ds-galo/backend/internal/photos/effects_test.go

package photos

import (
	"bytes"
	"encoding/json"
	"image"
	"math"
	"reflect"
	"testing"

	"github.com/disintegration/imaging"
)

func TestEffectsParams(t *testing.T) {
	t.Run("defaults", func(t *testing.T) {
		got := (EffectsParams{}).Normalized()
		if got.Texture != 0 || got.Clarity != 0 || got.Dehaze != 0 {
			t.Fatalf("strengths changed: %+v", got)
		}
		v := got.Vignette
		if v.Mode != VignetteLight || v.Midpoint != DefaultVignetteMidpoint ||
			v.Feather != DefaultVignetteFeather || v.Roundness != 0 || v.Highlights != 0 ||
			v.Color != DefaultVignetteColor || v.Amount != 0 {
			t.Fatalf("vignette defaults = %+v", v)
		}
		g := got.Grain
		if g.Amount != 0 || g.Size != DefaultGrainSize || g.Roughness != DefaultGrainRoughness {
			t.Fatalf("grain defaults = %+v", g)
		}
	})

	t.Run("clamps", func(t *testing.T) {
		got := EffectsParams{
			Texture: 250,
			Clarity: -250,
			Dehaze:  150,
			Vignette: VignetteParams{
				Mode: "sepia", Amount: -150, Midpoint: 150, Feather: 150,
				Roundness: -150, Highlights: 150, Color: "#AABBCC",
			},
			Grain: GrainParams{Amount: 150, Size: 150, Roughness: 150},
		}.Normalized()
		if got.Texture != MaxPercent || got.Clarity != -MaxPercent || got.Dehaze != MaxPercent {
			t.Fatalf("strengths = %v/%v/%v, want clamped", got.Texture, got.Clarity, got.Dehaze)
		}
		v := got.Vignette
		if v.Mode != VignetteLight {
			t.Fatalf("unknown mode = %q, want %q", v.Mode, VignetteLight)
		}
		if v.Amount != -MaxPercent || v.Midpoint != MaxPercent || v.Feather != MaxPercent ||
			v.Roundness != -MaxPercent || v.Highlights != MaxPercent {
			t.Fatalf("vignette not clamped: %+v", v)
		}
		if v.Color != "#aabbcc" {
			t.Fatalf("color = %q, want lower-case", v.Color)
		}
		if got.Grain.Amount != MaxPercent || got.Grain.Size != MaxPercent || got.Grain.Roughness != MaxPercent {
			t.Fatalf("grain not clamped: %+v", got.Grain)
		}
	})

	t.Run("modes and colors", func(t *testing.T) {
		cases := []struct {
			mode, want string
		}{
			{"", VignetteLight},
			{"light", VignetteLight},
			{"LIGHT", VignetteLight},
			{"color", VignetteColor},
			{"Colorize", VignetteColorize},
			{"unknown", VignetteLight},
		}
		for _, tc := range cases {
			if got := (VignetteParams{Mode: tc.mode}).Normalized().Mode; got != tc.want {
				t.Fatalf("mode %q = %q, want %q", tc.mode, got, tc.want)
			}
		}
		colors := []struct {
			in, want string
		}{
			{"#A1B2C3", "#a1b2c3"},
			{"#abcdef", "#abcdef"},
			{"#12345", DefaultVignetteColor},
			{"#gggggg", DefaultVignetteColor},
			{"aabbcc", DefaultVignetteColor},
			{"red", DefaultVignetteColor},
			{"", DefaultVignetteColor},
		}
		for _, tc := range colors {
			if got := normalizeHexColor(tc.in); got != tc.want {
				t.Fatalf("color %q = %q, want %q", tc.in, got, tc.want)
			}
		}
	})

	t.Run("idempotent", func(t *testing.T) {
		p := EffectsParams{
			Texture: 12.5, Clarity: -8, Dehaze: 33,
			Vignette: VignetteParams{Mode: "COLOR", Amount: -25, Midpoint: 40, Feather: 70, Roundness: 20, Highlights: 10, Color: "#FF8000"},
			Grain:    GrainParams{Amount: 45, Size: 33, Roughness: 66},
		}
		once := p.Normalized()
		twice := once.Normalized()
		if !reflect.DeepEqual(once, twice) {
			t.Fatalf("normalization is not idempotent:\n once = %+v\ntwice = %+v", once, twice)
		}
	})

	t.Run("is empty", func(t *testing.T) {
		cases := []struct {
			name  string
			p     EffectsParams
			empty bool
		}{
			{"zero", EffectsParams{}, true},
			{
				"geometry only",
				EffectsParams{
					Vignette: VignetteParams{
						Mode: VignetteColor, Midpoint: 20, Feather: 80,
						Roundness: 40, Highlights: 50, Color: "#ff0000",
					},
					Grain: GrainParams{Size: 90, Roughness: 90},
				},
				true,
			},
			{"texture", EffectsParams{Texture: 1}, false},
			{"clarity", EffectsParams{Clarity: -1}, false},
			{"dehaze", EffectsParams{Dehaze: 1}, false},
			{"vignette", EffectsParams{Vignette: VignetteParams{Amount: -1}}, false},
			{"grain", EffectsParams{Grain: GrainParams{Amount: 1}}, false},
		}
		for _, tc := range cases {
			if got := tc.p.IsEmpty(); got != tc.empty {
				t.Fatalf("%s: IsEmpty = %v, want %v", tc.name, got, tc.empty)
			}
		}
	})

	t.Run("explicit zero geometry", func(t *testing.T) {
		// An explicit zero is a valid geometry value and must survive both
		// parsing and normalization; omitted fields still get the defaults.
		p, err := ParseEditParams([]byte(`{"effects":{"vignette":{"mode":"light","amount":10,"midpoint":0,"feather":0},"grain":{"amount":10,"size":0,"roughness":0}}}`))
		if err != nil {
			t.Fatalf("parse explicit zero: %v", err)
		}
		if p.Effects.Vignette.Midpoint != 0 || p.Effects.Vignette.Feather != 0 {
			t.Fatalf("explicit zero vignette geometry was defaulted: %+v", p.Effects.Vignette)
		}
		if p.Effects.Grain.Size != 0 || p.Effects.Grain.Roughness != 0 {
			t.Fatalf("explicit zero grain geometry was defaulted: %+v", p.Effects.Grain)
		}

		omitted, err := ParseEditParams([]byte(`{"effects":{"vignette":{"amount":10},"grain":{"amount":10}}}`))
		if err != nil {
			t.Fatalf("parse omitted: %v", err)
		}
		if omitted.Effects.Vignette.Midpoint != DefaultVignetteMidpoint ||
			omitted.Effects.Vignette.Feather != DefaultVignetteFeather ||
			omitted.Effects.Grain.Size != DefaultGrainSize ||
			omitted.Effects.Grain.Roughness != DefaultGrainRoughness {
			t.Fatalf("omitted geometry was not defaulted: %+v", omitted.Effects)
		}
	})

	t.Run("json", func(t *testing.T) {
		p := EditParams{Effects: EffectsParams{Dehaze: 10}}
		data := p.JSON()
		var raw map[string]json.RawMessage
		if err := json.Unmarshal(data, &raw); err != nil {
			t.Fatalf("unmarshal: %v", err)
		}
		if _, ok := raw["effects"]; !ok {
			t.Fatalf("effects key missing from %s", data)
		}
		var effects EffectsParams
		if err := json.Unmarshal(raw["effects"], &effects); err != nil {
			t.Fatalf("unmarshal effects: %v", err)
		}
		if effects.Dehaze != 10 {
			t.Fatalf("dehaze = %v, want 10", effects.Dehaze)
		}
		var body string
		if err := json.Unmarshal(data, &body); err == nil {
			t.Fatalf("unexpected string body %q", body)
		}
	})

	t.Run("edit params count effects", func(t *testing.T) {
		if (EditParams{Effects: EffectsParams{Texture: 1}}).IsEmpty() {
			t.Fatal("params with texture must not be empty")
		}
		if (EditParams{Effects: EffectsParams{Vignette: VignetteParams{Mode: VignetteColor}}}).IsEmpty() != true {
			t.Fatal("vignette defaults without strength must count as empty")
		}
	})
}

func TestEditParamsLegacy(t *testing.T) {
	empty, err := ParseEditParams([]byte(`{}`))
	if err != nil {
		t.Fatalf("parse empty: %v", err)
	}
	if !empty.IsEmpty() || !empty.Effects.IsEmpty() {
		t.Fatalf("empty params = %+v", empty)
	}
	if empty.Effects.Vignette.Midpoint != DefaultVignetteMidpoint ||
		empty.Effects.Vignette.Color != DefaultVignetteColor ||
		empty.Effects.Grain.Size != DefaultGrainSize {
		t.Fatalf("legacy defaults not filled: %+v", empty.Effects)
	}

	legacy, err := ParseEditParams([]byte(`{"light":{"exposure":0.4},"color":{"temperature":15,"saturation":10}}`))
	if err != nil {
		t.Fatalf("parse legacy: %v", err)
	}
	if legacy.IsEmpty() {
		t.Fatal("legacy light/color params must not be empty")
	}
	if !legacy.Effects.IsEmpty() {
		t.Fatalf("params without effects key read as effects: %+v", legacy.Effects)
	}

	// The old pipeline (light, then color) renders exactly the same pixels when
	// the parameters carry no effects.
	src := gradientImage(16, 12)
	data := mustEncodePNG(t, src)
	got, err := decodeAndAdjust(data, legacy, editSeed("legacy"))
	if err != nil {
		t.Fatalf("decode and adjust: %v", err)
	}
	decoded, err := imaging.Decode(bytes.NewReader(data), imaging.AutoOrientation(true))
	if err != nil {
		t.Fatalf("decode: %v", err)
	}
	want := applyColor(applyLight(decoded, legacy.Light), legacy.Color)
	if !bytes.Equal(got.Pix, want.Pix) {
		t.Fatal("legacy render changed when effects are absent")
	}

	// An untouched photo decodes to the original pixels.
	plain, err := decodeAndAdjust(data, EditParams{}, editSeed("legacy"))
	if err != nil {
		t.Fatalf("decode and adjust untouched: %v", err)
	}
	if !bytes.Equal(plain.Pix, imaging.Clone(decoded).Pix) {
		t.Fatal("empty params changed the original")
	}
}

func TestBoxBlur(t *testing.T) {
	if got := boxBlur([]uint8{1, 2, 3, 4}, 2, 2, 0); !bytes.Equal(got, []uint8{1, 2, 3, 4}) {
		t.Fatalf("radius 0 = %v, want identity", got)
	}

	constant := make([]uint8, 9*7)
	for i := range constant {
		constant[i] = 77
	}
	blurred := boxBlur3(constant, 9, 7, 3)
	for i, v := range blurred {
		if v != 77 {
			t.Fatalf("constant image changed at %d: %d", i, v)
		}
	}

	w, h := 13, 9
	src := make([]uint8, w*h)
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			src[y*w+x] = uint8((x*37 + y*61) % 256)
		}
	}
	for _, radius := range []int{1, 2, 5} {
		if want, got := naiveBoxPass(src, w, h, radius), boxBlur(src, w, h, radius); !bytes.Equal(got, want) {
			t.Fatalf("radius %d: boxBlur = %v, want %v", radius, got, want)
		}
		want := src
		for i := 0; i < 3; i++ {
			want = naiveBoxPass(want, w, h, radius)
		}
		if got := boxBlur3(src, w, h, radius); !bytes.Equal(got, want) {
			t.Fatalf("radius %d: boxBlur3 = %v, want %v", radius, got, want)
		}
	}

	// A one-pixel image is a fixed point of any radius.
	if got := boxBlur3([]uint8{42}, 1, 1, 4); len(got) != 1 || got[0] != 42 {
		t.Fatalf("1x1 blur = %v, want [42]", got)
	}
}

// naiveBoxPass is the reference implementation of one box-blur pass: the
// window is sampled explicitly with edge-clamped coordinates.
func naiveBoxPass(src []uint8, w, h, radius int) []uint8 {
	window := 2*radius + 1
	tmp := make([]uint8, w*h)
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			sum := 0
			for kx := -radius; kx <= radius; kx++ {
				sum += int(src[y*w+clampInt(x+kx, 0, w-1)])
			}
			tmp[y*w+x] = uint8((sum + window/2) / window)
		}
	}
	out := make([]uint8, w*h)
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			sum := 0
			for ky := -radius; ky <= radius; ky++ {
				sum += int(tmp[clampInt(y+ky, 0, h-1)*w+x])
			}
			out[y*w+x] = uint8((sum + window/2) / window)
		}
	}
	return out
}

func TestTextureClarity(t *testing.T) {
	src := checkerImage(64, 64, 96, 160)
	base := lumaLocalVariance(src)
	for _, effect := range []string{"texture", "clarity"} {
		up := applyEffects(imaging.Clone(src), strengths(effect, 100), 1)
		down := applyEffects(imaging.Clone(src), strengths(effect, -100), 1)
		if got := lumaLocalVariance(up); got <= base {
			t.Fatalf("%s +100: local variance %v must grow above %v", effect, got, base)
		}
		if got := lumaLocalVariance(down); got >= base {
			t.Fatalf("%s -100: local variance %v must drop below %v", effect, got, base)
		}
	}
}

// lumaLocalVariance is the average variance of the luminance inside a 3x3
// window: the property that texture and clarity change.
func lumaLocalVariance(img *image.NRGBA) float64 {
	plane := lumaPlane(img)
	w, h := img.Rect.Dx(), img.Rect.Dy()
	var total float64
	count := 0
	for y := 1; y < h-1; y++ {
		for x := 1; x < w-1; x++ {
			var sum, sumSq float64
			for dy := -1; dy <= 1; dy++ {
				for dx := -1; dx <= 1; dx++ {
					v := float64(plane[(y+dy)*w+x+dx])
					sum += v
					sumSq += v * v
				}
			}
			mean := sum / 9
			total += sumSq/9 - mean*mean
			count++
		}
	}
	return total / float64(count)
}

// strengths builds a single-effect parameter block.
func strengths(effect string, value float64) EffectsParams {
	switch effect {
	case "texture":
		return EffectsParams{Texture: value}
	case "clarity":
		return EffectsParams{Clarity: value}
	}
	return EffectsParams{}
}

func TestDehaze(t *testing.T) {
	clear := hazyScene(128, 96)
	hazy := addHaze(clear, 0.45, 235)

	baseContrast := lumaStdDev(hazy)
	baseSaturation := meanSaturation(hazy)
	removed := applyEffects(imaging.Clone(hazy), EffectsParams{Dehaze: 100}, 1)
	t.Logf("contrast %.2f -> %.2f, saturation %.2f -> %.2f",
		baseContrast, lumaStdDev(removed), baseSaturation, meanSaturation(removed))
	if got := lumaStdDev(removed); got <= baseContrast {
		t.Fatalf("dehaze +100: contrast %v did not grow above %v", got, baseContrast)
	}
	if got := meanSaturation(removed); got <= baseSaturation {
		t.Fatalf("dehaze +100: saturation %v did not grow above %v", got, baseSaturation)
	}

	clearContrast := lumaStdDev(clear)
	clearSaturation := meanSaturation(clear)
	added := applyEffects(imaging.Clone(clear), EffectsParams{Dehaze: -100}, 1)
	t.Logf("clear contrast %.2f -> %.2f, saturation %.2f -> %.2f",
		clearContrast, lumaStdDev(added), clearSaturation, meanSaturation(added))
	if got := lumaStdDev(added); got >= clearContrast {
		t.Fatalf("dehaze -100: contrast %v did not drop below %v", got, clearContrast)
	}
	if got := meanSaturation(added); got >= clearSaturation {
		t.Fatalf("dehaze -100: saturation %v did not drop below %v", got, clearSaturation)
	}
}

func TestResizeTransmission(t *testing.T) {
	// A vertical step map: the top half is dark, the bottom half is bright.
	sw, sh := 16, 12
	transmission := make([]uint8, sw*sh)
	for y := 0; y < sh; y++ {
		for x := 0; x < sw; x++ {
			if y < sh/2 {
				transmission[y*sw+x] = 20
			} else {
				transmission[y*sw+x] = 230
			}
		}
	}
	gray := image.NewGray(image.Rect(0, 0, sw, sh))
	copy(gray.Pix, transmission)
	out := resizeTransmission(gray, 128, 96)
	if len(out) != 128*96 {
		t.Fatalf("resized map length = %d, want %d", len(out), 128*96)
	}
	// The transition band is blurred by the interpolation, so the edges are
	// checked away from the step.
	for y := 0; y < 96; y++ {
		if y >= 40 && y < 56 {
			continue
		}
		want := uint8(20)
		if y >= 56 {
			want = 230
		}
		for x := 0; x < 128; x++ {
			if got := out[y*128+x]; got < want-4 || got > want+4 {
				t.Fatalf("pixel %d,%d = %d, want around %d", x, y, got, want)
			}
		}
	}
}

func TestVignette(t *testing.T) {
	src := solidImage(64, 64, 128)
	const centerX, centerY = 32, 32
	const cornerX, cornerY = 0, 0

	t.Run("light mode", func(t *testing.T) {
		dark := applyEffects(imaging.Clone(src), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteLight, Amount: 80},
		}, 0)
		if pixelAt(dark, centerX, centerY) != 128 {
			t.Fatalf("center changed: %d", pixelAt(dark, centerX, centerY))
		}
		if pixelAt(dark, cornerX, cornerY) >= 128 {
			t.Fatalf("positive amount did not darken the corner: %d", pixelAt(dark, cornerX, cornerY))
		}

		bright := applyEffects(imaging.Clone(src), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteLight, Amount: -80},
		}, 0)
		if pixelAt(bright, centerX, centerY) != 128 {
			t.Fatalf("center changed: %d", pixelAt(bright, centerX, centerY))
		}
		if pixelAt(bright, cornerX, cornerY) <= 128 {
			t.Fatalf("negative amount did not brighten the corner: %d", pixelAt(bright, cornerX, cornerY))
		}
	})

	t.Run("zero amount is identity", func(t *testing.T) {
		out := applyEffects(imaging.Clone(src), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteColor, Amount: 0, Color: "#ff0000"},
		}, 0)
		if !bytes.Equal(out.Pix, src.Pix) {
			t.Fatal("zero vignette changed the image")
		}
	})

	t.Run("color mode tints the edges", func(t *testing.T) {
		out := applyEffects(imaging.Clone(src), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteColor, Amount: 100, Color: "#ff0000"},
		}, 0)
		r, g, b := rgbAt(out, cornerX, cornerY)
		if !(r > g && r > b) {
			t.Fatalf("color vignette corner = %d/%d/%d, want a red tint", r, g, b)
		}
		if r, g, b := rgbAt(out, centerX, centerY); r != 128 || g != 128 || b != 128 {
			t.Fatalf("color vignette changed the center: %d/%d/%d", r, g, b)
		}
	})

	t.Run("colorize mode fills the edges", func(t *testing.T) {
		out := applyEffects(imaging.Clone(src), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteColorize, Amount: 100, Color: "#ff0000"},
		}, 0)
		if r, g, b := rgbAt(out, cornerX, cornerY); r != 255 || g != 0 || b != 0 {
			t.Fatalf("colorize corner = %d/%d/%d, want 255/0/0", r, g, b)
		}
		if r, g, b := rgbAt(out, centerX, centerY); r != 128 || g != 128 || b != 128 {
			t.Fatalf("colorize changed the center: %d/%d/%d", r, g, b)
		}
	})

	t.Run("highlight protection", func(t *testing.T) {
		bright := solidImage(64, 64, 240)
		plain := applyEffects(imaging.Clone(bright), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteLight, Amount: 100},
		}, 0)
		protected := applyEffects(imaging.Clone(bright), EffectsParams{
			Vignette: VignetteParams{Mode: VignetteLight, Amount: 100, Highlights: 100},
		}, 0)
		if pixelAt(protected, cornerX, cornerY) <= pixelAt(plain, cornerX, cornerY) {
			t.Fatalf("highlight protection did not preserve the corner: %d <= %d",
				pixelAt(protected, cornerX, cornerY), pixelAt(plain, cornerX, cornerY))
		}
	})
}

func TestGrainDeterminism(t *testing.T) {
	src := gradientImage(64, 64)
	grain := EffectsParams{Grain: GrainParams{Amount: 70, Size: 30, Roughness: 60}}

	first := applyEffects(imaging.Clone(src), grain, 12345)
	second := applyEffects(imaging.Clone(src), grain, 12345)
	if !bytes.Equal(first.Pix, second.Pix) {
		t.Fatal("two renders with the same parameters and seed differ")
	}
	if bytes.Equal(first.Pix, src.Pix) {
		t.Fatal("grain did not change the image")
	}

	other := applyEffects(imaging.Clone(src), grain, 999)
	if bytes.Equal(first.Pix, other.Pix) {
		t.Fatal("a different seed produced the same grain")
	}

	zero := applyEffects(imaging.Clone(src), EffectsParams{
		Grain: GrainParams{Amount: 0, Size: 30, Roughness: 60},
	}, 12345)
	if !bytes.Equal(zero.Pix, src.Pix) {
		t.Fatal("zero grain strength changed the image")
	}

	// The full render path seeds the same noise from the photo id.
	data := mustEncodePNG(t, src)
	a, err := decodeAndAdjust(data, EditParams{Effects: grain}, editSeed("photo-1"))
	if err != nil {
		t.Fatalf("decode and adjust: %v", err)
	}
	b, err := decodeAndAdjust(data, EditParams{Effects: grain}, editSeed("photo-1"))
	if err != nil {
		t.Fatalf("decode and adjust again: %v", err)
	}
	if !bytes.Equal(a.Pix, b.Pix) {
		t.Fatal("repeated decodeAndAdjust renders differ")
	}
}

func TestEffectsKeepTransparentPixels(t *testing.T) {
	src := image.NewNRGBA(image.Rect(0, 0, 32, 32))
	for i := 0; i < len(src.Pix); i += 4 {
		src.Pix[i], src.Pix[i+1], src.Pix[i+2], src.Pix[i+3] = 200, 10, 30, 0
	}
	before := append([]uint8(nil), src.Pix...)
	out := applyEffects(src, EffectsParams{
		Texture: 60,
		Clarity: 60,
		Dehaze:  60,
		Vignette: VignetteParams{
			Mode: VignetteColorize, Amount: 100, Color: "#00ff00",
		},
		Grain: GrainParams{Amount: 100, Size: 40, Roughness: 80},
	}, 7)
	if !bytes.Equal(out.Pix, before) {
		t.Fatal("effects changed transparent pixels")
	}
}

// solidImage returns a uniform opaque image.
func solidImage(w, h int, value uint8) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, w, h))
	for i := 0; i < len(img.Pix); i += 4 {
		img.Pix[i] = value
		img.Pix[i+1] = value
		img.Pix[i+2] = value
		img.Pix[i+3] = 255
	}
	return img
}

// gradientImage returns an opaque image with smooth horizontal and vertical
// variation, so every effect has something to work with.
func gradientImage(w, h int) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, w, h))
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*img.Stride + x*4
			img.Pix[i] = uint8(20 + x*3)
			img.Pix[i+1] = uint8(30 + y*3)
			img.Pix[i+2] = uint8(40 + (x + y))
			img.Pix[i+3] = 255
		}
	}
	return img
}

// checkerImage returns a fine checkerboard, the high-frequency input of the
// local-contrast tests.
func checkerImage(w, h int, dark, light uint8) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, w, h))
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*img.Stride + x*4
			v := dark
			if (x+y)%2 == 0 {
				v = light
			}
			img.Pix[i] = v
			img.Pix[i+1] = v
			img.Pix[i+2] = v
			img.Pix[i+3] = 255
		}
	}
	return img
}

// hazyScene builds a synthetic landscape: a bright sky band and a colored
// checker with a brightness gradient. It represents the "clear" frame.
func hazyScene(w, h int) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, w, h))
	sky := h / 6
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*img.Stride + x*4
			if y < sky {
				img.Pix[i], img.Pix[i+1], img.Pix[i+2] = 235, 235, 235
				img.Pix[i+3] = 255
				continue
			}
			t := float64(x) / float64(w-1)
			if (x/16+y/16)%2 == 0 {
				img.Pix[i] = uint8(40 + 180*t)
				img.Pix[i+1] = uint8(40 + 60*t)
				img.Pix[i+2] = uint8(40 + 40*t)
			} else {
				img.Pix[i] = uint8(40 + 40*t)
				img.Pix[i+1] = uint8(40 + 120*t)
				img.Pix[i+2] = uint8(40 + 180*t)
			}
			img.Pix[i+3] = 255
		}
	}
	return img
}

// addHaze mixes the frame toward an atmospheric color.
func addHaze(src *image.NRGBA, haze float64, atmosphere uint8) *image.NRGBA {
	out := imaging.Clone(src)
	w, h := out.Rect.Dx(), out.Rect.Dy()
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*out.Stride + x*4
			for c := 0; c < 3; c++ {
				out.Pix[i+c] = uint8(float64(out.Pix[i+c])*(1-haze) + float64(atmosphere)*haze + 0.5)
			}
		}
	}
	return out
}

// lumaVariance is the global variance of the luminance.
func lumaVariance(img *image.NRGBA) float64 {
	plane := lumaPlane(img)
	var sum float64
	for _, v := range plane {
		sum += float64(v)
	}
	mean := sum / float64(len(plane))
	var variance float64
	for _, v := range plane {
		d := float64(v) - mean
		variance += d * d
	}
	return variance / float64(len(plane))
}

// lumaStdDev is the contrast measure of the dehaze test.
func lumaStdDev(img *image.NRGBA) float64 {
	return math.Sqrt(lumaVariance(img))
}

// meanSaturation is the average channel spread of the image.
func meanSaturation(img *image.NRGBA) float64 {
	w, h := img.Rect.Dx(), img.Rect.Dy()
	var sum float64
	for y := 0; y < h; y++ {
		for x := 0; x < w; x++ {
			i := y*img.Stride + x*4
			mx := max(img.Pix[i], max(img.Pix[i+1], img.Pix[i+2]))
			mn := min(img.Pix[i], min(img.Pix[i+1], img.Pix[i+2]))
			sum += float64(mx - mn)
		}
	}
	return sum / float64(w*h)
}

func pixelAt(img *image.NRGBA, x, y int) uint8 {
	return img.Pix[y*img.Stride+x*4]
}

func rgbAt(img *image.NRGBA, x, y int) (uint8, uint8, uint8) {
	i := y*img.Stride + x*4
	return img.Pix[i], img.Pix[i+1], img.Pix[i+2]
}