Hard-coded undersampling.

Adds uniform undersampling for the 3D rendered scene. GUI elements
are not undersampled, resulting in better playability for users
with low-performance platforms with readable fonts and formspecs.

The undersampling setting can be set to 0 (disabled), 2, 3, 4 pixels
which translates into a resolution reduction of x4, x9 or x16, and
is significant.

builtin/settingtypes.txt

@@ -362,6 +362,11 @@ texture_min_size (Minimum texture size for filters) int 64
 #    when set to higher number than 0.
 fsaa (FSAA) enum 0 0,1,2,4,8,16
 
+#    Undersampling is similar to using lower screen resolution, but it applies
+#    to the game world only, keeping the GUI intact.
+#    It should give significant performance boost at the cost of less detailed image.
+undersampling (Undersampling) enum 0 0,2,3,4
+
 [***Shaders]
 
 #    Shaders allow advanced visual effects and may increase performance on some video cards.

minetest.conf.example

@@ -404,6 +404,12 @@
 #    type: enum values: 0, 1, 2, 4, 8, 16
 # fsaa = 0
 
+#    Undersampling is similar to using lower screen resolution, but it applies
+#    to the game world only, keeping the GUI intact.
+#    It should give significant performance boost at the cost of less detailed image.
+#    type: enum values: 0, 2, 3, 4
+# undersampling = 0
+
 #### Shaders
 
 #    Shaders allow advanced visual effects and may increase performance on some video cards.

src/defaultsettings.cpp

@@ -108,6 +108,7 @@ void set_default_settings(Settings *settings)
 	settings->setDefault("show_debug", "true");
 #endif
 	settings->setDefault("fsaa", "0");
+	settings->setDefault("undersampling", "0");
 	settings->setDefault("enable_fog", "true");
 	settings->setDefault("fog_start", "0.4");
 	settings->setDefault("3d_mode", "none");

src/drawscene.cpp

@@ -459,10 +459,37 @@ void draw_pageflip_3d_mode(Camera& camera, bool show_hud,
 #endif
 }
 
-void draw_plain(Camera &camera, bool show_hud, Hud &hud,
+// returns (size / coef), rounded upwards
-		video::IVideoDriver *driver, bool draw_wield_tool,
+inline int scaledown(int coef, int size)
-		Client &client, gui::IGUIEnvironment *guienv)
 {
+	return (size + coef - 1) / coef;
+}
+
+void draw_plain(Camera &camera, bool show_hud,
+		Hud &hud, video::IVideoDriver *driver,
+		scene::ISceneManager *smgr, const v2u32 &screensize,
+		bool draw_wield_tool, Client &client, gui::IGUIEnvironment *guienv,
+		video::SColor skycolor)
+{
+	// Undersampling-specific stuff
+	static video::ITexture *image = NULL;
+	static v2u32 last_pixelated_size = v2u32(0, 0);
+	int undersampling = g_settings->getU16("undersampling");
+	v2u32 pixelated_size;
+	v2u32 dest_size;
+	if (undersampling > 0) {
+		pixelated_size = v2u32(scaledown(undersampling, screensize.X),
+				scaledown(undersampling, screensize.Y));
+		dest_size = v2u32(undersampling * pixelated_size.X, undersampling * pixelated_size.Y);
+		if (pixelated_size != last_pixelated_size) {
+			init_texture(driver, pixelated_size, &image, "mt_drawimage_img1");
+			last_pixelated_size = pixelated_size;
+		}
+		driver->setRenderTarget(image, true, true, skycolor);
+	}
+
+	// Render
+	smgr->drawAll();
 	driver->setTransform(video::ETS_WORLD, core::IdentityMatrix);
 	if (show_hud) {
 		hud.drawSelectionMesh();
@@ -470,6 +497,14 @@ void draw_plain(Camera &camera, bool show_hud, Hud &hud,
 			camera.drawWieldedTool();
 		}
 	}
+
+	// Upscale lowres render
+	if (undersampling > 0) {
+		driver->setRenderTarget(0, true, true);
+		driver->draw2DImage(image,
+				irr::core::rect<s32>(0, 0, dest_size.X, dest_size.Y),
+				irr::core::rect<s32>(0, 0, pixelated_size.X, pixelated_size.Y));
+	}
 }
 
 void draw_scene(video::IVideoDriver *driver, scene::ISceneManager *smgr,
@@ -496,8 +531,6 @@ void draw_scene(video::IVideoDriver *driver, scene::ISceneManager *smgr,
 
 	const std::string &draw_mode = g_settings->get("3d_mode");
 
-	smgr->drawAll();
-
 	if (draw_mode == "anaglyph")
 	{
 		draw_anaglyph_3d_mode(camera, show_hud, hud, driver,
@@ -531,7 +564,7 @@ void draw_scene(video::IVideoDriver *driver, scene::ISceneManager *smgr,
 	}
 	else {
 		draw_plain(camera, show_hud, hud, driver,
-				draw_wield_tool, client, guienv);
+				smgr, screensize, draw_wield_tool, client, guienv, skycolor);
 	}
 
 	/*

src/settings_translation_file.cpp

@@ -152,6 +152,8 @@ fake_function() {
 	gettext("When using bilinear/trilinear/anisotropic filters, low-resolution textures\ncan be blurred, so automatically upscale them with nearest-neighbor\ninterpolation to preserve crisp pixels.  This sets the minimum texture size\nfor the upscaled textures; higher values look sharper, but require more\nmemory.  Powers of 2 are recommended.  Setting this higher than 1 may not\nhave a visible effect unless bilinear/trilinear/anisotropic filtering is\nenabled.");
 	gettext("FSAA");
 	gettext("Experimental option, might cause visible spaces between blocks\nwhen set to higher number than 0.");
+	gettext("Undersampling");
+	gettext("Undersampling is similar to using lower screen resolution, but it applies\nto the game world only, keeping the GUI intact.\nIt should give significant performance boost at the cost of less detailed image.");
 	gettext("Shaders");
 	gettext("Shaders");
 	gettext("Shaders allow advanced visual effects and may increase performance on some video cards.\nThy only work with the OpenGL video backend.");