From 6baa77e7ae721a16c9009c3f8e0529825a1c5617 Mon Sep 17 00:00:00 2001
From: Jacek Olszak <jacekolszak@gmail.com>
Date: Fri, 25 Jul 2025 15:31:34 +0200
Subject: [PATCH] feat: use perceptual color comparison in pi.DecodeCanvas

Use perceptual color comparison in pi.DecodeCanvas for more accurate matching.
---
 internal/color.go                         |  19 ++++++----
 internal/test/decode/indexed-brighter.png | Bin 0 -> 161 bytes
 internal/test/decode/indexed.png          | Bin 0 -> 161 bytes
 internal/test/decode/rgb-brighter.png     | Bin 0 -> 133 bytes
 internal/test/decode/rgb.png              | Bin 0 -> 133 bytes
 surface_test.go                           |  41 ++++++++++++++++++++++
 6 files changed, 54 insertions(+), 6 deletions(-)
 create mode 100644 internal/test/decode/indexed-brighter.png
 create mode 100644 internal/test/decode/indexed.png
 create mode 100644 internal/test/decode/rgb-brighter.png
 create mode 100644 internal/test/decode/rgb.png

diff --git a/internal/color.go b/internal/color.go
index 6b2dcd5b..c945bdd6 100644
--- a/internal/color.go
+++ b/internal/color.go
@@ -30,16 +30,16 @@ func (c *ClosestColorPicker[RGB, Color]) IndexInPalette(rgba color.Color) (Color
 	for i, paletteCol := range c.Palette {
 		r, g, b, _ := rgba.RGBA()
 		r, g, b = r&0xff, g&0xff, b&0xff
-		r2, g2, b2 := paletteCol>>16&0xff, paletteCol>>8&0xff, paletteCol&0xff
-		if r == uint32(r2) && g == uint32(g2) && b == uint32(b2) {
+		r2, g2, b2 := uint32(paletteCol>>16&0xff),
+			uint32(paletteCol>>8&0xff),
+			uint32(paletteCol&0xff)
+		if r == r2 && g == g2 && b == b2 {
 			// found perfect match. Short circuit
 			closestColor = Color(i)
 			break
 		}
-		distance := math.Sqrt(
-			math.Pow(float64(r2-RGB(r)), 2) +
-				math.Pow(float64(g2-RGB(g)), 2) +
-				math.Pow(float64(b2-RGB(b)), 2))
+		distance := perceptualColorDistance(r, g, b, r2, g2, b2)
+
 		if distance < smallestDistance {
 			smallestDistance = distance
 			closestColor = Color(i)
@@ -50,3 +50,10 @@ func (c *ClosestColorPicker[RGB, Color]) IndexInPalette(rgba color.Color) (Color
 
 	return closestColor, nil
 }
+
+func perceptualColorDistance(r1, g1, b1, r2, g2, b2 uint32) float64 {
+	rd := float64(r1 - r2)
+	gd := float64(g1 - g2)
+	bd := float64(b1 - b2)
+	return math.Sqrt(0.299*rd*rd + 0.587*gd*gd + 0.114*bd*bd)
+}
diff --git a/internal/test/decode/indexed-brighter.png b/internal/test/decode/indexed-brighter.png
new file mode 100644
index 0000000000000000000000000000000000000000..3a6992a5b393978126b0e35e7549d20748fd373e
GIT binary patch
literal 161
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literal 0
HcmV?d00001

diff --git a/internal/test/decode/indexed.png b/internal/test/decode/indexed.png
new file mode 100644
index 0000000000000000000000000000000000000000..81f0ad571e8a9de0edca0a8e18236939346d0a0d
GIT binary patch
literal 161
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literal 0
HcmV?d00001

diff --git a/internal/test/decode/rgb-brighter.png b/internal/test/decode/rgb-brighter.png
new file mode 100644
index 0000000000000000000000000000000000000000..86e147624e7d11084f3582a0ec05e3f601fcbd13
GIT binary patch
literal 133
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literal 0
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diff --git a/internal/test/decode/rgb.png b/internal/test/decode/rgb.png
new file mode 100644
index 0000000000000000000000000000000000000000..e3894e7ff6a769a408baade470b052055b440ce7
GIT binary patch
literal 133
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literal 0
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diff --git a/surface_test.go b/surface_test.go
index 07fedeb9..9c7ee382 100644
--- a/surface_test.go
+++ b/surface_test.go
@@ -4,6 +4,9 @@
 package pi_test
 
 import (
+	_ "embed"
+	"github.com/elgopher/pi/pitest"
+	"github.com/stretchr/testify/require"
 	"math/rand"
 	"testing"
 
@@ -12,6 +15,44 @@ import (
 	"github.com/elgopher/pi"
 )
 
+var (
+	//go:embed "internal/test/decode/indexed.png"
+	indexedPNG []byte
+	//go:embed "internal/test/decode/rgb.png"
+	rgbPNG []byte
+	//go:embed "internal/test/decode/indexed-brighter.png"
+	brighterIndexedPNG []byte
+	//go:embed "internal/test/decode/rgb-brighter.png"
+	rgbBrighterPNG []byte
+)
+
+func TestDecodeCanvasOrErr(t *testing.T) {
+	tests := map[string][]byte{
+		"indexed png when palette is the same": indexedPNG,
+		"RGB png":                              rgbPNG,
+		"indexed png when palette is slightly brighter": brighterIndexedPNG,
+		"RGB png when palette is slightly brighter":     rgbBrighterPNG,
+	}
+
+	for testName, png := range tests {
+		t.Run(testName, func(t *testing.T) {
+			pi.Palette = pi.DecodePalette(indexedPNG)
+			// when
+			canvas, err := pi.DecodeCanvasOrErr(png)
+			// then
+			require.NoError(t, err)
+			expected := pi.NewCanvas(4, 4)
+			expected.SetAll(
+				0, 1, 2, 3,
+				4, 5, 6, 7,
+				8, 9, 10, 11,
+				12, 13, 14, 15,
+			)
+			pitest.AssertSurfaceEqual(t, expected, canvas)
+		})
+	}
+}
+
 func TestSet(t *testing.T) {
 	t.Run("should be noop when outside surface", func(t *testing.T) {
 		width := 2