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Source file src/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		YCbCrSubsampleRatio411
    20		YCbCrSubsampleRatio410
    21	)
    22	
    23	func (s YCbCrSubsampleRatio) String() string {
    24		switch s {
    25		case YCbCrSubsampleRatio444:
    26			return "YCbCrSubsampleRatio444"
    27		case YCbCrSubsampleRatio422:
    28			return "YCbCrSubsampleRatio422"
    29		case YCbCrSubsampleRatio420:
    30			return "YCbCrSubsampleRatio420"
    31		case YCbCrSubsampleRatio440:
    32			return "YCbCrSubsampleRatio440"
    33		case YCbCrSubsampleRatio411:
    34			return "YCbCrSubsampleRatio411"
    35		case YCbCrSubsampleRatio410:
    36			return "YCbCrSubsampleRatio410"
    37		}
    38		return "YCbCrSubsampleRatioUnknown"
    39	}
    40	
    41	// YCbCr is an in-memory image of Y'CbCr colors. There is one Y sample per
    42	// pixel, but each Cb and Cr sample can span one or more pixels.
    43	// YStride is the Y slice index delta between vertically adjacent pixels.
    44	// CStride is the Cb and Cr slice index delta between vertically adjacent pixels
    45	// that map to separate chroma samples.
    46	// It is not an absolute requirement, but YStride and len(Y) are typically
    47	// multiples of 8, and:
    48	//	For 4:4:4, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/1.
    49	//	For 4:2:2, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/2.
    50	//	For 4:2:0, CStride == YStride/2 && len(Cb) == len(Cr) == len(Y)/4.
    51	//	For 4:4:0, CStride == YStride/1 && len(Cb) == len(Cr) == len(Y)/2.
    52	//	For 4:1:1, CStride == YStride/4 && len(Cb) == len(Cr) == len(Y)/4.
    53	//	For 4:1:0, CStride == YStride/4 && len(Cb) == len(Cr) == len(Y)/8.
    54	type YCbCr struct {
    55		Y, Cb, Cr      []uint8
    56		YStride        int
    57		CStride        int
    58		SubsampleRatio YCbCrSubsampleRatio
    59		Rect           Rectangle
    60	}
    61	
    62	func (p *YCbCr) ColorModel() color.Model {
    63		return color.YCbCrModel
    64	}
    65	
    66	func (p *YCbCr) Bounds() Rectangle {
    67		return p.Rect
    68	}
    69	
    70	func (p *YCbCr) At(x, y int) color.Color {
    71		return p.YCbCrAt(x, y)
    72	}
    73	
    74	func (p *YCbCr) YCbCrAt(x, y int) color.YCbCr {
    75		if !(Point{x, y}.In(p.Rect)) {
    76			return color.YCbCr{}
    77		}
    78		yi := p.YOffset(x, y)
    79		ci := p.COffset(x, y)
    80		return color.YCbCr{
    81			p.Y[yi],
    82			p.Cb[ci],
    83			p.Cr[ci],
    84		}
    85	}
    86	
    87	// YOffset returns the index of the first element of Y that corresponds to
    88	// the pixel at (x, y).
    89	func (p *YCbCr) YOffset(x, y int) int {
    90		return (y-p.Rect.Min.Y)*p.YStride + (x - p.Rect.Min.X)
    91	}
    92	
    93	// COffset returns the index of the first element of Cb or Cr that corresponds
    94	// to the pixel at (x, y).
    95	func (p *YCbCr) COffset(x, y int) int {
    96		switch p.SubsampleRatio {
    97		case YCbCrSubsampleRatio422:
    98			return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
    99		case YCbCrSubsampleRatio420:
   100			return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
   101		case YCbCrSubsampleRatio440:
   102			return (y/2-p.Rect.Min.Y/2)*p.CStride + (x - p.Rect.Min.X)
   103		case YCbCrSubsampleRatio411:
   104			return (y-p.Rect.Min.Y)*p.CStride + (x/4 - p.Rect.Min.X/4)
   105		case YCbCrSubsampleRatio410:
   106			return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/4 - p.Rect.Min.X/4)
   107		}
   108		// Default to 4:4:4 subsampling.
   109		return (y-p.Rect.Min.Y)*p.CStride + (x - p.Rect.Min.X)
   110	}
   111	
   112	// SubImage returns an image representing the portion of the image p visible
   113	// through r. The returned value shares pixels with the original image.
   114	func (p *YCbCr) SubImage(r Rectangle) Image {
   115		r = r.Intersect(p.Rect)
   116		// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
   117		// either r1 or r2 if the intersection is empty. Without explicitly checking for
   118		// this, the Pix[i:] expression below can panic.
   119		if r.Empty() {
   120			return &YCbCr{
   121				SubsampleRatio: p.SubsampleRatio,
   122			}
   123		}
   124		yi := p.YOffset(r.Min.X, r.Min.Y)
   125		ci := p.COffset(r.Min.X, r.Min.Y)
   126		return &YCbCr{
   127			Y:              p.Y[yi:],
   128			Cb:             p.Cb[ci:],
   129			Cr:             p.Cr[ci:],
   130			SubsampleRatio: p.SubsampleRatio,
   131			YStride:        p.YStride,
   132			CStride:        p.CStride,
   133			Rect:           r,
   134		}
   135	}
   136	
   137	func (p *YCbCr) Opaque() bool {
   138		return true
   139	}
   140	
   141	// NewYCbCr returns a new YCbCr with the given bounds and subsample ratio.
   142	func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
   143		w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
   144		switch subsampleRatio {
   145		case YCbCrSubsampleRatio422:
   146			cw = (r.Max.X+1)/2 - r.Min.X/2
   147			ch = h
   148		case YCbCrSubsampleRatio420:
   149			cw = (r.Max.X+1)/2 - r.Min.X/2
   150			ch = (r.Max.Y+1)/2 - r.Min.Y/2
   151		case YCbCrSubsampleRatio440:
   152			cw = w
   153			ch = (r.Max.Y+1)/2 - r.Min.Y/2
   154		case YCbCrSubsampleRatio411:
   155			cw = (r.Max.X+3)/4 - r.Min.X/4
   156			ch = h
   157		case YCbCrSubsampleRatio410:
   158			cw = (r.Max.X+3)/4 - r.Min.X/4
   159			ch = (r.Max.Y+1)/2 - r.Min.Y/2
   160		default:
   161			// Default to 4:4:4 subsampling.
   162			cw = w
   163			ch = h
   164		}
   165		i0 := w*h + 0*cw*ch
   166		i1 := w*h + 1*cw*ch
   167		i2 := w*h + 2*cw*ch
   168		b := make([]byte, i2)
   169		return &YCbCr{
   170			Y:              b[:i0:i0],
   171			Cb:             b[i0:i1:i1],
   172			Cr:             b[i1:i2:i2],
   173			SubsampleRatio: subsampleRatio,
   174			YStride:        w,
   175			CStride:        cw,
   176			Rect:           r,
   177		}
   178	}
   179	

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