From c8106a7f6d235041b5ce156c20109a3c90364245 Mon Sep 17 00:00:00 2001
From: Gleb Mazovetskiy <glex.spb@gmail.com>
Date: Mon, 2 Jun 2025 06:19:45 +0100
Subject: [PATCH] Palette blending: minor cleanups

1. Removes stray comments.
2. Loops only over half of the matrix.
3. Adds tests for `paletteTransparencyLookupBlack16`.
---
 Source/engine/palette.cpp         | 13 -----------
 Source/utils/palette_blending.cpp | 36 +++++++++++++++++--------------
 test/palette_blending_test.cpp    | 19 ++++++++++++++++
 3 files changed, 39 insertions(+), 29 deletions(-)

diff --git a/Source/engine/palette.cpp b/Source/engine/palette.cpp
index 7a73d9a57..04935feb8 100644
--- a/Source/engine/palette.cpp
+++ b/Source/engine/palette.cpp
@@ -39,19 +39,6 @@ void LoadBrightness()
 	GetOptions().Graphics.brightness.SetValue(brightnessValue - brightnessValue % 5);
 }
 
-/**
- * @brief Generate lookup table for transparency
- *
- * This is based of the same technique found in Quake2.
- *
- * To mimic 50% transparency we figure out what colors in the existing palette are the best match for the combination of any 2 colors.
- * We save this into a lookup table for use during rendering.
- *
- * @param palette The colors to operate on
- * @param skipFrom Do not use colors between this index and skipTo
- * @param skipTo Do not use colors between skipFrom and this index
- * @param toUpdate Only update the first n colors
- */
 /**
  * @brief Cycle the given range of colors in the palette
  * @param from First color index of the range
diff --git a/Source/utils/palette_blending.cpp b/Source/utils/palette_blending.cpp
index 6b22051cc..e6af27090 100644
--- a/Source/utils/palette_blending.cpp
+++ b/Source/utils/palette_blending.cpp
@@ -53,20 +53,25 @@ RGB BlendColors(const SDL_Color &a, const SDL_Color &b)
 	};
 }
 
+#if DEVILUTIONX_PALETTE_TRANSPARENCY_BLACK_16_LUT
+void SetPaletteTransparencyLookupBlack16(unsigned i, unsigned j)
+{
+	paletteTransparencyLookupBlack16[i | (j << 8U)] = paletteTransparencyLookup[0][i] | (paletteTransparencyLookup[0][j] << 8U);
+}
+#endif
+
 } // namespace
 
 void GenerateBlendedLookupTable(SDL_Color palette[256], int skipFrom, int skipTo)
 {
 	for (unsigned i = 0; i < 256; i++) {
-		for (unsigned j = 0; j < 256; j++) {
-			if (i == j) { // No need to calculate transparency between 2 identical colors
-				paletteTransparencyLookup[i][j] = j;
-				continue;
-			}
-			if (i > j) { // Half the blends will be mirror identical ([i][j] is the same as [j][i]), so simply copy the existing combination.
-				paletteTransparencyLookup[i][j] = paletteTransparencyLookup[j][i];
-				continue;
-			}
+		paletteTransparencyLookup[i][i] = i;
+		unsigned j = 0;
+		for (; j < i; j++) {
+			paletteTransparencyLookup[i][j] = paletteTransparencyLookup[j][i];
+		}
+		++j;
+		for (; j < 256; j++) {
 			const uint8_t best = FindBestMatchForColor(palette, BlendColors(palette[i], palette[j]), skipFrom, skipTo);
 			paletteTransparencyLookup[i][j] = best;
 		}
@@ -74,9 +79,10 @@ void GenerateBlendedLookupTable(SDL_Color palette[256], int skipFrom, int skipTo
 
 #if DEVILUTIONX_PALETTE_TRANSPARENCY_BLACK_16_LUT
 	for (unsigned i = 0; i < 256; ++i) {
-		for (unsigned j = 0; j < 256; ++j) {
-			const uint16_t index = i | (j << 8U);
-			paletteTransparencyLookupBlack16[index] = paletteTransparencyLookup[0][i] | (paletteTransparencyLookup[0][j] << 8);
+		SetPaletteTransparencyLookupBlack16(i, i);
+		for (unsigned j = 0; j < i; ++j) {
+			SetPaletteTransparencyLookupBlack16(i, j);
+			SetPaletteTransparencyLookupBlack16(j, i);
 		}
 	}
 #endif
@@ -104,10 +110,8 @@ void UpdateTransparencyLookupBlack16(unsigned from, unsigned to)
 {
 	for (unsigned i = from; i <= to; i++) {
 		for (unsigned j = 0; j < 256; j++) {
-			const uint16_t index = i | (j << 8U);
-			const uint16_t reverseIndex = j | (i << 8U);
-			paletteTransparencyLookupBlack16[index] = paletteTransparencyLookup[0][i] | (paletteTransparencyLookup[0][j] << 8);
-			paletteTransparencyLookupBlack16[reverseIndex] = paletteTransparencyLookup[0][j] | (paletteTransparencyLookup[0][i] << 8);
+			SetPaletteTransparencyLookupBlack16(i, j);
+			SetPaletteTransparencyLookupBlack16(j, i);
 		}
 	}
 }
diff --git a/test/palette_blending_test.cpp b/test/palette_blending_test.cpp
index 73b0d79fc..984062878 100644
--- a/test/palette_blending_test.cpp
+++ b/test/palette_blending_test.cpp
@@ -47,6 +47,8 @@ TEST(GenerateBlendedLookupTableTest, BasicTest)
 
 	GenerateBlendedLookupTable(palette, /*skipFrom=*/-1, /*skipTo=*/-1);
 
+	EXPECT_EQ(paletteTransparencyLookup[100][100], 100);
+
 	EXPECT_THAT(palette[17], ColorIs(17, 0, 34));
 	EXPECT_THAT(palette[150], ColorIs(44, 44, 44));
 	EXPECT_THAT(palette[86], ColorIs(22, 22, 44));
@@ -58,6 +60,23 @@ TEST(GenerateBlendedLookupTableTest, BasicTest)
 	EXPECT_THAT(palette[15], ColorIs(15, 0, 30));
 	EXPECT_EQ(paletteTransparencyLookup[27][130], 15);
 	EXPECT_EQ(paletteTransparencyLookup[130][27], 15);
+
+	EXPECT_THAT(palette[0], ColorIs(0, 0, 0));
+	EXPECT_THAT(palette[100], ColorIs(36, 36, 72));
+	EXPECT_THAT(palette[82], ColorIs(18, 18, 36));
+	EXPECT_EQ(paletteTransparencyLookup[0][100], 82);
+	EXPECT_EQ(paletteTransparencyLookup[100][0], 82);
+
+	EXPECT_THAT(palette[0], ColorIs(0, 0, 0));
+	EXPECT_THAT(palette[200], ColorIs(24, 24, 16));
+	EXPECT_THAT(palette[196], ColorIs(12, 12, 8));
+	EXPECT_EQ(paletteTransparencyLookup[0][200], 196);
+	EXPECT_EQ(paletteTransparencyLookup[200][0], 196);
+
+#if DEVILUTIONX_PALETTE_TRANSPARENCY_BLACK_16_LUT
+	EXPECT_EQ(paletteTransparencyLookupBlack16[100 | (100 << 8)], 82 | (82 << 8));
+	EXPECT_EQ(paletteTransparencyLookupBlack16[100 | (200 << 8)], 82 | (196 << 8));
+#endif
 }
 
 } // namespace