github.com/bgentry/go@v0.0.0-20150121062915-6cf5a733d54d/src/image/ycbcr.go (about)

     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  	return p.YCbCrAt(x, y)
    64  }
    65  
    66  func (p *YCbCr) YCbCrAt(x, y int) color.YCbCr {
    67  	if !(Point{x, y}.In(p.Rect)) {
    68  		return color.YCbCr{}
    69  	}
    70  	yi := p.YOffset(x, y)
    71  	ci := p.COffset(x, y)
    72  	return color.YCbCr{
    73  		p.Y[yi],
    74  		p.Cb[ci],
    75  		p.Cr[ci],
    76  	}
    77  }
    78  
    79  // YOffset returns the index of the first element of Y that corresponds to
    80  // the pixel at (x, y).
    81  func (p *YCbCr) YOffset(x, y int) int {
    82  	return (y-p.Rect.Min.Y)*p.YStride + (x - p.Rect.Min.X)
    83  }
    84  
    85  // COffset returns the index of the first element of Cb or Cr that corresponds
    86  // to the pixel at (x, y).
    87  func (p *YCbCr) COffset(x, y int) int {
    88  	switch p.SubsampleRatio {
    89  	case YCbCrSubsampleRatio422:
    90  		return (y-p.Rect.Min.Y)*p.CStride + (x/2 - p.Rect.Min.X/2)
    91  	case YCbCrSubsampleRatio420:
    92  		return (y/2-p.Rect.Min.Y/2)*p.CStride + (x/2 - p.Rect.Min.X/2)
    93  	case YCbCrSubsampleRatio440:
    94  		return (y/2-p.Rect.Min.Y/2)*p.CStride + (x - p.Rect.Min.X)
    95  	}
    96  	// Default to 4:4:4 subsampling.
    97  	return (y-p.Rect.Min.Y)*p.CStride + (x - p.Rect.Min.X)
    98  }
    99  
   100  // SubImage returns an image representing the portion of the image p visible
   101  // through r. The returned value shares pixels with the original image.
   102  func (p *YCbCr) SubImage(r Rectangle) Image {
   103  	r = r.Intersect(p.Rect)
   104  	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
   105  	// either r1 or r2 if the intersection is empty. Without explicitly checking for
   106  	// this, the Pix[i:] expression below can panic.
   107  	if r.Empty() {
   108  		return &YCbCr{
   109  			SubsampleRatio: p.SubsampleRatio,
   110  		}
   111  	}
   112  	yi := p.YOffset(r.Min.X, r.Min.Y)
   113  	ci := p.COffset(r.Min.X, r.Min.Y)
   114  	return &YCbCr{
   115  		Y:              p.Y[yi:],
   116  		Cb:             p.Cb[ci:],
   117  		Cr:             p.Cr[ci:],
   118  		SubsampleRatio: p.SubsampleRatio,
   119  		YStride:        p.YStride,
   120  		CStride:        p.CStride,
   121  		Rect:           r,
   122  	}
   123  }
   124  
   125  func (p *YCbCr) Opaque() bool {
   126  	return true
   127  }
   128  
   129  // NewYCbCr returns a new YCbCr with the given bounds and subsample ratio.
   130  func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
   131  	w, h, cw, ch := r.Dx(), r.Dy(), 0, 0
   132  	switch subsampleRatio {
   133  	case YCbCrSubsampleRatio422:
   134  		cw = (r.Max.X+1)/2 - r.Min.X/2
   135  		ch = h
   136  	case YCbCrSubsampleRatio420:
   137  		cw = (r.Max.X+1)/2 - r.Min.X/2
   138  		ch = (r.Max.Y+1)/2 - r.Min.Y/2
   139  	case YCbCrSubsampleRatio440:
   140  		cw = w
   141  		ch = (r.Max.Y+1)/2 - r.Min.Y/2
   142  	default:
   143  		// Default to 4:4:4 subsampling.
   144  		cw = w
   145  		ch = h
   146  	}
   147  	i0 := w*h + 0*cw*ch
   148  	i1 := w*h + 1*cw*ch
   149  	i2 := w*h + 2*cw*ch
   150  	b := make([]byte, i2)
   151  	return &YCbCr{
   152  		Y:              b[:i0:i0],
   153  		Cb:             b[i0:i1:i1],
   154  		Cr:             b[i1:i2:i2],
   155  		SubsampleRatio: subsampleRatio,
   156  		YStride:        w,
   157  		CStride:        cw,
   158  		Rect:           r,
   159  	}
   160  }