Feature: Support converting 32bpp-only sprites to indexed 8bpp.

This uses nearest colour lookup to convert 32bpp-only sprites to indexed 8bpp on the fly. This provides a reasonable usable sprite instead of being incompatible.

src/palette.cpp

@@ -44,6 +44,9 @@ const uint PALETTE_BITS_OR = (1U << (PALETTE_SHIFT - 1));
 using PaletteLookup = std::array<uint8_t, 1U << (PALETTE_BITS * 3)>;
 static PaletteLookup _palette_lookup{};
 
+using ReshadeLookup = std::array<uint8_t, 1U << PALETTE_BITS>;
+static ReshadeLookup _reshade_lookup{};
+
 /**
  * Reduce bits per channel to PALETTE_BITS, and place value in the middle of the reduced range.
  * This is to counteract the information lost between bright and dark pixels, e.g if PALETTE_BITS was 2:
@@ -116,6 +119,27 @@ static uint8_t FindNearestColourIndex(uint8_t r, uint8_t g, uint8_t b)
 	return best_index;
 }
 
+/**
+ * Find nearest company colour palette index for a brightness level.
+ * @param pixel Pixel to find.
+ * @returns palette index of nearest colour.
+ */
+static uint8_t FindNearestColourReshadeIndex(uint8_t b)
+{
+	b = CrunchColour(b);
+
+	uint best_index = 0;
+	uint best_distance = UINT32_MAX;
+
+	for (uint i = PALETTE_INDEX_CC_START; i < PALETTE_INDEX_CC_END; i++) {
+		if (uint distance = CalculateColourDistance(_palette.palette[i], b, b, b); distance < best_distance) {
+			best_index = i;
+			best_distance = distance;
+		}
+	}
+	return best_index;
+}
+
 /**
  * Get nearest colour palette index from an RGB colour.
  * A search is performed if this colour is not already in the lookup table.
@@ -131,6 +155,19 @@ uint8_t GetNearestColourIndex(uint8_t r, uint8_t g, uint8_t b)
 	return _palette_lookup[key];
 }
 
+/**
+ * Get nearest colour palette index from a brightness level.
+ * A search is performed if this brightness level is not already in the lookup table.
+ * @param b Brightness component.
+ * @returns nearest colour palette index.
+ */
+uint8_t GetNearestColourReshadeIndex(uint8_t b)
+{
+	uint32_t key = (b >> PALETTE_SHIFT);
+	if (_reshade_lookup[key] == 0) _reshade_lookup[key] = FindNearestColourReshadeIndex(b);
+	return _reshade_lookup[key];
+}
+
 /**
  * Adjust brightness of colour.
  * @param colour Colour to adjust.

src/palette_func.h

@@ -21,6 +21,7 @@ bool CopyPalette(Palette &local_palette, bool force_copy = false);
 void GfxInitPalettes();
 
 uint8_t GetNearestColourIndex(uint8_t r, uint8_t g, uint8_t b);
+uint8_t GetNearestColourReshadeIndex(uint8_t b);
 
 inline uint8_t GetNearestColourIndex(const Colour colour)
 {

src/spritecache.cpp

@@ -10,6 +10,7 @@
 #include "stdafx.h"
 #include "random_access_file_type.h"
 #include "spriteloader/grf.hpp"
+#include "spriteloader/makeindexed.h"
 #include "gfx_func.h"
 #include "error.h"
 #include "error_func.h"
@@ -488,6 +489,11 @@ static void *ReadSprite(const SpriteCache *sc, SpriteID id, SpriteType sprite_ty
 	}
 	if (sprite_avail == 0) {
 		sprite_avail = sprite_loader.LoadSprite(sprite, file, file_pos, sprite_type, false, sc->control_flags, avail_8bpp, avail_32bpp);
+		if (sprite_type == SpriteType::Normal && avail_32bpp != 0 && !encoder->Is32BppSupported() && sprite_avail == 0) {
+			/* No 8bpp available, try converting from 32bpp. */
+			SpriteLoaderMakeIndexed make_indexed(sprite_loader);
+			sprite_avail = make_indexed.LoadSprite(sprite, file, file_pos, sprite_type, true, sc->control_flags, sprite_avail, avail_32bpp);
+		}
 	}
 
 	if (sprite_avail == 0) {

src/spriteloader/CMakeLists.txt

@@ -1,6 +1,8 @@
 add_files(
     grf.cpp
     grf.hpp
+    makeindexed.cpp
+    makeindexed.h
     sprite_file.cpp
     sprite_file_type.hpp
     spriteloader.hpp

src/spriteloader/makeindexed.cpp

@@ -0,0 +1,60 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file makeindexed.cpp Implementation for converting sprites from another source from 32bpp RGBA to indexed 8bpp. */
+
+#include "../stdafx.h"
+#include "../core/bitmath_func.hpp"
+#include "../core/math_func.hpp"
+#include "../gfx_func.h"
+#include "../palette_func.h"
+#include "makeindexed.h"
+
+#include "../safeguards.h"
+
+/**
+ * Convert in place a 32bpp sprite to 8bpp.
+ * @param sprite Sprite to convert.
+ */
+static void Convert32bppTo8bpp(SpriteLoader::Sprite &sprite)
+{
+	const auto *pixel_end = sprite.data + sprite.width * sprite.height;
+	for (auto *pixel = sprite.data; pixel != pixel_end; ++pixel) {
+		if (pixel->m != 0) {
+			/* Pixel has 8bpp mask, test if should be reshaded. */
+			uint8_t brightness = std::max({pixel->r, pixel->g, pixel->b});
+			if (brightness == 0 || brightness == 128) continue;
+
+			/* Update RGB component with reshaded palette colour, and enabled reshade. */
+			Colour c = AdjustBrightness(_cur_palette.palette[pixel->m], brightness);
+
+			if (IsInsideMM(pixel->m, 0xC6, 0xCE)) {
+				/* Dumb but simple brightness conversion. */
+				pixel->m = GetNearestColourReshadeIndex((c.r + c.g + c.b) / 3);
+			} else {
+				pixel->m = GetNearestColourIndex(c.r, c.g, c.b);
+			}
+		} else if (pixel->a < 128) {
+			/* Transparent pixel. */
+			pixel->m = 0;
+		} else {
+			/* Find nearest match from palette. */
+			pixel->m = GetNearestColourIndex(pixel->r, pixel->g, pixel->b);
+		}
+	}
+}
+
+uint8_t SpriteLoaderMakeIndexed::LoadSprite(SpriteLoader::SpriteCollection &sprite, SpriteFile &file, size_t file_pos, SpriteType sprite_type, bool, uint8_t control_flags, uint8_t &avail_8bpp, uint8_t &avail_32bpp)
+{
+	uint8_t avail = this->baseloader.LoadSprite(sprite, file, file_pos, sprite_type, true, control_flags, avail_8bpp, avail_32bpp);
+
+	for (ZoomLevel zoom = ZOOM_LVL_NORMAL; zoom != ZOOM_LVL_END; zoom++) {
+		if (HasBit(avail, zoom)) Convert32bppTo8bpp(sprite[zoom]);
+	}
+
+	return avail;
+}

src/spriteloader/makeindexed.h

@@ -0,0 +1,23 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file makeindexed.h Base for converting sprites from another source from 32bpp RGBA to indexed 8bpp. */
+
+#ifndef SPRITELOADER_MAKEINDEXED_H
+#define SPRITELOADER_MAKEINDEXED_H
+
+#include "spriteloader.hpp"
+
+/** Sprite loader for converting graphics coming from another source. */
+class SpriteLoaderMakeIndexed : public SpriteLoader {
+	SpriteLoader &baseloader;
+public:
+	SpriteLoaderMakeIndexed(SpriteLoader &baseloader) : baseloader(baseloader) {}
+	uint8_t LoadSprite(SpriteLoader::SpriteCollection &sprite, SpriteFile &file, size_t file_pos, SpriteType sprite_type, bool load_32bpp, uint8_t control_flags, uint8_t &avail_8bpp, uint8_t &avail_32bpp) override;
+};
+
+#endif /* SPRITELOADER_MAKEINDEXED_H */