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Source file src/image/draw/draw.go

     1	// Copyright 2009 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 draw provides image composition functions.
     6	//
     7	// See "The Go image/draw package" for an introduction to this package:
     8	// https://golang.org/doc/articles/image_draw.html
     9	package draw
    10	
    11	import (
    12		"image"
    13		"image/color"
    14		"image/internal/imageutil"
    15	)
    16	
    17	// m is the maximum color value returned by image.Color.RGBA.
    18	const m = 1<<16 - 1
    19	
    20	// Image is an image.Image with a Set method to change a single pixel.
    21	type Image interface {
    22		image.Image
    23		Set(x, y int, c color.Color)
    24	}
    25	
    26	// Quantizer produces a palette for an image.
    27	type Quantizer interface {
    28		// Quantize appends up to cap(p) - len(p) colors to p and returns the
    29		// updated palette suitable for converting m to a paletted image.
    30		Quantize(p color.Palette, m image.Image) color.Palette
    31	}
    32	
    33	// Op is a Porter-Duff compositing operator.
    34	type Op int
    35	
    36	const (
    37		// Over specifies ``(src in mask) over dst''.
    38		Over Op = iota
    39		// Src specifies ``src in mask''.
    40		Src
    41	)
    42	
    43	// Draw implements the Drawer interface by calling the Draw function with this
    44	// Op.
    45	func (op Op) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
    46		DrawMask(dst, r, src, sp, nil, image.Point{}, op)
    47	}
    48	
    49	// Drawer contains the Draw method.
    50	type Drawer interface {
    51		// Draw aligns r.Min in dst with sp in src and then replaces the
    52		// rectangle r in dst with the result of drawing src on dst.
    53		Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point)
    54	}
    55	
    56	// FloydSteinberg is a Drawer that is the Src Op with Floyd-Steinberg error
    57	// diffusion.
    58	var FloydSteinberg Drawer = floydSteinberg{}
    59	
    60	type floydSteinberg struct{}
    61	
    62	func (floydSteinberg) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
    63		clip(dst, &r, src, &sp, nil, nil)
    64		if r.Empty() {
    65			return
    66		}
    67		drawPaletted(dst, r, src, sp, true)
    68	}
    69	
    70	// clip clips r against each image's bounds (after translating into the
    71	// destination image's coordinate space) and shifts the points sp and mp by
    72	// the same amount as the change in r.Min.
    73	func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask image.Image, mp *image.Point) {
    74		orig := r.Min
    75		*r = r.Intersect(dst.Bounds())
    76		*r = r.Intersect(src.Bounds().Add(orig.Sub(*sp)))
    77		if mask != nil {
    78			*r = r.Intersect(mask.Bounds().Add(orig.Sub(*mp)))
    79		}
    80		dx := r.Min.X - orig.X
    81		dy := r.Min.Y - orig.Y
    82		if dx == 0 && dy == 0 {
    83			return
    84		}
    85		sp.X += dx
    86		sp.Y += dy
    87		if mp != nil {
    88			mp.X += dx
    89			mp.Y += dy
    90		}
    91	}
    92	
    93	func processBackward(dst Image, r image.Rectangle, src image.Image, sp image.Point) bool {
    94		return image.Image(dst) == src &&
    95			r.Overlaps(r.Add(sp.Sub(r.Min))) &&
    96			(sp.Y < r.Min.Y || (sp.Y == r.Min.Y && sp.X < r.Min.X))
    97	}
    98	
    99	// Draw calls DrawMask with a nil mask.
   100	func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point, op Op) {
   101		DrawMask(dst, r, src, sp, nil, image.Point{}, op)
   102	}
   103	
   104	// DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r
   105	// in dst with the result of a Porter-Duff composition. A nil mask is treated as opaque.
   106	func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
   107		clip(dst, &r, src, &sp, mask, &mp)
   108		if r.Empty() {
   109			return
   110		}
   111	
   112		// Fast paths for special cases. If none of them apply, then we fall back to a general but slow implementation.
   113		switch dst0 := dst.(type) {
   114		case *image.RGBA:
   115			if op == Over {
   116				if mask == nil {
   117					switch src0 := src.(type) {
   118					case *image.Uniform:
   119						sr, sg, sb, sa := src0.RGBA()
   120						if sa == 0xffff {
   121							drawFillSrc(dst0, r, sr, sg, sb, sa)
   122						} else {
   123							drawFillOver(dst0, r, sr, sg, sb, sa)
   124						}
   125						return
   126					case *image.RGBA:
   127						drawCopyOver(dst0, r, src0, sp)
   128						return
   129					case *image.NRGBA:
   130						drawNRGBAOver(dst0, r, src0, sp)
   131						return
   132					case *image.YCbCr:
   133						// An image.YCbCr is always fully opaque, and so if the
   134						// mask is nil (i.e. fully opaque) then the op is
   135						// effectively always Src. Similarly for image.Gray and
   136						// image.CMYK.
   137						if imageutil.DrawYCbCr(dst0, r, src0, sp) {
   138							return
   139						}
   140					case *image.Gray:
   141						drawGray(dst0, r, src0, sp)
   142						return
   143					case *image.CMYK:
   144						drawCMYK(dst0, r, src0, sp)
   145						return
   146					}
   147				} else if mask0, ok := mask.(*image.Alpha); ok {
   148					switch src0 := src.(type) {
   149					case *image.Uniform:
   150						drawGlyphOver(dst0, r, src0, mask0, mp)
   151						return
   152					}
   153				}
   154			} else {
   155				if mask == nil {
   156					switch src0 := src.(type) {
   157					case *image.Uniform:
   158						sr, sg, sb, sa := src0.RGBA()
   159						drawFillSrc(dst0, r, sr, sg, sb, sa)
   160						return
   161					case *image.RGBA:
   162						drawCopySrc(dst0, r, src0, sp)
   163						return
   164					case *image.NRGBA:
   165						drawNRGBASrc(dst0, r, src0, sp)
   166						return
   167					case *image.YCbCr:
   168						if imageutil.DrawYCbCr(dst0, r, src0, sp) {
   169							return
   170						}
   171					case *image.Gray:
   172						drawGray(dst0, r, src0, sp)
   173						return
   174					case *image.CMYK:
   175						drawCMYK(dst0, r, src0, sp)
   176						return
   177					}
   178				}
   179			}
   180			drawRGBA(dst0, r, src, sp, mask, mp, op)
   181			return
   182		case *image.Paletted:
   183			if op == Src && mask == nil && !processBackward(dst, r, src, sp) {
   184				drawPaletted(dst0, r, src, sp, false)
   185				return
   186			}
   187		}
   188	
   189		x0, x1, dx := r.Min.X, r.Max.X, 1
   190		y0, y1, dy := r.Min.Y, r.Max.Y, 1
   191		if processBackward(dst, r, src, sp) {
   192			x0, x1, dx = x1-1, x0-1, -1
   193			y0, y1, dy = y1-1, y0-1, -1
   194		}
   195	
   196		var out color.RGBA64
   197		sy := sp.Y + y0 - r.Min.Y
   198		my := mp.Y + y0 - r.Min.Y
   199		for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
   200			sx := sp.X + x0 - r.Min.X
   201			mx := mp.X + x0 - r.Min.X
   202			for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
   203				ma := uint32(m)
   204				if mask != nil {
   205					_, _, _, ma = mask.At(mx, my).RGBA()
   206				}
   207				switch {
   208				case ma == 0:
   209					if op == Over {
   210						// No-op.
   211					} else {
   212						dst.Set(x, y, color.Transparent)
   213					}
   214				case ma == m && op == Src:
   215					dst.Set(x, y, src.At(sx, sy))
   216				default:
   217					sr, sg, sb, sa := src.At(sx, sy).RGBA()
   218					if op == Over {
   219						dr, dg, db, da := dst.At(x, y).RGBA()
   220						a := m - (sa * ma / m)
   221						out.R = uint16((dr*a + sr*ma) / m)
   222						out.G = uint16((dg*a + sg*ma) / m)
   223						out.B = uint16((db*a + sb*ma) / m)
   224						out.A = uint16((da*a + sa*ma) / m)
   225					} else {
   226						out.R = uint16(sr * ma / m)
   227						out.G = uint16(sg * ma / m)
   228						out.B = uint16(sb * ma / m)
   229						out.A = uint16(sa * ma / m)
   230					}
   231					// The third argument is &out instead of out (and out is
   232					// declared outside of the inner loop) to avoid the implicit
   233					// conversion to color.Color here allocating memory in the
   234					// inner loop if sizeof(color.RGBA64) > sizeof(uintptr).
   235					dst.Set(x, y, &out)
   236				}
   237			}
   238		}
   239	}
   240	
   241	func drawFillOver(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
   242		// The 0x101 is here for the same reason as in drawRGBA.
   243		a := (m - sa) * 0x101
   244		i0 := dst.PixOffset(r.Min.X, r.Min.Y)
   245		i1 := i0 + r.Dx()*4
   246		for y := r.Min.Y; y != r.Max.Y; y++ {
   247			for i := i0; i < i1; i += 4 {
   248				dr := &dst.Pix[i+0]
   249				dg := &dst.Pix[i+1]
   250				db := &dst.Pix[i+2]
   251				da := &dst.Pix[i+3]
   252	
   253				*dr = uint8((uint32(*dr)*a/m + sr) >> 8)
   254				*dg = uint8((uint32(*dg)*a/m + sg) >> 8)
   255				*db = uint8((uint32(*db)*a/m + sb) >> 8)
   256				*da = uint8((uint32(*da)*a/m + sa) >> 8)
   257			}
   258			i0 += dst.Stride
   259			i1 += dst.Stride
   260		}
   261	}
   262	
   263	func drawFillSrc(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
   264		sr8 := uint8(sr >> 8)
   265		sg8 := uint8(sg >> 8)
   266		sb8 := uint8(sb >> 8)
   267		sa8 := uint8(sa >> 8)
   268		// The built-in copy function is faster than a straightforward for loop to fill the destination with
   269		// the color, but copy requires a slice source. We therefore use a for loop to fill the first row, and
   270		// then use the first row as the slice source for the remaining rows.
   271		i0 := dst.PixOffset(r.Min.X, r.Min.Y)
   272		i1 := i0 + r.Dx()*4
   273		for i := i0; i < i1; i += 4 {
   274			dst.Pix[i+0] = sr8
   275			dst.Pix[i+1] = sg8
   276			dst.Pix[i+2] = sb8
   277			dst.Pix[i+3] = sa8
   278		}
   279		firstRow := dst.Pix[i0:i1]
   280		for y := r.Min.Y + 1; y < r.Max.Y; y++ {
   281			i0 += dst.Stride
   282			i1 += dst.Stride
   283			copy(dst.Pix[i0:i1], firstRow)
   284		}
   285	}
   286	
   287	func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
   288		dx, dy := r.Dx(), r.Dy()
   289		d0 := dst.PixOffset(r.Min.X, r.Min.Y)
   290		s0 := src.PixOffset(sp.X, sp.Y)
   291		var (
   292			ddelta, sdelta int
   293			i0, i1, idelta int
   294		)
   295		if r.Min.Y < sp.Y || r.Min.Y == sp.Y && r.Min.X <= sp.X {
   296			ddelta = dst.Stride
   297			sdelta = src.Stride
   298			i0, i1, idelta = 0, dx*4, +4
   299		} else {
   300			// If the source start point is higher than the destination start point, or equal height but to the left,
   301			// then we compose the rows in right-to-left, bottom-up order instead of left-to-right, top-down.
   302			d0 += (dy - 1) * dst.Stride
   303			s0 += (dy - 1) * src.Stride
   304			ddelta = -dst.Stride
   305			sdelta = -src.Stride
   306			i0, i1, idelta = (dx-1)*4, -4, -4
   307		}
   308		for ; dy > 0; dy-- {
   309			dpix := dst.Pix[d0:]
   310			spix := src.Pix[s0:]
   311			for i := i0; i != i1; i += idelta {
   312				sr := uint32(spix[i+0]) * 0x101
   313				sg := uint32(spix[i+1]) * 0x101
   314				sb := uint32(spix[i+2]) * 0x101
   315				sa := uint32(spix[i+3]) * 0x101
   316	
   317				dr := &dpix[i+0]
   318				dg := &dpix[i+1]
   319				db := &dpix[i+2]
   320				da := &dpix[i+3]
   321	
   322				// The 0x101 is here for the same reason as in drawRGBA.
   323				a := (m - sa) * 0x101
   324	
   325				*dr = uint8((uint32(*dr)*a/m + sr) >> 8)
   326				*dg = uint8((uint32(*dg)*a/m + sg) >> 8)
   327				*db = uint8((uint32(*db)*a/m + sb) >> 8)
   328				*da = uint8((uint32(*da)*a/m + sa) >> 8)
   329			}
   330			d0 += ddelta
   331			s0 += sdelta
   332		}
   333	}
   334	
   335	func drawCopySrc(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
   336		n, dy := 4*r.Dx(), r.Dy()
   337		d0 := dst.PixOffset(r.Min.X, r.Min.Y)
   338		s0 := src.PixOffset(sp.X, sp.Y)
   339		var ddelta, sdelta int
   340		if r.Min.Y <= sp.Y {
   341			ddelta = dst.Stride
   342			sdelta = src.Stride
   343		} else {
   344			// If the source start point is higher than the destination start
   345			// point, then we compose the rows in bottom-up order instead of
   346			// top-down. Unlike the drawCopyOver function, we don't have to check
   347			// the x coordinates because the built-in copy function can handle
   348			// overlapping slices.
   349			d0 += (dy - 1) * dst.Stride
   350			s0 += (dy - 1) * src.Stride
   351			ddelta = -dst.Stride
   352			sdelta = -src.Stride
   353		}
   354		for ; dy > 0; dy-- {
   355			copy(dst.Pix[d0:d0+n], src.Pix[s0:s0+n])
   356			d0 += ddelta
   357			s0 += sdelta
   358		}
   359	}
   360	
   361	func drawNRGBAOver(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
   362		i0 := (r.Min.X - dst.Rect.Min.X) * 4
   363		i1 := (r.Max.X - dst.Rect.Min.X) * 4
   364		si0 := (sp.X - src.Rect.Min.X) * 4
   365		yMax := r.Max.Y - dst.Rect.Min.Y
   366	
   367		y := r.Min.Y - dst.Rect.Min.Y
   368		sy := sp.Y - src.Rect.Min.Y
   369		for ; y != yMax; y, sy = y+1, sy+1 {
   370			dpix := dst.Pix[y*dst.Stride:]
   371			spix := src.Pix[sy*src.Stride:]
   372	
   373			for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
   374				// Convert from non-premultiplied color to pre-multiplied color.
   375				sa := uint32(spix[si+3]) * 0x101
   376				sr := uint32(spix[si+0]) * sa / 0xff
   377				sg := uint32(spix[si+1]) * sa / 0xff
   378				sb := uint32(spix[si+2]) * sa / 0xff
   379	
   380				dr := uint32(dpix[i+0])
   381				dg := uint32(dpix[i+1])
   382				db := uint32(dpix[i+2])
   383				da := uint32(dpix[i+3])
   384	
   385				// The 0x101 is here for the same reason as in drawRGBA.
   386				a := (m - sa) * 0x101
   387	
   388				dpix[i+0] = uint8((dr*a/m + sr) >> 8)
   389				dpix[i+1] = uint8((dg*a/m + sg) >> 8)
   390				dpix[i+2] = uint8((db*a/m + sb) >> 8)
   391				dpix[i+3] = uint8((da*a/m + sa) >> 8)
   392			}
   393		}
   394	}
   395	
   396	func drawNRGBASrc(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
   397		i0 := (r.Min.X - dst.Rect.Min.X) * 4
   398		i1 := (r.Max.X - dst.Rect.Min.X) * 4
   399		si0 := (sp.X - src.Rect.Min.X) * 4
   400		yMax := r.Max.Y - dst.Rect.Min.Y
   401	
   402		y := r.Min.Y - dst.Rect.Min.Y
   403		sy := sp.Y - src.Rect.Min.Y
   404		for ; y != yMax; y, sy = y+1, sy+1 {
   405			dpix := dst.Pix[y*dst.Stride:]
   406			spix := src.Pix[sy*src.Stride:]
   407	
   408			for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
   409				// Convert from non-premultiplied color to pre-multiplied color.
   410				sa := uint32(spix[si+3]) * 0x101
   411				sr := uint32(spix[si+0]) * sa / 0xff
   412				sg := uint32(spix[si+1]) * sa / 0xff
   413				sb := uint32(spix[si+2]) * sa / 0xff
   414	
   415				dpix[i+0] = uint8(sr >> 8)
   416				dpix[i+1] = uint8(sg >> 8)
   417				dpix[i+2] = uint8(sb >> 8)
   418				dpix[i+3] = uint8(sa >> 8)
   419			}
   420		}
   421	}
   422	
   423	func drawGray(dst *image.RGBA, r image.Rectangle, src *image.Gray, sp image.Point) {
   424		i0 := (r.Min.X - dst.Rect.Min.X) * 4
   425		i1 := (r.Max.X - dst.Rect.Min.X) * 4
   426		si0 := (sp.X - src.Rect.Min.X) * 1
   427		yMax := r.Max.Y - dst.Rect.Min.Y
   428	
   429		y := r.Min.Y - dst.Rect.Min.Y
   430		sy := sp.Y - src.Rect.Min.Y
   431		for ; y != yMax; y, sy = y+1, sy+1 {
   432			dpix := dst.Pix[y*dst.Stride:]
   433			spix := src.Pix[sy*src.Stride:]
   434	
   435			for i, si := i0, si0; i < i1; i, si = i+4, si+1 {
   436				p := spix[si]
   437				dpix[i+0] = p
   438				dpix[i+1] = p
   439				dpix[i+2] = p
   440				dpix[i+3] = 255
   441			}
   442		}
   443	}
   444	
   445	func drawCMYK(dst *image.RGBA, r image.Rectangle, src *image.CMYK, sp image.Point) {
   446		i0 := (r.Min.X - dst.Rect.Min.X) * 4
   447		i1 := (r.Max.X - dst.Rect.Min.X) * 4
   448		si0 := (sp.X - src.Rect.Min.X) * 4
   449		yMax := r.Max.Y - dst.Rect.Min.Y
   450	
   451		y := r.Min.Y - dst.Rect.Min.Y
   452		sy := sp.Y - src.Rect.Min.Y
   453		for ; y != yMax; y, sy = y+1, sy+1 {
   454			dpix := dst.Pix[y*dst.Stride:]
   455			spix := src.Pix[sy*src.Stride:]
   456	
   457			for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
   458				dpix[i+0], dpix[i+1], dpix[i+2] =
   459					color.CMYKToRGB(spix[si+0], spix[si+1], spix[si+2], spix[si+3])
   460				dpix[i+3] = 255
   461			}
   462		}
   463	}
   464	
   465	func drawGlyphOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform, mask *image.Alpha, mp image.Point) {
   466		i0 := dst.PixOffset(r.Min.X, r.Min.Y)
   467		i1 := i0 + r.Dx()*4
   468		mi0 := mask.PixOffset(mp.X, mp.Y)
   469		sr, sg, sb, sa := src.RGBA()
   470		for y, my := r.Min.Y, mp.Y; y != r.Max.Y; y, my = y+1, my+1 {
   471			for i, mi := i0, mi0; i < i1; i, mi = i+4, mi+1 {
   472				ma := uint32(mask.Pix[mi])
   473				if ma == 0 {
   474					continue
   475				}
   476				ma |= ma << 8
   477	
   478				dr := &dst.Pix[i+0]
   479				dg := &dst.Pix[i+1]
   480				db := &dst.Pix[i+2]
   481				da := &dst.Pix[i+3]
   482	
   483				// The 0x101 is here for the same reason as in drawRGBA.
   484				a := (m - (sa * ma / m)) * 0x101
   485	
   486				*dr = uint8((uint32(*dr)*a + sr*ma) / m >> 8)
   487				*dg = uint8((uint32(*dg)*a + sg*ma) / m >> 8)
   488				*db = uint8((uint32(*db)*a + sb*ma) / m >> 8)
   489				*da = uint8((uint32(*da)*a + sa*ma) / m >> 8)
   490			}
   491			i0 += dst.Stride
   492			i1 += dst.Stride
   493			mi0 += mask.Stride
   494		}
   495	}
   496	
   497	func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
   498		x0, x1, dx := r.Min.X, r.Max.X, 1
   499		y0, y1, dy := r.Min.Y, r.Max.Y, 1
   500		if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
   501			if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
   502				x0, x1, dx = x1-1, x0-1, -1
   503				y0, y1, dy = y1-1, y0-1, -1
   504			}
   505		}
   506	
   507		sy := sp.Y + y0 - r.Min.Y
   508		my := mp.Y + y0 - r.Min.Y
   509		sx0 := sp.X + x0 - r.Min.X
   510		mx0 := mp.X + x0 - r.Min.X
   511		sx1 := sx0 + (x1 - x0)
   512		i0 := dst.PixOffset(x0, y0)
   513		di := dx * 4
   514		for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
   515			for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
   516				ma := uint32(m)
   517				if mask != nil {
   518					_, _, _, ma = mask.At(mx, my).RGBA()
   519				}
   520				sr, sg, sb, sa := src.At(sx, sy).RGBA()
   521				if op == Over {
   522					dr := uint32(dst.Pix[i+0])
   523					dg := uint32(dst.Pix[i+1])
   524					db := uint32(dst.Pix[i+2])
   525					da := uint32(dst.Pix[i+3])
   526	
   527					// dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
   528					// We work in 16-bit color, and so would normally do:
   529					// dr |= dr << 8
   530					// and similarly for dg, db and da, but instead we multiply a
   531					// (which is a 16-bit color, ranging in [0,65535]) by 0x101.
   532					// This yields the same result, but is fewer arithmetic operations.
   533					a := (m - (sa * ma / m)) * 0x101
   534	
   535					dst.Pix[i+0] = uint8((dr*a + sr*ma) / m >> 8)
   536					dst.Pix[i+1] = uint8((dg*a + sg*ma) / m >> 8)
   537					dst.Pix[i+2] = uint8((db*a + sb*ma) / m >> 8)
   538					dst.Pix[i+3] = uint8((da*a + sa*ma) / m >> 8)
   539	
   540				} else {
   541					dst.Pix[i+0] = uint8(sr * ma / m >> 8)
   542					dst.Pix[i+1] = uint8(sg * ma / m >> 8)
   543					dst.Pix[i+2] = uint8(sb * ma / m >> 8)
   544					dst.Pix[i+3] = uint8(sa * ma / m >> 8)
   545				}
   546			}
   547			i0 += dy * dst.Stride
   548		}
   549	}
   550	
   551	// clamp clamps i to the interval [0, 0xffff].
   552	func clamp(i int32) int32 {
   553		if i < 0 {
   554			return 0
   555		}
   556		if i > 0xffff {
   557			return 0xffff
   558		}
   559		return i
   560	}
   561	
   562	// sqDiff returns the squared-difference of x and y, shifted by 2 so that
   563	// adding four of those won't overflow a uint32.
   564	//
   565	// x and y are both assumed to be in the range [0, 0xffff].
   566	func sqDiff(x, y int32) uint32 {
   567		var d uint32
   568		if x > y {
   569			d = uint32(x - y)
   570		} else {
   571			d = uint32(y - x)
   572		}
   573		return (d * d) >> 2
   574	}
   575	
   576	func drawPaletted(dst Image, r image.Rectangle, src image.Image, sp image.Point, floydSteinberg bool) {
   577		// TODO(nigeltao): handle the case where the dst and src overlap.
   578		// Does it even make sense to try and do Floyd-Steinberg whilst
   579		// walking the image backward (right-to-left bottom-to-top)?
   580	
   581		// If dst is an *image.Paletted, we have a fast path for dst.Set and
   582		// dst.At. The dst.Set equivalent is a batch version of the algorithm
   583		// used by color.Palette's Index method in image/color/color.go, plus
   584		// optional Floyd-Steinberg error diffusion.
   585		palette, pix, stride := [][4]int32(nil), []byte(nil), 0
   586		if p, ok := dst.(*image.Paletted); ok {
   587			palette = make([][4]int32, len(p.Palette))
   588			for i, col := range p.Palette {
   589				r, g, b, a := col.RGBA()
   590				palette[i][0] = int32(r)
   591				palette[i][1] = int32(g)
   592				palette[i][2] = int32(b)
   593				palette[i][3] = int32(a)
   594			}
   595			pix, stride = p.Pix[p.PixOffset(r.Min.X, r.Min.Y):], p.Stride
   596		}
   597	
   598		// quantErrorCurr and quantErrorNext are the Floyd-Steinberg quantization
   599		// errors that have been propagated to the pixels in the current and next
   600		// rows. The +2 simplifies calculation near the edges.
   601		var quantErrorCurr, quantErrorNext [][4]int32
   602		if floydSteinberg {
   603			quantErrorCurr = make([][4]int32, r.Dx()+2)
   604			quantErrorNext = make([][4]int32, r.Dx()+2)
   605		}
   606	
   607		// Loop over each source pixel.
   608		out := color.RGBA64{A: 0xffff}
   609		for y := 0; y != r.Dy(); y++ {
   610			for x := 0; x != r.Dx(); x++ {
   611				// er, eg and eb are the pixel's R,G,B values plus the
   612				// optional Floyd-Steinberg error.
   613				sr, sg, sb, sa := src.At(sp.X+x, sp.Y+y).RGBA()
   614				er, eg, eb, ea := int32(sr), int32(sg), int32(sb), int32(sa)
   615				if floydSteinberg {
   616					er = clamp(er + quantErrorCurr[x+1][0]/16)
   617					eg = clamp(eg + quantErrorCurr[x+1][1]/16)
   618					eb = clamp(eb + quantErrorCurr[x+1][2]/16)
   619					ea = clamp(ea + quantErrorCurr[x+1][3]/16)
   620				}
   621	
   622				if palette != nil {
   623					// Find the closest palette color in Euclidean R,G,B,A space:
   624					// the one that minimizes sum-squared-difference.
   625					// TODO(nigeltao): consider smarter algorithms.
   626					bestIndex, bestSum := 0, uint32(1<<32-1)
   627					for index, p := range palette {
   628						sum := sqDiff(er, p[0]) + sqDiff(eg, p[1]) + sqDiff(eb, p[2]) + sqDiff(ea, p[3])
   629						if sum < bestSum {
   630							bestIndex, bestSum = index, sum
   631							if sum == 0 {
   632								break
   633							}
   634						}
   635					}
   636					pix[y*stride+x] = byte(bestIndex)
   637	
   638					if !floydSteinberg {
   639						continue
   640					}
   641					er -= palette[bestIndex][0]
   642					eg -= palette[bestIndex][1]
   643					eb -= palette[bestIndex][2]
   644					ea -= palette[bestIndex][3]
   645	
   646				} else {
   647					out.R = uint16(er)
   648					out.G = uint16(eg)
   649					out.B = uint16(eb)
   650					out.A = uint16(ea)
   651					// The third argument is &out instead of out (and out is
   652					// declared outside of the inner loop) to avoid the implicit
   653					// conversion to color.Color here allocating memory in the
   654					// inner loop if sizeof(color.RGBA64) > sizeof(uintptr).
   655					dst.Set(r.Min.X+x, r.Min.Y+y, &out)
   656	
   657					if !floydSteinberg {
   658						continue
   659					}
   660					sr, sg, sb, sa = dst.At(r.Min.X+x, r.Min.Y+y).RGBA()
   661					er -= int32(sr)
   662					eg -= int32(sg)
   663					eb -= int32(sb)
   664					ea -= int32(sa)
   665				}
   666	
   667				// Propagate the Floyd-Steinberg quantization error.
   668				quantErrorNext[x+0][0] += er * 3
   669				quantErrorNext[x+0][1] += eg * 3
   670				quantErrorNext[x+0][2] += eb * 3
   671				quantErrorNext[x+0][3] += ea * 3
   672				quantErrorNext[x+1][0] += er * 5
   673				quantErrorNext[x+1][1] += eg * 5
   674				quantErrorNext[x+1][2] += eb * 5
   675				quantErrorNext[x+1][3] += ea * 5
   676				quantErrorNext[x+2][0] += er * 1
   677				quantErrorNext[x+2][1] += eg * 1
   678				quantErrorNext[x+2][2] += eb * 1
   679				quantErrorNext[x+2][3] += ea * 1
   680				quantErrorCurr[x+2][0] += er * 7
   681				quantErrorCurr[x+2][1] += eg * 7
   682				quantErrorCurr[x+2][2] += eb * 7
   683				quantErrorCurr[x+2][3] += ea * 7
   684			}
   685	
   686			// Recycle the quantization error buffers.
   687			if floydSteinberg {
   688				quantErrorCurr, quantErrorNext = quantErrorNext, quantErrorCurr
   689				for i := range quantErrorNext {
   690					quantErrorNext[i] = [4]int32{}
   691				}
   692			}
   693		}
   694	}
   695	

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