Custom Mip Map generation with SSIM-based perceptual downscaling

Currently, the lower-resolution mip maps in Minetest are generated without gamma correction and are therefore often too dark.
Furthermore, common mip maps are downscaled without preserving the image contrast.

After this change, it is possible to choose between the default mip map generation
and a new custom mip map generation algorithm which preserves image sharpness and average brightness.

Author: HybridDog

builtin/settingtypes.txt

@@ -407,9 +407,18 @@ enable_3d_clouds (3D clouds) bool true
 [**Filtering and Antialiasing]
 
 #    Use mipmaps when scaling textures. May slightly increase performance,
-#    especially when using a high-resolution texture pack.
-#    Gamma-correct downscaling is not supported.
-mip_map (Mipmapping) bool false
+#    especially when using a high-resolution texture pack or a high
+#    texture_min_size setting. It may also increase join times if textures are
+#    large.
+#
+#    * off    - Disable mipmapping (default)
+#
+#    * simple - Use the default mipmap generation algorithm, which scales
+#               textures down without gamma-correction
+#
+#    * sharp  - Use a custom mipmap generation algorithm, which scales textures
+#               down while preserving the contrast and average brightness
+mip_map (Mipmapping) enum off off,simple,sharp
 
 #    Use bilinear filtering when scaling textures.
 bilinear_filter (Bilinear filtering) bool false

src/client/CMakeLists.txt

@@ -55,6 +55,7 @@ set(client_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/hud.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/imagefilters.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/inputhandler.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/mipmap_generation.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/joystick_controller.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/keycode.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/localplayer.cpp

src/client/game.cpp

@@ -1108,7 +1108,8 @@ bool Game::startup(bool *kill,
 	driver = device->getVideoDriver();
 	smgr = m_rendering_engine->get_scene_manager();
 
-	driver->setTextureCreationFlag(video::ETCF_CREATE_MIP_MAPS, g_settings->getBool("mip_map"));
+	driver->setTextureCreationFlag(video::ETCF_CREATE_MIP_MAPS,
+		g_settings->get("mip_map") != "off");
 
 	smgr->getParameters()->setAttribute(scene::OBJ_LOADER_IGNORE_MATERIAL_FILES, true);
 
@@ -1445,7 +1446,7 @@ void Game::copyServerClientCache()
 {
 	// It would be possible to let the client directly read the media files
 	// from where the server knows they are. But aside from being more complicated
-	// it would also *not* fill the media cache and cause slower joining of 
+	// it would also *not* fill the media cache and cause slower joining of
 	// remote servers.
 	// (Imagine that you launch a game once locally and then connect to a server.)
 

src/client/imagesource.cpp

@@ -1528,7 +1528,7 @@ bool ImageSource::generateImagePart(std::string_view part_of_name,
 			CHECK_BASEIMG();
 
 			// Apply the "clean transparent" filter, if needed
-			if (m_setting_mipmap || m_setting_bilinear_filter ||
+			if (m_setting_mip_map_enabled || m_setting_bilinear_filter ||
 				m_setting_trilinear_filter || m_setting_anisotropic_filter)
 				imageCleanTransparent(baseimg, 127);
 

src/client/imagesource.h

@@ -59,7 +59,7 @@ struct ImageSource {
 	static video::SColor getImageAverageColor(const video::IImage &image);
 
 	ImageSource() :
-		m_setting_mipmap{g_settings->getBool("mip_map")},
+		m_setting_mip_map_enabled{g_settings->get("mip_map") != "off"},
 		m_setting_trilinear_filter{g_settings->getBool("trilinear_filter")},
 		m_setting_bilinear_filter{g_settings->getBool("bilinear_filter")},
 		m_setting_anisotropic_filter{g_settings->getBool("anisotropic_filter")}
@@ -74,7 +74,7 @@ struct ImageSource {
 			std::set<std::string> &source_image_names);
 
 	// Cached settings needed for making textures from meshes
-	bool m_setting_mipmap;
+	bool m_setting_mip_map_enabled;
 	bool m_setting_trilinear_filter;
 	bool m_setting_bilinear_filter;
 	bool m_setting_anisotropic_filter;