Adding upstream version 1.17.13.upstream/1.17.13upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 730 insertions, 0 deletions

diff --git a/src/image/draw/draw.go b/src/image/draw/draw.go
new file mode 100644
index 0000000..13f6668
--- /dev/null
+++ b/src/image/draw/draw.go
@@ -0,0 +1,730 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package draw provides image composition functions.
+//
+// See "The Go image/draw package" for an introduction to this package:
+// https://golang.org/doc/articles/image_draw.html
+package draw
+
+import (
+	"image"
+	"image/color"
+	"image/internal/imageutil"
+)
+
+// m is the maximum color value returned by image.Color.RGBA.
+const m = 1<<16 - 1
+
+// Image is an image.Image with a Set method to change a single pixel.
+type Image interface {
+	image.Image
+	Set(x, y int, c color.Color)
+}
+
+// RGBA64Image extends both the Image and image.RGBA64Image interfaces with a
+// SetRGBA64 method to change a single pixel. SetRGBA64 is equivalent to
+// calling Set, but it can avoid allocations from converting concrete color
+// types to the color.Color interface type.
+type RGBA64Image interface {
+	image.RGBA64Image
+	Set(x, y int, c color.Color)
+	SetRGBA64(x, y int, c color.RGBA64)
+}
+
+// Quantizer produces a palette for an image.
+type Quantizer interface {
+	// Quantize appends up to cap(p) - len(p) colors to p and returns the
+	// updated palette suitable for converting m to a paletted image.
+	Quantize(p color.Palette, m image.Image) color.Palette
+}
+
+// Op is a Porter-Duff compositing operator.
+type Op int
+
+const (
+	// Over specifies ``(src in mask) over dst''.
+	Over Op = iota
+	// Src specifies ``src in mask''.
+	Src
+)
+
+// Draw implements the Drawer interface by calling the Draw function with this
+// Op.
+func (op Op) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
+	DrawMask(dst, r, src, sp, nil, image.Point{}, op)
+}
+
+// Drawer contains the Draw method.
+type Drawer interface {
+	// Draw aligns r.Min in dst with sp in src and then replaces the
+	// rectangle r in dst with the result of drawing src on dst.
+	Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point)
+}
+
+// FloydSteinberg is a Drawer that is the Src Op with Floyd-Steinberg error
+// diffusion.
+var FloydSteinberg Drawer = floydSteinberg{}
+
+type floydSteinberg struct{}
+
+func (floydSteinberg) Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point) {
+	clip(dst, &r, src, &sp, nil, nil)
+	if r.Empty() {
+		return
+	}
+	drawPaletted(dst, r, src, sp, true)
+}
+
+// clip clips r against each image's bounds (after translating into the
+// destination image's coordinate space) and shifts the points sp and mp by
+// the same amount as the change in r.Min.
+func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask image.Image, mp *image.Point) {
+	orig := r.Min
+	*r = r.Intersect(dst.Bounds())
+	*r = r.Intersect(src.Bounds().Add(orig.Sub(*sp)))
+	if mask != nil {
+		*r = r.Intersect(mask.Bounds().Add(orig.Sub(*mp)))
+	}
+	dx := r.Min.X - orig.X
+	dy := r.Min.Y - orig.Y
+	if dx == 0 && dy == 0 {
+		return
+	}
+	sp.X += dx
+	sp.Y += dy
+	if mp != nil {
+		mp.X += dx
+		mp.Y += dy
+	}
+}
+
+func processBackward(dst image.Image, r image.Rectangle, src image.Image, sp image.Point) bool {
+	return dst == src &&
+		r.Overlaps(r.Add(sp.Sub(r.Min))) &&
+		(sp.Y < r.Min.Y || (sp.Y == r.Min.Y && sp.X < r.Min.X))
+}
+
+// Draw calls DrawMask with a nil mask.
+func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point, op Op) {
+	DrawMask(dst, r, src, sp, nil, image.Point{}, op)
+}
+
+// DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r
+// in dst with the result of a Porter-Duff composition. A nil mask is treated as opaque.
+func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
+	clip(dst, &r, src, &sp, mask, &mp)
+	if r.Empty() {
+		return
+	}
+
+	// Fast paths for special cases. If none of them apply, then we fall back to a general but slow implementation.
+	switch dst0 := dst.(type) {
+	case *image.RGBA:
+		if op == Over {
+			if mask == nil {
+				switch src0 := src.(type) {
+				case *image.Uniform:
+					sr, sg, sb, sa := src0.RGBA()
+					if sa == 0xffff {
+						drawFillSrc(dst0, r, sr, sg, sb, sa)
+					} else {
+						drawFillOver(dst0, r, sr, sg, sb, sa)
+					}
+					return
+				case *image.RGBA:
+					drawCopyOver(dst0, r, src0, sp)
+					return
+				case *image.NRGBA:
+					drawNRGBAOver(dst0, r, src0, sp)
+					return
+				case *image.YCbCr:
+					// An image.YCbCr is always fully opaque, and so if the
+					// mask is nil (i.e. fully opaque) then the op is
+					// effectively always Src. Similarly for image.Gray and
+					// image.CMYK.
+					if imageutil.DrawYCbCr(dst0, r, src0, sp) {
+						return
+					}
+				case *image.Gray:
+					drawGray(dst0, r, src0, sp)
+					return
+				case *image.CMYK:
+					drawCMYK(dst0, r, src0, sp)
+					return
+				}
+			} else if mask0, ok := mask.(*image.Alpha); ok {
+				switch src0 := src.(type) {
+				case *image.Uniform:
+					drawGlyphOver(dst0, r, src0, mask0, mp)
+					return
+				}
+			}
+		} else {
+			if mask == nil {
+				switch src0 := src.(type) {
+				case *image.Uniform:
+					sr, sg, sb, sa := src0.RGBA()
+					drawFillSrc(dst0, r, sr, sg, sb, sa)
+					return
+				case *image.RGBA:
+					drawCopySrc(dst0, r, src0, sp)
+					return
+				case *image.NRGBA:
+					drawNRGBASrc(dst0, r, src0, sp)
+					return
+				case *image.YCbCr:
+					if imageutil.DrawYCbCr(dst0, r, src0, sp) {
+						return
+					}
+				case *image.Gray:
+					drawGray(dst0, r, src0, sp)
+					return
+				case *image.CMYK:
+					drawCMYK(dst0, r, src0, sp)
+					return
+				}
+			}
+		}
+		drawRGBA(dst0, r, src, sp, mask, mp, op)
+		return
+	case *image.Paletted:
+		if op == Src && mask == nil {
+			if src0, ok := src.(*image.Uniform); ok {
+				colorIndex := uint8(dst0.Palette.Index(src0.C))
+				i0 := dst0.PixOffset(r.Min.X, r.Min.Y)
+				i1 := i0 + r.Dx()
+				for i := i0; i < i1; i++ {
+					dst0.Pix[i] = colorIndex
+				}
+				firstRow := dst0.Pix[i0:i1]
+				for y := r.Min.Y + 1; y < r.Max.Y; y++ {
+					i0 += dst0.Stride
+					i1 += dst0.Stride
+					copy(dst0.Pix[i0:i1], firstRow)
+				}
+				return
+			} else if !processBackward(dst, r, src, sp) {
+				drawPaletted(dst0, r, src, sp, false)
+				return
+			}
+		}
+	}
+
+	x0, x1, dx := r.Min.X, r.Max.X, 1
+	y0, y1, dy := r.Min.Y, r.Max.Y, 1
+	if processBackward(dst, r, src, sp) {
+		x0, x1, dx = x1-1, x0-1, -1
+		y0, y1, dy = y1-1, y0-1, -1
+	}
+
+	var out color.RGBA64
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		sx := sp.X + x0 - r.Min.X
+		mx := mp.X + x0 - r.Min.X
+		for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
+			ma := uint32(m)
+			if mask != nil {
+				_, _, _, ma = mask.At(mx, my).RGBA()
+			}
+			switch {
+			case ma == 0:
+				if op == Over {
+					// No-op.
+				} else {
+					dst.Set(x, y, color.Transparent)
+				}
+			case ma == m && op == Src:
+				dst.Set(x, y, src.At(sx, sy))
+			default:
+				sr, sg, sb, sa := src.At(sx, sy).RGBA()
+				if op == Over {
+					dr, dg, db, da := dst.At(x, y).RGBA()
+					a := m - (sa * ma / m)
+					out.R = uint16((dr*a + sr*ma) / m)
+					out.G = uint16((dg*a + sg*ma) / m)
+					out.B = uint16((db*a + sb*ma) / m)
+					out.A = uint16((da*a + sa*ma) / m)
+				} else {
+					out.R = uint16(sr * ma / m)
+					out.G = uint16(sg * ma / m)
+					out.B = uint16(sb * ma / m)
+					out.A = uint16(sa * ma / m)
+				}
+				// The third argument is &out instead of out (and out is
+				// declared outside of the inner loop) to avoid the implicit
+				// conversion to color.Color here allocating memory in the
+				// inner loop if sizeof(color.RGBA64) > sizeof(uintptr).
+				dst.Set(x, y, &out)
+			}
+		}
+	}
+}
+
+func drawFillOver(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
+	// The 0x101 is here for the same reason as in drawRGBA.
+	a := (m - sa) * 0x101
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	i1 := i0 + r.Dx()*4
+	for y := r.Min.Y; y != r.Max.Y; y++ {
+		for i := i0; i < i1; i += 4 {
+			dr := &dst.Pix[i+0]
+			dg := &dst.Pix[i+1]
+			db := &dst.Pix[i+2]
+			da := &dst.Pix[i+3]
+
+			*dr = uint8((uint32(*dr)*a/m + sr) >> 8)
+			*dg = uint8((uint32(*dg)*a/m + sg) >> 8)
+			*db = uint8((uint32(*db)*a/m + sb) >> 8)
+			*da = uint8((uint32(*da)*a/m + sa) >> 8)
+		}
+		i0 += dst.Stride
+		i1 += dst.Stride
+	}
+}
+
+func drawFillSrc(dst *image.RGBA, r image.Rectangle, sr, sg, sb, sa uint32) {
+	sr8 := uint8(sr >> 8)
+	sg8 := uint8(sg >> 8)
+	sb8 := uint8(sb >> 8)
+	sa8 := uint8(sa >> 8)
+	// The built-in copy function is faster than a straightforward for loop to fill the destination with
+	// the color, but copy requires a slice source. We therefore use a for loop to fill the first row, and
+	// then use the first row as the slice source for the remaining rows.
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	i1 := i0 + r.Dx()*4
+	for i := i0; i < i1; i += 4 {
+		dst.Pix[i+0] = sr8
+		dst.Pix[i+1] = sg8
+		dst.Pix[i+2] = sb8
+		dst.Pix[i+3] = sa8
+	}
+	firstRow := dst.Pix[i0:i1]
+	for y := r.Min.Y + 1; y < r.Max.Y; y++ {
+		i0 += dst.Stride
+		i1 += dst.Stride
+		copy(dst.Pix[i0:i1], firstRow)
+	}
+}
+
+func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
+	dx, dy := r.Dx(), r.Dy()
+	d0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	s0 := src.PixOffset(sp.X, sp.Y)
+	var (
+		ddelta, sdelta int
+		i0, i1, idelta int
+	)
+	if r.Min.Y < sp.Y || r.Min.Y == sp.Y && r.Min.X <= sp.X {
+		ddelta = dst.Stride
+		sdelta = src.Stride
+		i0, i1, idelta = 0, dx*4, +4
+	} else {
+		// If the source start point is higher than the destination start point, or equal height but to the left,
+		// then we compose the rows in right-to-left, bottom-up order instead of left-to-right, top-down.
+		d0 += (dy - 1) * dst.Stride
+		s0 += (dy - 1) * src.Stride
+		ddelta = -dst.Stride
+		sdelta = -src.Stride
+		i0, i1, idelta = (dx-1)*4, -4, -4
+	}
+	for ; dy > 0; dy-- {
+		dpix := dst.Pix[d0:]
+		spix := src.Pix[s0:]
+		for i := i0; i != i1; i += idelta {
+			s := spix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			sr := uint32(s[0]) * 0x101
+			sg := uint32(s[1]) * 0x101
+			sb := uint32(s[2]) * 0x101
+			sa := uint32(s[3]) * 0x101
+
+			// The 0x101 is here for the same reason as in drawRGBA.
+			a := (m - sa) * 0x101
+
+			d := dpix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			d[0] = uint8((uint32(d[0])*a/m + sr) >> 8)
+			d[1] = uint8((uint32(d[1])*a/m + sg) >> 8)
+			d[2] = uint8((uint32(d[2])*a/m + sb) >> 8)
+			d[3] = uint8((uint32(d[3])*a/m + sa) >> 8)
+		}
+		d0 += ddelta
+		s0 += sdelta
+	}
+}
+
+func drawCopySrc(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
+	n, dy := 4*r.Dx(), r.Dy()
+	d0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	s0 := src.PixOffset(sp.X, sp.Y)
+	var ddelta, sdelta int
+	if r.Min.Y <= sp.Y {
+		ddelta = dst.Stride
+		sdelta = src.Stride
+	} else {
+		// If the source start point is higher than the destination start
+		// point, then we compose the rows in bottom-up order instead of
+		// top-down. Unlike the drawCopyOver function, we don't have to check
+		// the x coordinates because the built-in copy function can handle
+		// overlapping slices.
+		d0 += (dy - 1) * dst.Stride
+		s0 += (dy - 1) * src.Stride
+		ddelta = -dst.Stride
+		sdelta = -src.Stride
+	}
+	for ; dy > 0; dy-- {
+		copy(dst.Pix[d0:d0+n], src.Pix[s0:s0+n])
+		d0 += ddelta
+		s0 += sdelta
+	}
+}
+
+func drawNRGBAOver(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
+	i0 := (r.Min.X - dst.Rect.Min.X) * 4
+	i1 := (r.Max.X - dst.Rect.Min.X) * 4
+	si0 := (sp.X - src.Rect.Min.X) * 4
+	yMax := r.Max.Y - dst.Rect.Min.Y
+
+	y := r.Min.Y - dst.Rect.Min.Y
+	sy := sp.Y - src.Rect.Min.Y
+	for ; y != yMax; y, sy = y+1, sy+1 {
+		dpix := dst.Pix[y*dst.Stride:]
+		spix := src.Pix[sy*src.Stride:]
+
+		for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
+			// Convert from non-premultiplied color to pre-multiplied color.
+			s := spix[si : si+4 : si+4] // Small cap improves performance, see https://golang.org/issue/27857
+			sa := uint32(s[3]) * 0x101
+			sr := uint32(s[0]) * sa / 0xff
+			sg := uint32(s[1]) * sa / 0xff
+			sb := uint32(s[2]) * sa / 0xff
+
+			d := dpix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			dr := uint32(d[0])
+			dg := uint32(d[1])
+			db := uint32(d[2])
+			da := uint32(d[3])
+
+			// The 0x101 is here for the same reason as in drawRGBA.
+			a := (m - sa) * 0x101
+
+			d[0] = uint8((dr*a/m + sr) >> 8)
+			d[1] = uint8((dg*a/m + sg) >> 8)
+			d[2] = uint8((db*a/m + sb) >> 8)
+			d[3] = uint8((da*a/m + sa) >> 8)
+		}
+	}
+}
+
+func drawNRGBASrc(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
+	i0 := (r.Min.X - dst.Rect.Min.X) * 4
+	i1 := (r.Max.X - dst.Rect.Min.X) * 4
+	si0 := (sp.X - src.Rect.Min.X) * 4
+	yMax := r.Max.Y - dst.Rect.Min.Y
+
+	y := r.Min.Y - dst.Rect.Min.Y
+	sy := sp.Y - src.Rect.Min.Y
+	for ; y != yMax; y, sy = y+1, sy+1 {
+		dpix := dst.Pix[y*dst.Stride:]
+		spix := src.Pix[sy*src.Stride:]
+
+		for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
+			// Convert from non-premultiplied color to pre-multiplied color.
+			s := spix[si : si+4 : si+4] // Small cap improves performance, see https://golang.org/issue/27857
+			sa := uint32(s[3]) * 0x101
+			sr := uint32(s[0]) * sa / 0xff
+			sg := uint32(s[1]) * sa / 0xff
+			sb := uint32(s[2]) * sa / 0xff
+
+			d := dpix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			d[0] = uint8(sr >> 8)
+			d[1] = uint8(sg >> 8)
+			d[2] = uint8(sb >> 8)
+			d[3] = uint8(sa >> 8)
+		}
+	}
+}
+
+func drawGray(dst *image.RGBA, r image.Rectangle, src *image.Gray, sp image.Point) {
+	i0 := (r.Min.X - dst.Rect.Min.X) * 4
+	i1 := (r.Max.X - dst.Rect.Min.X) * 4
+	si0 := (sp.X - src.Rect.Min.X) * 1
+	yMax := r.Max.Y - dst.Rect.Min.Y
+
+	y := r.Min.Y - dst.Rect.Min.Y
+	sy := sp.Y - src.Rect.Min.Y
+	for ; y != yMax; y, sy = y+1, sy+1 {
+		dpix := dst.Pix[y*dst.Stride:]
+		spix := src.Pix[sy*src.Stride:]
+
+		for i, si := i0, si0; i < i1; i, si = i+4, si+1 {
+			p := spix[si]
+			d := dpix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			d[0] = p
+			d[1] = p
+			d[2] = p
+			d[3] = 255
+		}
+	}
+}
+
+func drawCMYK(dst *image.RGBA, r image.Rectangle, src *image.CMYK, sp image.Point) {
+	i0 := (r.Min.X - dst.Rect.Min.X) * 4
+	i1 := (r.Max.X - dst.Rect.Min.X) * 4
+	si0 := (sp.X - src.Rect.Min.X) * 4
+	yMax := r.Max.Y - dst.Rect.Min.Y
+
+	y := r.Min.Y - dst.Rect.Min.Y
+	sy := sp.Y - src.Rect.Min.Y
+	for ; y != yMax; y, sy = y+1, sy+1 {
+		dpix := dst.Pix[y*dst.Stride:]
+		spix := src.Pix[sy*src.Stride:]
+
+		for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
+			s := spix[si : si+4 : si+4] // Small cap improves performance, see https://golang.org/issue/27857
+			d := dpix[i : i+4 : i+4]
+			d[0], d[1], d[2] = color.CMYKToRGB(s[0], s[1], s[2], s[3])
+			d[3] = 255
+		}
+	}
+}
+
+func drawGlyphOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform, mask *image.Alpha, mp image.Point) {
+	i0 := dst.PixOffset(r.Min.X, r.Min.Y)
+	i1 := i0 + r.Dx()*4
+	mi0 := mask.PixOffset(mp.X, mp.Y)
+	sr, sg, sb, sa := src.RGBA()
+	for y, my := r.Min.Y, mp.Y; y != r.Max.Y; y, my = y+1, my+1 {
+		for i, mi := i0, mi0; i < i1; i, mi = i+4, mi+1 {
+			ma := uint32(mask.Pix[mi])
+			if ma == 0 {
+				continue
+			}
+			ma |= ma << 8
+
+			// The 0x101 is here for the same reason as in drawRGBA.
+			a := (m - (sa * ma / m)) * 0x101
+
+			d := dst.Pix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			d[0] = uint8((uint32(d[0])*a + sr*ma) / m >> 8)
+			d[1] = uint8((uint32(d[1])*a + sg*ma) / m >> 8)
+			d[2] = uint8((uint32(d[2])*a + sb*ma) / m >> 8)
+			d[3] = uint8((uint32(d[3])*a + sa*ma) / m >> 8)
+		}
+		i0 += dst.Stride
+		i1 += dst.Stride
+		mi0 += mask.Stride
+	}
+}
+
+func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
+	x0, x1, dx := r.Min.X, r.Max.X, 1
+	y0, y1, dy := r.Min.Y, r.Max.Y, 1
+	if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
+		if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
+			x0, x1, dx = x1-1, x0-1, -1
+			y0, y1, dy = y1-1, y0-1, -1
+		}
+	}
+
+	sy := sp.Y + y0 - r.Min.Y
+	my := mp.Y + y0 - r.Min.Y
+	sx0 := sp.X + x0 - r.Min.X
+	mx0 := mp.X + x0 - r.Min.X
+	sx1 := sx0 + (x1 - x0)
+	i0 := dst.PixOffset(x0, y0)
+	di := dx * 4
+	for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
+		for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
+			ma := uint32(m)
+			if mask != nil {
+				_, _, _, ma = mask.At(mx, my).RGBA()
+			}
+			sr, sg, sb, sa := src.At(sx, sy).RGBA()
+			d := dst.Pix[i : i+4 : i+4] // Small cap improves performance, see https://golang.org/issue/27857
+			if op == Over {
+				dr := uint32(d[0])
+				dg := uint32(d[1])
+				db := uint32(d[2])
+				da := uint32(d[3])
+
+				// dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
+				// We work in 16-bit color, and so would normally do:
+				// dr |= dr << 8
+				// and similarly for dg, db and da, but instead we multiply a
+				// (which is a 16-bit color, ranging in [0,65535]) by 0x101.
+				// This yields the same result, but is fewer arithmetic operations.
+				a := (m - (sa * ma / m)) * 0x101
+
+				d[0] = uint8((dr*a + sr*ma) / m >> 8)
+				d[1] = uint8((dg*a + sg*ma) / m >> 8)
+				d[2] = uint8((db*a + sb*ma) / m >> 8)
+				d[3] = uint8((da*a + sa*ma) / m >> 8)
+
+			} else {
+				d[0] = uint8(sr * ma / m >> 8)
+				d[1] = uint8(sg * ma / m >> 8)
+				d[2] = uint8(sb * ma / m >> 8)
+				d[3] = uint8(sa * ma / m >> 8)
+			}
+		}
+		i0 += dy * dst.Stride
+	}
+}
+
+// clamp clamps i to the interval [0, 0xffff].
+func clamp(i int32) int32 {
+	if i < 0 {
+		return 0
+	}
+	if i > 0xffff {
+		return 0xffff
+	}
+	return i
+}
+
+// sqDiff returns the squared-difference of x and y, shifted by 2 so that
+// adding four of those won't overflow a uint32.
+//
+// x and y are both assumed to be in the range [0, 0xffff].
+func sqDiff(x, y int32) uint32 {
+	// This is an optimized code relying on the overflow/wrap around
+	// properties of unsigned integers operations guaranteed by the language
+	// spec. See sqDiff from the image/color package for more details.
+	d := uint32(x - y)
+	return (d * d) >> 2
+}
+
+func drawPaletted(dst Image, r image.Rectangle, src image.Image, sp image.Point, floydSteinberg bool) {
+	// TODO(nigeltao): handle the case where the dst and src overlap.
+	// Does it even make sense to try and do Floyd-Steinberg whilst
+	// walking the image backward (right-to-left bottom-to-top)?
+
+	// If dst is an *image.Paletted, we have a fast path for dst.Set and
+	// dst.At. The dst.Set equivalent is a batch version of the algorithm
+	// used by color.Palette's Index method in image/color/color.go, plus
+	// optional Floyd-Steinberg error diffusion.
+	palette, pix, stride := [][4]int32(nil), []byte(nil), 0
+	if p, ok := dst.(*image.Paletted); ok {
+		palette = make([][4]int32, len(p.Palette))
+		for i, col := range p.Palette {
+			r, g, b, a := col.RGBA()
+			palette[i][0] = int32(r)
+			palette[i][1] = int32(g)
+			palette[i][2] = int32(b)
+			palette[i][3] = int32(a)
+		}
+		pix, stride = p.Pix[p.PixOffset(r.Min.X, r.Min.Y):], p.Stride
+	}
+
+	// quantErrorCurr and quantErrorNext are the Floyd-Steinberg quantization
+	// errors that have been propagated to the pixels in the current and next
+	// rows. The +2 simplifies calculation near the edges.
+	var quantErrorCurr, quantErrorNext [][4]int32
+	if floydSteinberg {
+		quantErrorCurr = make([][4]int32, r.Dx()+2)
+		quantErrorNext = make([][4]int32, r.Dx()+2)
+	}
+	pxRGBA := func(x, y int) (r, g, b, a uint32) { return src.At(x, y).RGBA() }
+	// Fast paths for special cases to avoid excessive use of the color.Color
+	// interface which escapes to the heap but need to be discovered for
+	// each pixel on r. See also https://golang.org/issues/15759.
+	switch src0 := src.(type) {
+	case *image.RGBA:
+		pxRGBA = func(x, y int) (r, g, b, a uint32) { return src0.RGBAAt(x, y).RGBA() }
+	case *image.NRGBA:
+		pxRGBA = func(x, y int) (r, g, b, a uint32) { return src0.NRGBAAt(x, y).RGBA() }
+	case *image.YCbCr:
+		pxRGBA = func(x, y int) (r, g, b, a uint32) { return src0.YCbCrAt(x, y).RGBA() }
+	}
+
+	// Loop over each source pixel.
+	out := color.RGBA64{A: 0xffff}
+	for y := 0; y != r.Dy(); y++ {
+		for x := 0; x != r.Dx(); x++ {
+			// er, eg and eb are the pixel's R,G,B values plus the
+			// optional Floyd-Steinberg error.
+			sr, sg, sb, sa := pxRGBA(sp.X+x, sp.Y+y)
+			er, eg, eb, ea := int32(sr), int32(sg), int32(sb), int32(sa)
+			if floydSteinberg {
+				er = clamp(er + quantErrorCurr[x+1][0]/16)
+				eg = clamp(eg + quantErrorCurr[x+1][1]/16)
+				eb = clamp(eb + quantErrorCurr[x+1][2]/16)
+				ea = clamp(ea + quantErrorCurr[x+1][3]/16)
+			}
+
+			if palette != nil {
+				// Find the closest palette color in Euclidean R,G,B,A space:
+				// the one that minimizes sum-squared-difference.
+				// TODO(nigeltao): consider smarter algorithms.
+				bestIndex, bestSum := 0, uint32(1<<32-1)
+				for index, p := range palette {
+					sum := sqDiff(er, p[0]) + sqDiff(eg, p[1]) + sqDiff(eb, p[2]) + sqDiff(ea, p[3])
+					if sum < bestSum {
+						bestIndex, bestSum = index, sum
+						if sum == 0 {
+							break
+						}
+					}
+				}
+				pix[y*stride+x] = byte(bestIndex)
+
+				if !floydSteinberg {
+					continue
+				}
+				er -= palette[bestIndex][0]
+				eg -= palette[bestIndex][1]
+				eb -= palette[bestIndex][2]
+				ea -= palette[bestIndex][3]
+
+			} else {
+				out.R = uint16(er)
+				out.G = uint16(eg)
+				out.B = uint16(eb)
+				out.A = uint16(ea)
+				// The third argument is &out instead of out (and out is
+				// declared outside of the inner loop) to avoid the implicit
+				// conversion to color.Color here allocating memory in the
+				// inner loop if sizeof(color.RGBA64) > sizeof(uintptr).
+				dst.Set(r.Min.X+x, r.Min.Y+y, &out)
+
+				if !floydSteinberg {
+					continue
+				}
+				sr, sg, sb, sa = dst.At(r.Min.X+x, r.Min.Y+y).RGBA()
+				er -= int32(sr)
+				eg -= int32(sg)
+				eb -= int32(sb)
+				ea -= int32(sa)
+			}
+
+			// Propagate the Floyd-Steinberg quantization error.
+			quantErrorNext[x+0][0] += er * 3
+			quantErrorNext[x+0][1] += eg * 3
+			quantErrorNext[x+0][2] += eb * 3
+			quantErrorNext[x+0][3] += ea * 3
+			quantErrorNext[x+1][0] += er * 5
+			quantErrorNext[x+1][1] += eg * 5
+			quantErrorNext[x+1][2] += eb * 5
+			quantErrorNext[x+1][3] += ea * 5
+			quantErrorNext[x+2][0] += er * 1
+			quantErrorNext[x+2][1] += eg * 1
+			quantErrorNext[x+2][2] += eb * 1
+			quantErrorNext[x+2][3] += ea * 1
+			quantErrorCurr[x+2][0] += er * 7
+			quantErrorCurr[x+2][1] += eg * 7
+			quantErrorCurr[x+2][2] += eb * 7
+			quantErrorCurr[x+2][3] += ea * 7
+		}
+
+		// Recycle the quantization error buffers.
+		if floydSteinberg {
+			quantErrorCurr, quantErrorNext = quantErrorNext, quantErrorCurr
+			for i := range quantErrorNext {
+				quantErrorNext[i] = [4]int32{}
+			}
+		}
+	}
+}