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