Add a setting to group transparency sorted triangles by buffer (#15115)

builtin/settingtypes.txt

@@ -1843,6 +1843,11 @@ video_driver (Video driver) enum  ,opengl,opengl3,ogles2
 #    Set to 0 to disable it entirely.
 transparency_sorting_distance (Transparency Sorting Distance) int 16 0 128
 
+#    Draw transparency sorted triangles grouped by their mesh buffers.
+#    This breaks transparency sorting between mesh buffers, but avoids situations
+#    where transparency sorting would be very slow otherwise.
+transparency_sorting_group_by_buffers (Transparency Sorting Group by Buffers) bool true
+
 #    Radius of cloud area stated in number of 64 node cloud squares.
 #    Values larger than 26 will start to produce sharp cutoffs at cloud area corners.
 cloud_radius (Cloud radius) int 12 1 62

src/client/clientmap.cpp

@@ -73,6 +73,7 @@ static const std::string ClientMap_settings[] = {
 	"trilinear_filter",
 	"bilinear_filter",
 	"anisotropic_filter",
+	"transparency_sorting_group_by_buffers",
 	"transparency_sorting_distance",
 	"occlusion_culler",
 	"enable_raytraced_culling",
@@ -115,6 +116,9 @@ void ClientMap::onSettingChanged(std::string_view name, bool all)
 		m_cache_bilinear_filter   = g_settings->getBool("bilinear_filter");
 	if (all || name == "anisotropic_filter")
 		m_cache_anistropic_filter = g_settings->getBool("anisotropic_filter");
+	if (all || name == "transparency_sorting_group_by_buffers")
+		m_cache_transparency_sorting_group_by_buffers =
+				g_settings->getBool("transparency_sorting_group_by_buffers");
 	if (all || name == "transparency_sorting_distance")
 		m_cache_transparency_sorting_distance = g_settings->getU16("transparency_sorting_distance");
 	if (all || name == "occlusion_culler")
@@ -1337,7 +1341,8 @@ void ClientMap::updateTransparentMeshBuffers()
 			}
 
 			if (do_sort_block) {
-				blockmesh->updateTransparentBuffers(m_camera_position, block->getPos());
+				blockmesh->updateTransparentBuffers(m_camera_position, block->getPos(),
+						m_cache_transparency_sorting_group_by_buffers);
 				++sorted_blocks;
 			} else {
 				blockmesh->consolidateTransparentBuffers();

src/client/clientmap.h

@@ -182,6 +182,7 @@ private:
 	bool m_cache_trilinear_filter;
 	bool m_cache_bilinear_filter;
 	bool m_cache_anistropic_filter;
+	bool m_cache_transparency_sorting_group_by_buffers;
 	u16 m_cache_transparency_sorting_distance;
 
 	bool m_loops_occlusion_culler;

src/client/mapblock_mesh.cpp

@@ -3,6 +3,7 @@
 // Copyright (C) 2010-2013 celeron55, Perttu Ahola <celeron55@gmail.com>
 
 #include "mapblock_mesh.h"
+#include "CMeshBuffer.h"
 #include "client.h"
 #include "mapblock.h"
 #include "map.h"
@@ -818,7 +819,8 @@ bool MapBlockMesh::animate(bool faraway, float time, int crack,
 	return true;
 }
 
-void MapBlockMesh::updateTransparentBuffers(v3f camera_pos, v3s16 block_pos)
+void MapBlockMesh::updateTransparentBuffers(v3f camera_pos, v3s16 block_pos,
+		bool group_by_buffers)
 {
 	// nothing to do if the entire block is opaque
 	if (m_transparent_triangles.empty())
@@ -834,24 +836,56 @@ void MapBlockMesh::updateTransparentBuffers(v3f camera_pos, v3s16 block_pos)
 	m_transparent_buffers_consolidated = false;
 	m_transparent_buffers.clear();
 
+	std::vector<std::pair<scene::SMeshBuffer *, std::vector<u16>>> ordered_strains;
+	std::unordered_map<scene::SMeshBuffer *, size_t> strain_idxs;
+
+	if (group_by_buffers) {
+		// find (reversed) order for strains, by iterating front-to-back
+		// (if a buffer A has a triangle nearer than all triangles of another
+		// buffer B, A should be drawn in front of (=after) B)
+		scene::SMeshBuffer *current_buffer = nullptr;
+		for (auto it = triangle_refs.rbegin(); it != triangle_refs.rend(); ++it) {
+			const auto &t = m_transparent_triangles[*it];
+			if (current_buffer == t.buffer)
+				continue;
+			current_buffer = t.buffer;
+			auto [_it2, is_new] =
+				strain_idxs.emplace(current_buffer, ordered_strains.size());
+			if (is_new)
+				ordered_strains.emplace_back(current_buffer, std::vector<u16>{});
+		}
+	}
+
+	// find order for triangles, by iterating back-to-front
 	scene::SMeshBuffer *current_buffer = nullptr;
-	std::vector<u16> current_strain;
+	std::vector<u16> *current_strain = nullptr;
 	for (auto i : triangle_refs) {
 		const auto &t = m_transparent_triangles[i];
 		if (current_buffer != t.buffer) {
-			if (current_buffer) {
-				m_transparent_buffers.emplace_back(current_buffer, std::move(current_strain));
-				current_strain.clear();
-			}
 			current_buffer = t.buffer;
+			if (group_by_buffers) {
+				auto it = strain_idxs.find(current_buffer);
+				assert(it != strain_idxs.end());
+				current_strain = &ordered_strains[it->second].second;
+			} else {
+				ordered_strains.emplace_back(current_buffer, std::vector<u16>{});
+				current_strain = &ordered_strains.back().second;
+			}
 		}
-		current_strain.push_back(t.p1);
-		current_strain.push_back(t.p2);
-		current_strain.push_back(t.p3);
+		current_strain->push_back(t.p1);
+		current_strain->push_back(t.p2);
+		current_strain->push_back(t.p3);
 	}
 
-	if (!current_strain.empty())
-		m_transparent_buffers.emplace_back(current_buffer, std::move(current_strain));
+	m_transparent_buffers.reserve(ordered_strains.size());
+	if (group_by_buffers) {
+		// the order was reversed
+		for (auto it = ordered_strains.rbegin(); it != ordered_strains.rend(); ++it)
+			m_transparent_buffers.emplace_back(it->first, std::move(it->second));
+	} else {
+		for (auto it = ordered_strains.begin(); it != ordered_strains.end(); ++it)
+			m_transparent_buffers.emplace_back(it->first, std::move(it->second));
+	}
 }
 
 void MapBlockMesh::consolidateTransparentBuffers()

src/client/mapblock_mesh.h

@@ -209,8 +209,14 @@ public:
 	/// Center of the bounding-sphere, in BS-space, relative to block pos.
 	v3f getBoundingSphereCenter() const { return m_bounding_sphere_center; }
 
-	/// update transparent buffers to render towards the camera
-	void updateTransparentBuffers(v3f camera_pos, v3s16 block_pos);
+	/** Update transparent buffers to render towards the camera.
+	 * @param group_by_buffers If true, triangles in the same buffer are batched
+	 *     into the same PartialMeshBuffer, resulting in fewer draw calls, but
+	 *     wrong order. Triangles within a single buffer are still ordered, and
+	 *     buffers are ordered relative to each other (with respect to their nearest
+	 *     triangle).
+	 */
+	void updateTransparentBuffers(v3f camera_pos, v3s16 block_pos, bool group_by_buffers);
 	void consolidateTransparentBuffers();
 
 	/// get the list of transparent buffers

src/defaultsettings.cpp

@@ -302,6 +302,7 @@ void set_default_settings()
 	settings->setDefault("arm_inertia", "true");
 	settings->setDefault("show_nametag_backgrounds", "true");
 	settings->setDefault("show_block_bounds_radius_near", "4");
+	settings->setDefault("transparency_sorting_group_by_buffers", "true");
 	settings->setDefault("transparency_sorting_distance", "16");
 
 	settings->setDefault("enable_minimap", "true");