// 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 image implements a basic 2-D image library.
//
// See "The Go image package" for an introduction to this package:
// http://blog.golang.org/2011/09/go-image-package.html
package image

import (
	"image/color"
)

// Config holds an image's color model and dimensions.
type Config struct {
	ColorModel    color.Model
	Width, Height int
}

// Image is a finite rectangular grid of Colors drawn from a color model.
type Image interface {
	// ColorModel returns the Image's color model.
	ColorModel() color.Model
	// Bounds returns the domain for which At can return non-zero color.
	// The bounds do not necessarily contain the point (0, 0).
	Bounds() Rectangle
	// At returns the color of the pixel at (x, y).
	// At(Bounds().Min.X, Bounds().Min.Y) returns the upper-left pixel of the grid.
	// At(Bounds().Max.X-1, Bounds().Max.Y-1) returns the lower-right one.
	At(x, y int) color.Color
}

// PalettedImage is an image whose colors may come from a limited palette.
// If m is a PalettedImage and m.ColorModel() returns a PalettedColorModel p,
// then m.At(x, y) should be equivalent to p[m.ColorIndexAt(x, y)]. If m's
// color model is not a PalettedColorModel, then ColorIndexAt's behavior is
// undefined.
type PalettedImage interface {
	// ColorIndexAt returns the palette index of the pixel at (x, y).
	ColorIndexAt(x, y int) uint8
	Image
}

// RGBA is an in-memory image of RGBAColor values.
type RGBA struct {
	// Pix holds the image's pixels, in R, G, B, A order. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*4].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *RGBA) ColorModel() color.Model { return color.RGBAModel }

func (p *RGBA) Bounds() Rectangle { return p.Rect }

func (p *RGBA) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.RGBA{}
	}
	i := p.PixOffset(x, y)
	return color.RGBA{p.Pix[i+0], p.Pix[i+1], p.Pix[i+2], p.Pix[i+3]}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *RGBA) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
}

func (p *RGBA) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.RGBAModel.Convert(c).(color.RGBA)
	p.Pix[i+0] = c1.R
	p.Pix[i+1] = c1.G
	p.Pix[i+2] = c1.B
	p.Pix[i+3] = c1.A
}

func (p *RGBA) SetRGBA(x, y int, c color.RGBA) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = c.R
	p.Pix[i+1] = c.G
	p.Pix[i+2] = c.B
	p.Pix[i+3] = c.A
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *RGBA) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &RGBA{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &RGBA{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *RGBA) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 3, p.Rect.Dx()*4
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i += 4 {
			if p.Pix[i] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewRGBA returns a new RGBA with the given bounds.
func NewRGBA(r Rectangle) *RGBA {
	w, h := r.Dx(), r.Dy()
	buf := make([]uint8, 4*w*h)
	return &RGBA{buf, 4 * w, r}
}

// RGBA64 is an in-memory image of RGBA64Color values.
type RGBA64 struct {
	// Pix holds the image's pixels, in R, G, B, A order and big-endian format. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*8].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *RGBA64) ColorModel() color.Model { return color.RGBA64Model }

func (p *RGBA64) Bounds() Rectangle { return p.Rect }

func (p *RGBA64) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.RGBA64{}
	}
	i := p.PixOffset(x, y)
	return color.RGBA64{
		uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1]),
		uint16(p.Pix[i+2])<<8 | uint16(p.Pix[i+3]),
		uint16(p.Pix[i+4])<<8 | uint16(p.Pix[i+5]),
		uint16(p.Pix[i+6])<<8 | uint16(p.Pix[i+7]),
	}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *RGBA64) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
}

func (p *RGBA64) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.RGBA64Model.Convert(c).(color.RGBA64)
	p.Pix[i+0] = uint8(c1.R >> 8)
	p.Pix[i+1] = uint8(c1.R)
	p.Pix[i+2] = uint8(c1.G >> 8)
	p.Pix[i+3] = uint8(c1.G)
	p.Pix[i+4] = uint8(c1.B >> 8)
	p.Pix[i+5] = uint8(c1.B)
	p.Pix[i+6] = uint8(c1.A >> 8)
	p.Pix[i+7] = uint8(c1.A)
}

func (p *RGBA64) SetRGBA64(x, y int, c color.RGBA64) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = uint8(c.R >> 8)
	p.Pix[i+1] = uint8(c.R)
	p.Pix[i+2] = uint8(c.G >> 8)
	p.Pix[i+3] = uint8(c.G)
	p.Pix[i+4] = uint8(c.B >> 8)
	p.Pix[i+5] = uint8(c.B)
	p.Pix[i+6] = uint8(c.A >> 8)
	p.Pix[i+7] = uint8(c.A)
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *RGBA64) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &RGBA64{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &RGBA64{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *RGBA64) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 6, p.Rect.Dx()*8
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i += 8 {
			if p.Pix[i+0] != 0xff || p.Pix[i+1] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewRGBA64 returns a new RGBA64 with the given bounds.
func NewRGBA64(r Rectangle) *RGBA64 {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 8*w*h)
	return &RGBA64{pix, 8 * w, r}
}

// NRGBA is an in-memory image of NRGBAColor values.
type NRGBA struct {
	// Pix holds the image's pixels, in R, G, B, A order. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*4].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *NRGBA) ColorModel() color.Model { return color.NRGBAModel }

func (p *NRGBA) Bounds() Rectangle { return p.Rect }

func (p *NRGBA) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.NRGBA{}
	}
	i := p.PixOffset(x, y)
	return color.NRGBA{p.Pix[i+0], p.Pix[i+1], p.Pix[i+2], p.Pix[i+3]}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *NRGBA) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*4
}

func (p *NRGBA) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.NRGBAModel.Convert(c).(color.NRGBA)
	p.Pix[i+0] = c1.R
	p.Pix[i+1] = c1.G
	p.Pix[i+2] = c1.B
	p.Pix[i+3] = c1.A
}

func (p *NRGBA) SetNRGBA(x, y int, c color.NRGBA) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = c.R
	p.Pix[i+1] = c.G
	p.Pix[i+2] = c.B
	p.Pix[i+3] = c.A
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *NRGBA) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &NRGBA{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &NRGBA{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *NRGBA) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 3, p.Rect.Dx()*4
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i += 4 {
			if p.Pix[i] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewNRGBA returns a new NRGBA with the given bounds.
func NewNRGBA(r Rectangle) *NRGBA {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 4*w*h)
	return &NRGBA{pix, 4 * w, r}
}

// NRGBA64 is an in-memory image of NRGBA64Color values.
type NRGBA64 struct {
	// Pix holds the image's pixels, in R, G, B, A order and big-endian format. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*8].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *NRGBA64) ColorModel() color.Model { return color.NRGBA64Model }

func (p *NRGBA64) Bounds() Rectangle { return p.Rect }

func (p *NRGBA64) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.NRGBA64{}
	}
	i := p.PixOffset(x, y)
	return color.NRGBA64{
		uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1]),
		uint16(p.Pix[i+2])<<8 | uint16(p.Pix[i+3]),
		uint16(p.Pix[i+4])<<8 | uint16(p.Pix[i+5]),
		uint16(p.Pix[i+6])<<8 | uint16(p.Pix[i+7]),
	}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *NRGBA64) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*8
}

func (p *NRGBA64) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.NRGBA64Model.Convert(c).(color.NRGBA64)
	p.Pix[i+0] = uint8(c1.R >> 8)
	p.Pix[i+1] = uint8(c1.R)
	p.Pix[i+2] = uint8(c1.G >> 8)
	p.Pix[i+3] = uint8(c1.G)
	p.Pix[i+4] = uint8(c1.B >> 8)
	p.Pix[i+5] = uint8(c1.B)
	p.Pix[i+6] = uint8(c1.A >> 8)
	p.Pix[i+7] = uint8(c1.A)
}

func (p *NRGBA64) SetNRGBA64(x, y int, c color.NRGBA64) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = uint8(c.R >> 8)
	p.Pix[i+1] = uint8(c.R)
	p.Pix[i+2] = uint8(c.G >> 8)
	p.Pix[i+3] = uint8(c.G)
	p.Pix[i+4] = uint8(c.B >> 8)
	p.Pix[i+5] = uint8(c.B)
	p.Pix[i+6] = uint8(c.A >> 8)
	p.Pix[i+7] = uint8(c.A)
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *NRGBA64) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &NRGBA64{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &NRGBA64{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *NRGBA64) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 6, p.Rect.Dx()*8
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i += 8 {
			if p.Pix[i+0] != 0xff || p.Pix[i+1] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewNRGBA64 returns a new NRGBA64 with the given bounds.
func NewNRGBA64(r Rectangle) *NRGBA64 {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 8*w*h)
	return &NRGBA64{pix, 8 * w, r}
}

// Alpha is an in-memory image of AlphaColor values.
type Alpha struct {
	// Pix holds the image's pixels, as alpha values. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*1].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *Alpha) ColorModel() color.Model { return color.AlphaModel }

func (p *Alpha) Bounds() Rectangle { return p.Rect }

func (p *Alpha) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.Alpha{}
	}
	i := p.PixOffset(x, y)
	return color.Alpha{p.Pix[i]}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *Alpha) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
}

func (p *Alpha) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = color.AlphaModel.Convert(c).(color.Alpha).A
}

func (p *Alpha) SetAlpha(x, y int, c color.Alpha) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = c.A
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *Alpha) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &Alpha{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &Alpha{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *Alpha) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 0, p.Rect.Dx()
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i++ {
			if p.Pix[i] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewAlpha returns a new Alpha with the given bounds.
func NewAlpha(r Rectangle) *Alpha {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 1*w*h)
	return &Alpha{pix, 1 * w, r}
}

// Alpha16 is an in-memory image of Alpha16Color values.
type Alpha16 struct {
	// Pix holds the image's pixels, as alpha values in big-endian format. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*2].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *Alpha16) ColorModel() color.Model { return color.Alpha16Model }

func (p *Alpha16) Bounds() Rectangle { return p.Rect }

func (p *Alpha16) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.Alpha16{}
	}
	i := p.PixOffset(x, y)
	return color.Alpha16{uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1])}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *Alpha16) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
}

func (p *Alpha16) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.Alpha16Model.Convert(c).(color.Alpha16)
	p.Pix[i+0] = uint8(c1.A >> 8)
	p.Pix[i+1] = uint8(c1.A)
}

func (p *Alpha16) SetAlpha16(x, y int, c color.Alpha16) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = uint8(c.A >> 8)
	p.Pix[i+1] = uint8(c.A)
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *Alpha16) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &Alpha16{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &Alpha16{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *Alpha16) Opaque() bool {
	if p.Rect.Empty() {
		return true
	}
	i0, i1 := 0, p.Rect.Dx()*2
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for i := i0; i < i1; i += 2 {
			if p.Pix[i+0] != 0xff || p.Pix[i+1] != 0xff {
				return false
			}
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	return true
}

// NewAlpha16 returns a new Alpha16 with the given bounds.
func NewAlpha16(r Rectangle) *Alpha16 {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 2*w*h)
	return &Alpha16{pix, 2 * w, r}
}

// Gray is an in-memory image of GrayColor values.
type Gray struct {
	// Pix holds the image's pixels, as gray values. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*1].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *Gray) ColorModel() color.Model { return color.GrayModel }

func (p *Gray) Bounds() Rectangle { return p.Rect }

func (p *Gray) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.Gray{}
	}
	i := p.PixOffset(x, y)
	return color.Gray{p.Pix[i]}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *Gray) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
}

func (p *Gray) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = color.GrayModel.Convert(c).(color.Gray).Y
}

func (p *Gray) SetGray(x, y int, c color.Gray) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = c.Y
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *Gray) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &Gray{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &Gray{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *Gray) Opaque() bool {
	return true
}

// NewGray returns a new Gray with the given bounds.
func NewGray(r Rectangle) *Gray {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 1*w*h)
	return &Gray{pix, 1 * w, r}
}

// Gray16 is an in-memory image of Gray16Color values.
type Gray16 struct {
	// Pix holds the image's pixels, as gray values in big-endian format. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*2].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
}

func (p *Gray16) ColorModel() color.Model { return color.Gray16Model }

func (p *Gray16) Bounds() Rectangle { return p.Rect }

func (p *Gray16) At(x, y int) color.Color {
	if !(Point{x, y}.In(p.Rect)) {
		return color.Gray16{}
	}
	i := p.PixOffset(x, y)
	return color.Gray16{uint16(p.Pix[i+0])<<8 | uint16(p.Pix[i+1])}
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *Gray16) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*2
}

func (p *Gray16) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	c1 := color.Gray16Model.Convert(c).(color.Gray16)
	p.Pix[i+0] = uint8(c1.Y >> 8)
	p.Pix[i+1] = uint8(c1.Y)
}

func (p *Gray16) SetGray16(x, y int, c color.Gray16) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i+0] = uint8(c.Y >> 8)
	p.Pix[i+1] = uint8(c.Y)
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *Gray16) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &Gray16{}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &Gray16{
		Pix:    p.Pix[i:],
		Stride: p.Stride,
		Rect:   r,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *Gray16) Opaque() bool {
	return true
}

// NewGray16 returns a new Gray16 with the given bounds.
func NewGray16(r Rectangle) *Gray16 {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 2*w*h)
	return &Gray16{pix, 2 * w, r}
}

// Paletted is an in-memory image of uint8 indices into a given palette.
type Paletted struct {
	// Pix holds the image's pixels, as palette indices. The pixel at
	// (x, y) starts at Pix[(y-Rect.Min.Y)*Stride + (x-Rect.Min.X)*1].
	Pix []uint8
	// Stride is the Pix stride (in bytes) between vertically adjacent pixels.
	Stride int
	// Rect is the image's bounds.
	Rect Rectangle
	// Palette is the image's palette.
	Palette color.Palette
}

func (p *Paletted) ColorModel() color.Model { return p.Palette }

func (p *Paletted) Bounds() Rectangle { return p.Rect }

func (p *Paletted) At(x, y int) color.Color {
	if len(p.Palette) == 0 {
		return nil
	}
	if !(Point{x, y}.In(p.Rect)) {
		return p.Palette[0]
	}
	i := p.PixOffset(x, y)
	return p.Palette[p.Pix[i]]
}

// PixOffset returns the index of the first element of Pix that corresponds to
// the pixel at (x, y).
func (p *Paletted) PixOffset(x, y int) int {
	return (y-p.Rect.Min.Y)*p.Stride + (x-p.Rect.Min.X)*1
}

func (p *Paletted) Set(x, y int, c color.Color) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = uint8(p.Palette.Index(c))
}

func (p *Paletted) ColorIndexAt(x, y int) uint8 {
	if !(Point{x, y}.In(p.Rect)) {
		return 0
	}
	i := p.PixOffset(x, y)
	return p.Pix[i]
}

func (p *Paletted) SetColorIndex(x, y int, index uint8) {
	if !(Point{x, y}.In(p.Rect)) {
		return
	}
	i := p.PixOffset(x, y)
	p.Pix[i] = index
}

// SubImage returns an image representing the portion of the image p visible
// through r. The returned value shares pixels with the original image.
func (p *Paletted) SubImage(r Rectangle) Image {
	r = r.Intersect(p.Rect)
	// If r1 and r2 are Rectangles, r1.Intersect(r2) is not guaranteed to be inside
	// either r1 or r2 if the intersection is empty. Without explicitly checking for
	// this, the Pix[i:] expression below can panic.
	if r.Empty() {
		return &Paletted{
			Palette: p.Palette,
		}
	}
	i := p.PixOffset(r.Min.X, r.Min.Y)
	return &Paletted{
		Pix:     p.Pix[i:],
		Stride:  p.Stride,
		Rect:    p.Rect.Intersect(r),
		Palette: p.Palette,
	}
}

// Opaque scans the entire image and returns whether or not it is fully opaque.
func (p *Paletted) Opaque() bool {
	var present [256]bool
	i0, i1 := 0, p.Rect.Dx()
	for y := p.Rect.Min.Y; y < p.Rect.Max.Y; y++ {
		for _, c := range p.Pix[i0:i1] {
			present[c] = true
		}
		i0 += p.Stride
		i1 += p.Stride
	}
	for i, c := range p.Palette {
		if !present[i] {
			continue
		}
		_, _, _, a := c.RGBA()
		if a != 0xffff {
			return false
		}
	}
	return true
}

// NewPaletted returns a new Paletted with the given width, height and palette.
func NewPaletted(r Rectangle, p color.Palette) *Paletted {
	w, h := r.Dx(), r.Dy()
	pix := make([]uint8, 1*w*h)
	return &Paletted{pix, 1 * w, r, p}
}