Index: ps/trunk/source/graphics/MapIO.cpp
===================================================================
--- ps/trunk/source/graphics/MapIO.cpp	(revision 25842)
+++ ps/trunk/source/graphics/MapIO.cpp	(revision 25843)
@@ -1,98 +1,108 @@
 /* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. 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, either version 2 of the License, or
  * (at your option) any later version.
  *
  * 0 A.D. 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 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 #include "precompiled.h"
 
 #include "MapIO.h"
 
 #include "graphics/Patch.h"
 #include "lib/allocators/shared_ptr.h"
 #include "lib/file/file.h"
 #include "lib/file/vfs/vfs_path.h"
 #include "lib/os_path.h"
 #include "lib/status.h"
 #include "lib/tex/tex.h"
 #include "maths/MathUtil.h"
 #include "ps/Filesystem.h"
 
 #include <algorithm>
 #include <vector>
 
 Status ParseHeightmapImage(const std::shared_ptr<u8>& fileData, size_t fileSize, std::vector<u16>& heightmap);
 
 Status LoadHeightmapImageVfs(const VfsPath& filepath, std::vector<u16>& heightmap)
 {
 	std::shared_ptr<u8> fileData;
 	size_t fileSize;
 
 	RETURN_STATUS_IF_ERR(g_VFS->LoadFile(filepath, fileData, fileSize));
 
 	return ParseHeightmapImage(fileData, fileSize, heightmap);
 }
 
 Status LoadHeightmapImageOs(const OsPath& filepath, std::vector<u16>& heightmap)
 {
 	File file;
 	RETURN_STATUS_IF_ERR(file.Open(OsString(filepath), O_RDONLY));
 
 	size_t fileSize = lseek(file.Descriptor(), 0, SEEK_END);
 	lseek(file.Descriptor(), 0, SEEK_SET);
 
 	std::shared_ptr<u8> fileData;
 	RETURN_STATUS_IF_ERR(AllocateAligned(fileData, fileSize, maxSectorSize));
 
 	Status readvalue = read(file.Descriptor(), fileData.get(), fileSize);
 	file.Close();
 
 	RETURN_STATUS_IF_ERR(readvalue);
 
 	return ParseHeightmapImage(fileData, fileSize, heightmap);
 }
 
 Status ParseHeightmapImage(const std::shared_ptr<u8>& fileData, size_t fileSize, std::vector<u16>& heightmap)
 {
 	// Decode to a raw pixel format
 	Tex tex;
 	RETURN_STATUS_IF_ERR(tex.decode(fileData, fileSize));
 
-	// Convert to uncompressed BGRA with no mipmaps
+	// Convert to uncompressed BGRA with no mipmaps.
+	// Note that grayscale images don't get converted - they remain grayscale, 8-bits per pixel.
 	RETURN_STATUS_IF_ERR(tex.transform_to((tex.m_Flags | TEX_BGR | TEX_ALPHA) & ~(TEX_DXT | TEX_MIPMAPS)));
 
 	// Pick smallest side of texture; truncate if not divisible by PATCH_SIZE
 	ssize_t tileSize = std::min(tex.m_Width, tex.m_Height);
 	tileSize -= tileSize % PATCH_SIZE;
 
 	u8* mapdata = tex.get_data();
 	ssize_t bytesPP = tex.m_Bpp / 8;
 	ssize_t mapLineSkip = tex.m_Width * bytesPP;
 
 	// Copy image data into the heightmap
 	heightmap.resize(SQR(tileSize + 1));
-	for (ssize_t y = 0; y < tileSize + 1; ++y)
-		for (ssize_t x = 0; x < tileSize + 1; ++x)
-		{
-			// Repeat the last pixel of the image for the last vertex of the heightmap
-			int offset = std::min(y, tileSize - 1) * mapLineSkip + std::min(x, tileSize - 1) * bytesPP;
-
-			heightmap[(tileSize - y) * (tileSize + 1) + x] = static_cast<u16>(256) * std::max({
-				mapdata[offset],
-				mapdata[offset + bytesPP],
-				mapdata[offset + bytesPP * 2]});
-		}
+	// if hoisted out of the loop as a micro-optimisation that doesn't harm readability much.
+	if (bytesPP == 1)
+		for (ssize_t y = 0; y < tileSize + 1; ++y)
+			for (ssize_t x = 0; x < tileSize + 1; ++x)
+			{
+				// Repeat the last pixel of the image for the last vertex of the heightmap
+				int offset = std::min(y, tileSize - 1) * mapLineSkip + std::min(x, tileSize - 1);
+				heightmap[(tileSize - y) * (tileSize + 1) + x] = static_cast<u16>(256) * mapdata[offset];
+			}
+	else if (bytesPP == 4)
+		for (ssize_t y = 0; y < tileSize + 1; ++y)
+			for (ssize_t x = 0; x < tileSize + 1; ++x)
+			{
+				// Repeat the last pixel of the image for the last vertex of the heightmap
+				int offset = std::min(y, tileSize - 1) * mapLineSkip + std::min(x, tileSize - 1) * bytesPP;
+				heightmap[(tileSize - y) * (tileSize + 1) + x] = static_cast<u16>(256) * std::max({
+					mapdata[offset],
+					mapdata[offset + 1],
+					mapdata[offset + 2]});
+			}
 
 	return INFO::OK;
 }