62	func (p *YCbCr) At(x, y int) color.Color {
    63		if !(Point{x, y}.In(p.Rect)) {
    64			return color.YCbCr{}
    65		}
    66		yi := p.YOffset(x, y)
    67		ci := p.COffset(x, y)
    68		return color.YCbCr{
    69			p.Y[yi],
    70			p.Cb[ci],
    71			p.Cr[ci],
    72		}
    73	}
    74	
    75	// YOffset returns the index of the first element of Y that corresponds to
    76	// the pixel at (x, y).
    77	func (p *YCbCr) YOffset(x, y int) int {
    78		return (y-p.Rect.Min.Y)*p.YStride + (x - p.Rect.Min.X)
    79	}
    80	
    81	// COffset returns the index of the first element of Cb or Cr that corresponds
    82	// to the pixel at (x, y).
    83	func (p *YCbCr) COffset(x, y int) int {
    84		switch p.SubsampleRatio {
    85		case YCbCrSubsampleRatio422:
    86			return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
    87		case YCbCrSubsampleRatio420:
    88			return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
    89		case YCbCrSubsampleRatio440:
    90			return (y/2-p.Rect.Min.Y/2)*p.CStride + (x - p.Rect.Min.X)
    91		}
    92		// Default to 4:4:4 subsampling.
    93		return (y-p.Rect.Min.Y)*p.CStride + (x - p.Rect.Min.X)
    94	}
    95	
    96	// SubImage returns an image representing the portion of the image p visible
    97	// through r. The returned value shares pixels with the original image.
    98	func (p *YCbCr) SubImage(r Rectangle) Image {
    99		r = r.Intersect(p.Rect)
   100		// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
   101		// either r1 or r2 if the intersection is empty. Without explicitly checking for
   102		// this, the Pix[i:] expression below can panic.
   103		if r.Empty() {
   104			return &YCbCr{
   105				SubsampleRatio: p.SubsampleRatio,
   106			}
   107		}
   108		yi := p.YOffset(r.Min.X, r.Min.Y)
   109		ci := p.COffset(r.Min.X, r.Min.Y)
   110		return &YCbCr{
   111			Y:              p.Y[yi:],
   112			Cb:             p.Cb[ci:],
   113			Cr:             p.Cr[ci:],
   114			SubsampleRatio: p.SubsampleRatio,
   115			YStride:        p.YStride,
   116			CStride:        p.CStride,
   117			Rect:           r,
   118		}
   119	}
   120	
   121	func (p *YCbCr) Opaque() bool {
   122		return true
   123	}
   124	
   125	// NewYCbCr returns a new YCbCr with the given bounds and subsample ratio.
   126	func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
   127		w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
   128		switch subsampleRatio {
   129		case YCbCrSubsampleRatio422:
   130			cw = (r.Max.X+1)/2 - r.Min.X/2
   131			ch = h
   132		case YCbCrSubsampleRatio420:
   133			cw = (r.Max.X+1)/2 - r.Min.X/2
   134			ch = (r.Max.Y+1)/2 - r.Min.Y/2
   135		case YCbCrSubsampleRatio440:
   136			cw = w
   137			ch = (r.Max.Y+1)/2 - r.Min.Y/2
   138		default:
   139			// Default to 4:4:4 subsampling.
   140			cw = w
   141			ch = h
   142		}
   143		b := make([]byte, w*h+2*cw*ch)
   144		return &YCbCr{
   145			Y:              b[:w*h],
   146			Cb:             b[w*h+0*cw*ch : w*h+1*cw*ch],
   147			Cr:             b[w*h+1*cw*ch : w*h+2*cw*ch],
   148			SubsampleRatio: subsampleRatio,
   149			YStride:        w,
   150			CStride:        cw,
   151			Rect:           r,
   152		}
   153	}
   154	

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