Source file src/image/ycbcr.go

Documentation: image

     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  func yCbCrSize(r Rectangle, subsampleRatio YCbCrSubsampleRatio) (w, h, cw, ch int) {
   142  	w, h = r.Dx(), r.Dy()
   143  	switch subsampleRatio {
   144  	case YCbCrSubsampleRatio422:
   145  		cw = (r.Max.X+1)/2 - r.Min.X/2
   146  		ch = h
   147  	case YCbCrSubsampleRatio420:
   148  		cw = (r.Max.X+1)/2 - r.Min.X/2
   149  		ch = (r.Max.Y+1)/2 - r.Min.Y/2
   150  	case YCbCrSubsampleRatio440:
   151  		cw = w
   152  		ch = (r.Max.Y+1)/2 - r.Min.Y/2
   153  	case YCbCrSubsampleRatio411:
   154  		cw = (r.Max.X+3)/4 - r.Min.X/4
   155  		ch = h
   156  	case YCbCrSubsampleRatio410:
   157  		cw = (r.Max.X+3)/4 - r.Min.X/4
   158  		ch = (r.Max.Y+1)/2 - r.Min.Y/2
   159  	default:
   160  		// Default to 4:4:4 subsampling.
   161  		cw = w
   162  		ch = h
   163  	}
   164  	return
   165  }
   166  
   167  // NewYCbCr returns a new YCbCr image with the given bounds and subsample
   168  // ratio.
   169  func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
   170  	w, h, cw, ch := yCbCrSize(r, subsampleRatio)
   171  	i0 := w*h + 0*cw*ch
   172  	i1 := w*h + 1*cw*ch
   173  	i2 := w*h + 2*cw*ch
   174  	b := make([]byte, i2)
   175  	return &YCbCr{
   176  		Y:              b[:i0:i0],
   177  		Cb:             b[i0:i1:i1],
   178  		Cr:             b[i1:i2:i2],
   179  		SubsampleRatio: subsampleRatio,
   180  		YStride:        w,
   181  		CStride:        cw,
   182  		Rect:           r,
   183  	}
   184  }
   185  
   186  // NYCbCrA is an in-memory image of non-alpha-premultiplied Y'CbCr-with-alpha
   187  // colors. A and AStride are analogous to the Y and YStride fields of the
   188  // embedded YCbCr.
   189  type NYCbCrA struct {
   190  	YCbCr
   191  	A       []uint8
   192  	AStride int
   193  }
   194  
   195  func (p *NYCbCrA) ColorModel() color.Model {
   196  	return color.NYCbCrAModel
   197  }
   198  
   199  func (p *NYCbCrA) At(x, y int) color.Color {
   200  	return p.NYCbCrAAt(x, y)
   201  }
   202  
   203  func (p *NYCbCrA) NYCbCrAAt(x, y int) color.NYCbCrA {
   204  	if !(Point{X: x, Y: y}.In(p.Rect)) {
   205  		return color.NYCbCrA{}
   206  	}
   207  	yi := p.YOffset(x, y)
   208  	ci := p.COffset(x, y)
   209  	ai := p.AOffset(x, y)
   210  	return color.NYCbCrA{
   211  		color.YCbCr{
   212  			Y:  p.Y[yi],
   213  			Cb: p.Cb[ci],
   214  			Cr: p.Cr[ci],
   215  		},
   216  		p.A[ai],
   217  	}
   218  }
   219  
   220  // AOffset returns the index of the first element of A that corresponds to the
   221  // pixel at (x, y).
   222  func (p *NYCbCrA) AOffset(x, y int) int {
   223  	return (y-p.Rect.Min.Y)*p.AStride + (x - p.Rect.Min.X)
   224  }
   225  
   226  // SubImage returns an image representing the portion of the image p visible
   227  // through r. The returned value shares pixels with the original image.
   228  func (p *NYCbCrA) SubImage(r Rectangle) Image {
   229  	r = r.Intersect(p.Rect)
   230  	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
   231  	// either r1 or r2 if the intersection is empty. Without explicitly checking for
   232  	// this, the Pix[i:] expression below can panic.
   233  	if r.Empty() {
   234  		return &NYCbCrA{
   235  			YCbCr: YCbCr{
   236  				SubsampleRatio: p.SubsampleRatio,
   237  			},
   238  		}
   239  	}
   240  	yi := p.YOffset(r.Min.X, r.Min.Y)
   241  	ci := p.COffset(r.Min.X, r.Min.Y)
   242  	ai := p.AOffset(r.Min.X, r.Min.Y)
   243  	return &NYCbCrA{
   244  		YCbCr: YCbCr{
   245  			Y:              p.Y[yi:],
   246  			Cb:             p.Cb[ci:],
   247  			Cr:             p.Cr[ci:],
   248  			SubsampleRatio: p.SubsampleRatio,
   249  			YStride:        p.YStride,
   250  			CStride:        p.CStride,
   251  			Rect:           r,
   252  		},
   253  		A:       p.A[ai:],
   254  		AStride: p.AStride,
   255  	}
   256  }
   257  
   258  // Opaque scans the entire image and reports whether it is fully opaque.
   259  func (p *NYCbCrA) Opaque() bool {
   260  	if p.Rect.Empty() {
   261  		return true
   262  	}
   263  	i0, i1 := 0, p.Rect.Dx()
   264  	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
   265  		for _, a := range p.A[i0:i1] {
   266  			if a != 0xff {
   267  				return false
   268  			}
   269  		}
   270  		i0 += p.AStride
   271  		i1 += p.AStride
   272  	}
   273  	return true
   274  }
   275  
   276  // NewNYCbCrA returns a new NYCbCrA image with the given bounds and subsample
   277  // ratio.
   278  func NewNYCbCrA(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *NYCbCrA {
   279  	w, h, cw, ch := yCbCrSize(r, subsampleRatio)
   280  	i0 := 1*w*h + 0*cw*ch
   281  	i1 := 1*w*h + 1*cw*ch
   282  	i2 := 1*w*h + 2*cw*ch
   283  	i3 := 2*w*h + 2*cw*ch
   284  	b := make([]byte, i3)
   285  	return &NYCbCrA{
   286  		YCbCr: YCbCr{
   287  			Y:              b[:i0:i0],
   288  			Cb:             b[i0:i1:i1],
   289  			Cr:             b[i1:i2:i2],
   290  			SubsampleRatio: subsampleRatio,
   291  			YStride:        w,
   292  			CStride:        cw,
   293  			Rect:           r,
   294  		},
   295  		A:       b[i2:],
   296  		AStride: w,
   297  	}
   298  }
   299  

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