Add depth sorting for node faces (#11696)

Use BSP tree to order transparent triangles
https://en.wikipedia.org/wiki/Binary_space_partitioning

builtin/settingtypes.txt

@@ -858,6 +858,10 @@ autoscale_mode (Autoscaling mode) enum disable disable,enable,force
 #    A restart is required after changing this.
 show_entity_selectionbox (Show entity selection boxes) bool false
 
+#    Distance in nodes at which transparency depth sorting is enabled
+#    Use this to limit the performance impact of transparency depth sorting
+transparency_sorting_distance (Transparency Sorting Distance) int 16 0 128
+
 [*Menus]
 
 #    Use a cloud animation for the main menu background.

src/client/clientmap.cpp

@@ -97,9 +97,32 @@ ClientMap::ClientMap(
 	m_cache_trilinear_filter  = g_settings->getBool("trilinear_filter");
 	m_cache_bilinear_filter   = g_settings->getBool("bilinear_filter");
 	m_cache_anistropic_filter = g_settings->getBool("anisotropic_filter");
+	m_cache_transparency_sorting_distance = g_settings->getU16("transparency_sorting_distance");
 
 }
 
+void ClientMap::updateCamera(v3f pos, v3f dir, f32 fov, v3s16 offset)
+{
+	v3s16 previous_node = floatToInt(m_camera_position, BS) + m_camera_offset;
+	v3s16 previous_block = getContainerPos(previous_node, MAP_BLOCKSIZE);
+
+	m_camera_position = pos;
+	m_camera_direction = dir;
+	m_camera_fov = fov;
+	m_camera_offset = offset;
+
+	v3s16 current_node = floatToInt(m_camera_position, BS) + m_camera_offset;
+	v3s16 current_block = getContainerPos(current_node, MAP_BLOCKSIZE);
+
+	// reorder the blocks when camera crosses block boundary
+	if (previous_block != current_block)
+		m_needs_update_drawlist = true;
+
+	// reorder transparent meshes when camera crosses node boundary
+	if (previous_node != current_node)
+		m_needs_update_transparent_meshes = true;
+}
+
 MapSector * ClientMap::emergeSector(v2s16 p2d)
 {
 	// Check that it doesn't exist already
@@ -323,22 +346,17 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 	u32 mesh_animate_count = 0;
 	//u32 mesh_animate_count_far = 0;
 
+	/*
+		Update transparent meshes
+	*/
+	if (is_transparent_pass)
+		updateTransparentMeshBuffers();
+
 	/*
 		Draw the selected MapBlocks
 	*/
 
 	MeshBufListList grouped_buffers;
-
-	struct DrawDescriptor {
-		v3s16 m_pos;
-		scene::IMeshBuffer *m_buffer;
-		bool m_reuse_material;
-
-		DrawDescriptor(const v3s16 &pos, scene::IMeshBuffer *buffer, bool reuse_material) :
-			m_pos(pos), m_buffer(buffer), m_reuse_material(reuse_material)
-		{}
-	};
-
 	std::vector<DrawDescriptor> draw_order;
 	video::SMaterial previous_material;
 
@@ -375,7 +393,15 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 		/*
 			Get the meshbuffers of the block
 		*/
-		{
+		if (is_transparent_pass) {
+			// In transparent pass, the mesh will give us
+			// the partial buffers in the correct order
+			for (auto &buffer : block->mesh->getTransparentBuffers())
+				draw_order.emplace_back(block_pos, &buffer);
+		}
+		else {
+			// otherwise, group buffers across meshes
+			// using MeshBufListList
 			MapBlockMesh *mapBlockMesh = block->mesh;
 			assert(mapBlockMesh);
 
@@ -391,38 +417,17 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 					video::IMaterialRenderer* rnd =
 							driver->getMaterialRenderer(material.MaterialType);
 					bool transparent = (rnd && rnd->isTransparent());
-					if (transparent == is_transparent_pass) {
+					if (!transparent) {
 						if (buf->getVertexCount() == 0)
 							errorstream << "Block [" << analyze_block(block)
 									<< "] contains an empty meshbuf" << std::endl;
 
-						material.setFlag(video::EMF_TRILINEAR_FILTER,
-							m_cache_trilinear_filter);
-						material.setFlag(video::EMF_BILINEAR_FILTER,
-							m_cache_bilinear_filter);
-						material.setFlag(video::EMF_ANISOTROPIC_FILTER,
-							m_cache_anistropic_filter);
-						material.setFlag(video::EMF_WIREFRAME,
-							m_control.show_wireframe);
-
-						if (is_transparent_pass) {
-							// Same comparison as in MeshBufListList
-							bool new_material = material.getTexture(0) != previous_material.getTexture(0) ||
-									material != previous_material;
-
-							draw_order.emplace_back(block_pos, buf, !new_material);
-
-							if (new_material)
-								previous_material = material;
-						}
-						else {
 						grouped_buffers.add(buf, block_pos, layer);
 					}
 				}
 			}
 		}
 	}
-	}
 
 	// Capture draw order for all solid meshes
 	for (auto &lists : grouped_buffers.lists) {
@@ -442,8 +447,17 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 
 	// Render all mesh buffers in order
 	drawcall_count += draw_order.size();
+
 	for (auto &descriptor : draw_order) {
-		scene::IMeshBuffer *buf = descriptor.m_buffer;
+		scene::IMeshBuffer *buf;
+		
+		if (descriptor.m_use_partial_buffer) {
+			descriptor.m_partial_buffer->beforeDraw();
+			buf = descriptor.m_partial_buffer->getBuffer();
+		}
+		else {
+			buf = descriptor.m_buffer;
+		}
 
 		// Check and abort if the machine is swapping a lot
 		if (draw.getTimerTime() > 2000) {
@@ -454,6 +468,17 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 
 		if (!descriptor.m_reuse_material) {
 			auto &material = buf->getMaterial();
+
+			// Apply filter settings
+			material.setFlag(video::EMF_TRILINEAR_FILTER,
+				m_cache_trilinear_filter);
+			material.setFlag(video::EMF_BILINEAR_FILTER,
+				m_cache_bilinear_filter);
+			material.setFlag(video::EMF_ANISOTROPIC_FILTER,
+				m_cache_anistropic_filter);
+			material.setFlag(video::EMF_WIREFRAME,
+				m_control.show_wireframe);
+
 			// pass the shadow map texture to the buffer texture
 			ShadowRenderer *shadow = m_rendering_engine->get_shadow_renderer();
 			if (shadow && shadow->is_active()) {
@@ -475,7 +500,7 @@ void ClientMap::renderMap(video::IVideoDriver* driver, s32 pass)
 
 		driver->setTransform(video::ETS_WORLD, m);
 		driver->drawMeshBuffer(buf);
-		vertex_count += buf->getVertexCount();
+		vertex_count += buf->getIndexCount();
 	}
 
 	g_profiler->avg(prefix + "draw meshes [ms]", draw.stop(true));
@@ -698,7 +723,9 @@ void ClientMap::renderMapShadows(video::IVideoDriver *driver,
 	u32 drawcall_count = 0;
 	u32 vertex_count = 0;
 
-	MeshBufListList drawbufs;
+	MeshBufListList grouped_buffers;
+	std::vector<DrawDescriptor> draw_order;
+
 
 	int count = 0;
 	int low_bound = is_transparent_pass ? 0 : m_drawlist_shadow.size() / total_frames * frame;
@@ -727,7 +754,15 @@ void ClientMap::renderMapShadows(video::IVideoDriver *driver,
 		/*
 			Get the meshbuffers of the block
 		*/
-		{
+		if (is_transparent_pass) {
+			// In transparent pass, the mesh will give us
+			// the partial buffers in the correct order
+			for (auto &buffer : block->mesh->getTransparentBuffers())
+				draw_order.emplace_back(block_pos, &buffer);
+		}
+		else {
+			// otherwise, group buffers across meshes
+			// using MeshBufListList
 			MapBlockMesh *mapBlockMesh = block->mesh;
 			assert(mapBlockMesh);
 
@@ -742,30 +777,57 @@ void ClientMap::renderMapShadows(video::IVideoDriver *driver,
 					video::SMaterial &mat = buf->getMaterial();
 					auto rnd = driver->getMaterialRenderer(mat.MaterialType);
 					bool transparent = rnd && rnd->isTransparent();
-					if (transparent == is_transparent_pass)
+					if (!transparent)
-						drawbufs.add(buf, block_pos, layer);
+						grouped_buffers.add(buf, block_pos, layer);
 				}
 			}
 		}
 	}
 
+	u32 buffer_count = 0;
+	for (auto &lists : grouped_buffers.lists)
+		for (MeshBufList &list : lists)
+			buffer_count += list.bufs.size();
+	
+	draw_order.reserve(draw_order.size() + buffer_count);
+	
+	// Capture draw order for all solid meshes
+	for (auto &lists : grouped_buffers.lists) {
+		for (MeshBufList &list : lists) {
+			// iterate in reverse to draw closest blocks first
+			for (auto it = list.bufs.rbegin(); it != list.bufs.rend(); ++it)
+				draw_order.emplace_back(it->first, it->second, it != list.bufs.rbegin());
+		}
+	}
+
 	TimeTaker draw("Drawing shadow mesh buffers");
 
 	core::matrix4 m; // Model matrix
 	v3f offset = intToFloat(m_camera_offset, BS);
+	u32 material_swaps = 0;
+
+	// Render all mesh buffers in order
+	drawcall_count += draw_order.size();
+
+	for (auto &descriptor : draw_order) {
+		scene::IMeshBuffer *buf;
+		
+		if (descriptor.m_use_partial_buffer) {
+			descriptor.m_partial_buffer->beforeDraw();
+			buf = descriptor.m_partial_buffer->getBuffer();
+		}
+		else {
+			buf = descriptor.m_buffer;
+		}
 
-	// Render all layers in order
-	for (auto &lists : drawbufs.lists) {
-		for (MeshBufList &list : lists) {
 		// Check and abort if the machine is swapping a lot
 		if (draw.getTimerTime() > 1000) {
 			infostream << "ClientMap::renderMapShadows(): Rendering "
 					"took >1s, returning." << std::endl;
 			break;
 		}
-			for (auto &pair : list.bufs) {
-				scene::IMeshBuffer *buf = pair.second;
 
+		if (!descriptor.m_reuse_material) {
 			// override some material properties
 			video::SMaterial local_material = buf->getMaterial();
 			local_material.MaterialType = material.MaterialType;
@@ -774,17 +836,15 @@ void ClientMap::renderMapShadows(video::IVideoDriver *driver,
 			local_material.BlendOperation = material.BlendOperation;
 			local_material.Lighting = false;
 			driver->setMaterial(local_material);
+			++material_swaps;
+		}
 
-				v3f block_wpos = intToFloat(pair.first * MAP_BLOCKSIZE, BS);
+		v3f block_wpos = intToFloat(descriptor.m_pos * MAP_BLOCKSIZE, BS);
 		m.setTranslation(block_wpos - offset);
 
 		driver->setTransform(video::ETS_WORLD, m);
 		driver->drawMeshBuffer(buf);
-				vertex_count += buf->getVertexCount();
+		vertex_count += buf->getIndexCount();
-			}
-
-			drawcall_count += list.bufs.size();
-		}
 	}
 
 	// restore the driver material state 
@@ -796,6 +856,7 @@ void ClientMap::renderMapShadows(video::IVideoDriver *driver,
 	g_profiler->avg(prefix + "draw meshes [ms]", draw.stop(true));
 	g_profiler->avg(prefix + "vertices drawn [#]", vertex_count);
 	g_profiler->avg(prefix + "drawcalls [#]", drawcall_count);
+	g_profiler->avg(prefix + "material swaps [#]", material_swaps);
 }
 
 /*
@@ -891,3 +952,40 @@ void ClientMap::updateDrawListShadow(const v3f &shadow_light_pos, const v3f &sha
 	g_profiler->avg("SHADOW MapBlocks drawn [#]", m_drawlist_shadow.size());
 	g_profiler->avg("SHADOW MapBlocks loaded [#]", blocks_loaded);
 }
+
+void ClientMap::updateTransparentMeshBuffers()
+{
+	ScopeProfiler sp(g_profiler, "CM::updateTransparentMeshBuffers", SPT_AVG);
+	u32 sorted_blocks = 0;
+	u32 unsorted_blocks = 0;
+	f32 sorting_distance_sq = pow(m_cache_transparency_sorting_distance * BS, 2.0f);
+
+
+	// Update the order of transparent mesh buffers in each mesh
+	for (auto it = m_drawlist.begin(); it != m_drawlist.end(); it++) {
+		MapBlock* block = it->second;
+		if (!block->mesh)
+			continue;
+		
+		if (m_needs_update_transparent_meshes || 
+				block->mesh->getTransparentBuffers().size() == 0) {
+
+			v3s16 block_pos = block->getPos();
+			v3f block_pos_f = intToFloat(block_pos * MAP_BLOCKSIZE + MAP_BLOCKSIZE / 2, BS);
+			f32 distance = m_camera_position.getDistanceFromSQ(block_pos_f);
+			if (distance <= sorting_distance_sq) {
+				block->mesh->updateTransparentBuffers(m_camera_position, block_pos);
+				++sorted_blocks;
+			}
+			else {
+				block->mesh->consolidateTransparentBuffers();
+				++unsorted_blocks;
+			}
+		}
+	}
+
+	g_profiler->avg("CM::Transparent Buffers - Sorted", sorted_blocks);
+	g_profiler->avg("CM::Transparent Buffers - Unsorted", unsorted_blocks);
+	m_needs_update_transparent_meshes = false;
+}
+

src/client/clientmap.h

@@ -56,6 +56,7 @@ struct MeshBufListList
 
 class Client;
 class ITextureSource;
+class PartialMeshBuffer;
 
 /*
 	ClientMap
@@ -85,21 +86,7 @@ public:
 		ISceneNode::drop();
 	}
 
-	void updateCamera(const v3f &pos, const v3f &dir, f32 fov, const v3s16 &offset)
+	void updateCamera(v3f pos, v3f dir, f32 fov, v3s16 offset);
-	{
-		v3s16 previous_block = getContainerPos(floatToInt(m_camera_position, BS) + m_camera_offset, MAP_BLOCKSIZE);
-
-		m_camera_position = pos;
-		m_camera_direction = dir;
-		m_camera_fov = fov;
-		m_camera_offset = offset;
-
-		v3s16 current_block = getContainerPos(floatToInt(m_camera_position, BS) + m_camera_offset, MAP_BLOCKSIZE);
-
-		// reorder the blocks when camera crosses block boundary
-		if (previous_block != current_block)
-			m_needs_update_drawlist = true;
-	}
 
 	/*
 		Forcefully get a sector from somewhere
@@ -150,6 +137,10 @@ public:
 	f32 getCameraFov() const { return m_camera_fov; }
 
 private:
+
+	// update the vertex order in transparent mesh buffers
+	void updateTransparentMeshBuffers();
+
 	// Orders blocks by distance to the camera
 	class MapBlockComparer
 	{
@@ -167,6 +158,26 @@ private:
 		v3s16 m_camera_block;
 	};
 
+
+	// reference to a mesh buffer used when rendering the map.
+	struct DrawDescriptor {
+		v3s16 m_pos;
+		union {
+			scene::IMeshBuffer *m_buffer;
+			const PartialMeshBuffer *m_partial_buffer;
+		};
+		bool m_reuse_material:1;
+		bool m_use_partial_buffer:1;
+
+		DrawDescriptor(v3s16 pos, scene::IMeshBuffer *buffer, bool reuse_material) :
+			m_pos(pos), m_buffer(buffer), m_reuse_material(reuse_material), m_use_partial_buffer(false)
+		{}
+
+		DrawDescriptor(v3s16 pos, const PartialMeshBuffer *buffer) :
+			m_pos(pos), m_partial_buffer(buffer), m_reuse_material(false), m_use_partial_buffer(true)
+		{}
+	};
+
 	Client *m_client;
 	RenderingEngine *m_rendering_engine;
 
@@ -179,6 +190,7 @@ private:
 	v3f m_camera_direction = v3f(0,0,1);
 	f32 m_camera_fov = M_PI;
 	v3s16 m_camera_offset;
+	bool m_needs_update_transparent_meshes = true;
 
 	std::map<v3s16, MapBlock*, MapBlockComparer> m_drawlist;
 	std::map<v3s16, MapBlock*> m_drawlist_shadow;
@@ -190,4 +202,5 @@ private:
 	bool m_cache_bilinear_filter;
 	bool m_cache_anistropic_filter;
 	bool m_added_to_shadow_renderer{false};
+	u16 m_cache_transparency_sorting_distance;
 };

src/client/content_mapblock.cpp

@@ -381,12 +381,12 @@ void MapblockMeshGenerator::drawAutoLightedCuboid(aabb3f box, const f32 *txc,
 		box.MinEdge *= f->visual_scale;
 		box.MaxEdge *= f->visual_scale;
 	}
-	box.MinEdge += origin;
-	box.MaxEdge += origin;
 	if (!txc) {
 		generateCuboidTextureCoords(box, texture_coord_buf);
 		txc = texture_coord_buf;
 	}
+	box.MinEdge += origin;
+	box.MaxEdge += origin;
 	if (!tiles) {
 		tiles = &tile;
 		tile_count = 1;
@@ -1377,6 +1377,59 @@ void MapblockMeshGenerator::drawNodeboxNode()
 
 	std::vector<aabb3f> boxes;
 	n.getNodeBoxes(nodedef, &boxes, neighbors_set);
+
+	bool isTransparent = false;
+
+	for (const TileSpec &tile : tiles) {
+		if (tile.layers[0].isTransparent()) {
+			isTransparent = true;
+			break;
+		}
+	}
+
+	if (isTransparent) {
+		std::vector<float> sections;
+		// Preallocate 8 default splits + Min&Max for each nodebox
+		sections.reserve(8 + 2 * boxes.size());
+
+		for (int axis = 0; axis < 3; axis++) {
+			// identify sections
+
+			if (axis == 0) {
+				// Default split at node bounds, up to 3 nodes in each direction
+				for (float s = -3.5f * BS; s < 4.0f * BS; s += 1.0f * BS)
+					sections.push_back(s);
+			}
+			else {
+				// Avoid readding the same 8 default splits for Y and Z
+				sections.resize(8);
+			}
+
+			// Add edges of existing node boxes, rounded to 1E-3
+			for (size_t i = 0; i < boxes.size(); i++) {
+				sections.push_back(std::floor(boxes[i].MinEdge[axis] * 1E3) * 1E-3);
+				sections.push_back(std::floor(boxes[i].MaxEdge[axis] * 1E3) * 1E-3);
+			}
+
+			// split the boxes at recorded sections
+			// limit splits to avoid runaway crash if inner loop adds infinite splits
+			// due to e.g. precision problems.
+			// 100 is just an arbitrary, reasonably high number.
+			for (size_t i = 0; i < boxes.size() && i < 100; i++) {
+				aabb3f *box = &boxes[i];
+				for (float section : sections) {
+					if (box->MinEdge[axis] < section && box->MaxEdge[axis] > section) {
+						aabb3f copy(*box);
+						copy.MinEdge[axis] = section;
+						box->MaxEdge[axis] = section;
+						boxes.push_back(copy);
+						box = &boxes[i]; // find new address of the box in case of reallocation
+					}
+				}
+			}
+		}
+	}
+
 	for (auto &box : boxes)
 		drawAutoLightedCuboid(box, nullptr, tiles, 6);
 }

src/client/mapblock_mesh.cpp

@@ -30,6 +30,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "client/meshgen/collector.h"
 #include "client/renderingengine.h"
 #include <array>
+#include <algorithm>
 
 /*
 	MeshMakeData
@@ -1003,6 +1004,173 @@ static void applyTileColor(PreMeshBuffer &pmb)
 	}
 }
 
+/*
+	MapBlockBspTree
+*/
+
+void MapBlockBspTree::buildTree(const std::vector<MeshTriangle> *triangles)
+{
+	this->triangles = triangles;
+
+	nodes.clear();
+
+	// assert that triangle index can fit into s32
+	assert(triangles->size() <= 0x7FFFFFFFL);
+	std::vector<s32> indexes;
+	indexes.reserve(triangles->size());
+	for (u32 i = 0; i < triangles->size(); i++)
+		indexes.push_back(i);
+
+	root = buildTree(v3f(1, 0, 0), v3f(85, 85, 85), 40, indexes, 0);
+}
+
+/**
+ * @brief Find a candidate plane to split a set of triangles in two
+ * 
+ * The candidate plane is represented by one of the triangles from the set.
+ * 
+ * @param list Vector of indexes of the triangles in the set
+ * @param triangles Vector of all triangles in the BSP tree
+ * @return Address of the triangle that represents the proposed split plane
+ */
+static const MeshTriangle *findSplitCandidate(const std::vector<s32> &list, const std::vector<MeshTriangle> &triangles)
+{
+	// find the center of the cluster.
+	v3f center(0, 0, 0);
+	size_t n = list.size();
+	for (s32 i : list) {
+		center += triangles[i].centroid / n;
+	}
+
+	// find the triangle with the largest area and closest to the center
+	const MeshTriangle *candidate_triangle = &triangles[list[0]];
+	const MeshTriangle *ith_triangle;
+	for (s32 i : list) {
+		ith_triangle = &triangles[i];
+		if (ith_triangle->areaSQ > candidate_triangle->areaSQ ||
+				(ith_triangle->areaSQ == candidate_triangle->areaSQ &&
+				ith_triangle->centroid.getDistanceFromSQ(center) < candidate_triangle->centroid.getDistanceFromSQ(center))) {
+			candidate_triangle = ith_triangle;
+		}
+	}
+	return candidate_triangle;
+}
+
+s32 MapBlockBspTree::buildTree(v3f normal, v3f origin, float delta, const std::vector<s32> &list, u32 depth)
+{
+	// if the list is empty, don't bother
+	if (list.empty())
+		return -1;
+
+	// if there is only one triangle, or the delta is insanely small, this is a leaf node
+	if (list.size() == 1 || delta < 0.01) {
+		nodes.emplace_back(normal, origin, list, -1, -1);
+		return nodes.size() - 1;
+	}
+
+	std::vector<s32> front_list;
+	std::vector<s32> back_list;
+	std::vector<s32> node_list;
+
+	// split the list
+	for (s32 i : list) {
+		const MeshTriangle &triangle = (*triangles)[i];
+		float factor = normal.dotProduct(triangle.centroid - origin);
+		if (factor == 0)
+			node_list.push_back(i);
+		else if (factor > 0)
+			front_list.push_back(i);
+		else
+			back_list.push_back(i);
+	}
+
+	// define the new split-plane
+	v3f candidate_normal(normal.Z, normal.X, normal.Y);
+	float candidate_delta = delta;
+	if (depth % 3 == 2)
+		candidate_delta /= 2;
+
+	s32 front_index = -1;
+	s32 back_index = -1;
+
+	if (!front_list.empty()) {
+		v3f next_normal = candidate_normal;
+		v3f next_origin = origin + delta * normal;
+		float next_delta = candidate_delta;
+		if (next_delta < 10) {
+			const MeshTriangle *candidate = findSplitCandidate(front_list, *triangles);
+			next_normal = candidate->getNormal();
+			next_origin = candidate->centroid;
+		}
+		front_index = buildTree(next_normal, next_origin, next_delta, front_list, depth + 1);
+
+		// if there are no other triangles, don't create a new node
+		if (back_list.empty() && node_list.empty())
+			return front_index;
+	}
+
+	if (!back_list.empty()) {
+		v3f next_normal = candidate_normal;
+		v3f next_origin = origin - delta * normal;
+		float next_delta = candidate_delta;
+		if (next_delta < 10) {
+			const MeshTriangle *candidate = findSplitCandidate(back_list, *triangles);
+			next_normal = candidate->getNormal();
+			next_origin = candidate->centroid;
+		}
+
+		back_index = buildTree(next_normal, next_origin, next_delta, back_list, depth + 1);
+
+		// if there are no other triangles, don't create a new node
+		if (front_list.empty() && node_list.empty())
+			return back_index;
+	}
+
+	nodes.emplace_back(normal, origin, node_list, front_index, back_index);
+
+	return nodes.size() - 1;
+}
+
+void MapBlockBspTree::traverse(s32 node, v3f viewpoint, std::vector<s32> &output) const
+{
+	if (node < 0) return; // recursion break;
+
+	const TreeNode &n = nodes[node];
+	float factor = n.normal.dotProduct(viewpoint - n.origin);
+
+	if (factor > 0)
+		traverse(n.back_ref, viewpoint, output);
+	else
+		traverse(n.front_ref, viewpoint, output);
+
+	if (factor != 0)
+		for (s32 i : n.triangle_refs)
+			output.push_back(i);
+
+	if (factor > 0)
+		traverse(n.front_ref, viewpoint, output);
+	else
+		traverse(n.back_ref, viewpoint, output);
+}
+
+
+
+/*
+	PartialMeshBuffer
+*/
+
+void PartialMeshBuffer::beforeDraw() const
+{
+	// Patch the indexes in the mesh buffer before draw
+
+	m_buffer->Indices.clear();
+	if (!m_vertex_indexes.empty()) {
+		for (auto index : m_vertex_indexes)
+			m_buffer->Indices.push_back(index);
+	}
+	m_buffer->setDirty(scene::EBT_INDEX);
+}
+
 /*
 	MapBlockMesh
 */
@@ -1173,8 +1341,31 @@ MapBlockMesh::MapBlockMesh(MeshMakeData *data, v3s16 camera_offset):
 
 			scene::SMeshBuffer *buf = new scene::SMeshBuffer();
 			buf->Material = material;
+			switch (p.layer.material_type) {
+			// list of transparent materials taken from tile.h
+			case TILE_MATERIAL_ALPHA:
+			case TILE_MATERIAL_LIQUID_TRANSPARENT:
+			case TILE_MATERIAL_WAVING_LIQUID_TRANSPARENT:
+				{
+					buf->append(&p.vertices[0], p.vertices.size(),
+						&p.indices[0], 0);
+
+					MeshTriangle t;
+					t.buffer = buf;
+					for (u32 i = 0; i < p.indices.size(); i += 3) {
+						t.p1 = p.indices[i];
+						t.p2 = p.indices[i + 1];
+						t.p3 = p.indices[i + 2];
+						t.updateAttributes();
+						m_transparent_triangles.push_back(t);
+					}
+				}
+				break;
+			default:
 				buf->append(&p.vertices[0], p.vertices.size(),
 					&p.indices[0], p.indices.size());
+				break;
+			}
 			mesh->addMeshBuffer(buf);
 			buf->drop();
 		}
@@ -1187,6 +1378,7 @@ MapBlockMesh::MapBlockMesh(MeshMakeData *data, v3s16 camera_offset):
 	}
 
 	//std::cout<<"added "<<fastfaces.getSize()<<" faces."<<std::endl;
+	m_bsp_tree.buildTree(&m_transparent_triangles);
 
 	// Check if animation is required for this mesh
 	m_has_animation =
@@ -1298,6 +1490,67 @@ bool MapBlockMesh::animate(bool faraway, float time, int crack,
 	return true;
 }
 
+void MapBlockMesh::updateTransparentBuffers(v3f camera_pos, v3s16 block_pos)
+{
+	// nothing to do if the entire block is opaque
+	if (m_transparent_triangles.empty())
+		return;
+
+	v3f block_posf = intToFloat(block_pos * MAP_BLOCKSIZE, BS);
+	v3f rel_camera_pos = camera_pos - block_posf;
+
+	std::vector<s32> triangle_refs;
+	m_bsp_tree.traverse(rel_camera_pos, triangle_refs);
+
+	// arrange index sequences into partial buffers
+	m_transparent_buffers.clear();
+
+	scene::SMeshBuffer *current_buffer = nullptr;
+	std::vector<u16> current_strain;
+	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, current_strain);
+				current_strain.clear();
+			}
+			current_buffer = t.buffer;
+		}
+		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, current_strain);
+}
+
+void MapBlockMesh::consolidateTransparentBuffers()
+{
+	m_transparent_buffers.clear();
+
+	scene::SMeshBuffer *current_buffer = nullptr;
+	std::vector<u16> current_strain;
+
+	// use the fact that m_transparent_triangles is already arranged by buffer
+	for (const auto &t : m_transparent_triangles) {
+		if (current_buffer != t.buffer) {
+			if (current_buffer != nullptr) {
+				this->m_transparent_buffers.emplace_back(current_buffer, current_strain);
+				current_strain.clear();
+			}
+			current_buffer = t.buffer;
+		}
+		current_strain.push_back(t.p1);
+		current_strain.push_back(t.p2);
+		current_strain.push_back(t.p3);
+	}
+
+	if (!current_strain.empty()) {
+		this->m_transparent_buffers.emplace_back(current_buffer, current_strain);
+	}
+}
+
 video::SColor encode_light(u16 light, u8 emissive_light)
 {
 	// Get components

src/client/mapblock_mesh.h

@@ -71,6 +71,91 @@ struct MeshMakeData
 	void setSmoothLighting(bool smooth_lighting);
 };
 
+// represents a triangle as indexes into the vertex buffer in SMeshBuffer
+class MeshTriangle
+{
+public:
+	scene::SMeshBuffer *buffer;
+	u16 p1, p2, p3;
+	v3f centroid;
+	float areaSQ;
+
+	void updateAttributes()
+	{
+		v3f v1 = buffer->getPosition(p1);
+		v3f v2 = buffer->getPosition(p2);
+		v3f v3 = buffer->getPosition(p3);
+
+		centroid = (v1 + v2 + v3) / 3;
+		areaSQ = (v2-v1).crossProduct(v3-v1).getLengthSQ() / 4;
+	}
+
+	v3f getNormal() const {
+		v3f v1 = buffer->getPosition(p1);
+		v3f v2 = buffer->getPosition(p2);
+		v3f v3 = buffer->getPosition(p3);
+
+		return (v2-v1).crossProduct(v3-v1);
+	}
+};
+
+/**
+ * Implements a binary space partitioning tree 
+ * See also: https://en.wikipedia.org/wiki/Binary_space_partitioning
+ */
+class MapBlockBspTree
+{
+public:
+	MapBlockBspTree() {}
+
+	void buildTree(const std::vector<MeshTriangle> *triangles);
+
+	void traverse(v3f viewpoint, std::vector<s32> &output) const
+	{
+		traverse(root, viewpoint, output);
+	}
+
+private:
+	// Tree node definition;
+	struct TreeNode
+	{
+		v3f normal;
+		v3f origin;
+		std::vector<s32> triangle_refs;
+		s32 front_ref;
+		s32 back_ref;
+
+		TreeNode() = default;
+		TreeNode(v3f normal, v3f origin, const std::vector<s32> &triangle_refs, s32 front_ref, s32 back_ref) :
+				normal(normal), origin(origin), triangle_refs(triangle_refs), front_ref(front_ref), back_ref(back_ref)
+		{}
+	};
+
+
+	s32 buildTree(v3f normal, v3f origin, float delta, const std::vector<s32> &list, u32 depth);
+	void traverse(s32 node, v3f viewpoint, std::vector<s32> &output) const;
+
+	const std::vector<MeshTriangle> *triangles = nullptr; // this reference is managed externally
+	std::vector<TreeNode> nodes; // list of nodes
+	s32 root = -1; // index of the root node
+};
+
+class PartialMeshBuffer
+{
+public:
+	PartialMeshBuffer(scene::SMeshBuffer *buffer, const std::vector<u16> &vertex_indexes) :
+			m_buffer(buffer), m_vertex_indexes(vertex_indexes)
+	{}
+
+	scene::IMeshBuffer *getBuffer() const { return m_buffer; }
+	const std::vector<u16> &getVertexIndexes() const { return m_vertex_indexes; }
+
+	void beforeDraw() const;
+private:
+	scene::SMeshBuffer *m_buffer;
+	std::vector<u16> m_vertex_indexes;
+};
+
 /*
 	Holds a mesh for a mapblock.
 
@@ -125,6 +210,15 @@ public:
 			m_animation_force_timer--;
 	}
 
+	/// update transparent buffers to render towards the camera
+	void updateTransparentBuffers(v3f camera_pos, v3s16 block_pos);
+	void consolidateTransparentBuffers();
+
+	/// get the list of transparent buffers
+	const std::vector<PartialMeshBuffer> &getTransparentBuffers() const
+	{
+		return this->m_transparent_buffers;
+	}
 private:
 	scene::IMesh *m_mesh[MAX_TILE_LAYERS];
 	MinimapMapblock *m_minimap_mapblock;
@@ -158,6 +252,13 @@ private:
 	// of sunlit vertices
 	// Keys are pairs of (mesh index, buffer index in the mesh)
 	std::map<std::pair<u8, u32>, std::map<u32, video::SColor > > m_daynight_diffs;
+
+	// list of all semitransparent triangles in the mapblock
+	std::vector<MeshTriangle> m_transparent_triangles;
+	// Binary Space Partitioning tree for the block
+	MapBlockBspTree m_bsp_tree;
+	// Ordered list of references to parts of transparent buffers to draw
+	std::vector<PartialMeshBuffer> m_transparent_buffers;
 };
 
 /*!

src/client/tile.h

@@ -260,6 +260,18 @@ struct TileLayer
 			&& (material_flags & MATERIAL_FLAG_TILEABLE_VERTICAL);
 	}
 
+	bool isTransparent() const
+	{
+		switch (material_type) {
+		case TILE_MATERIAL_BASIC:
+		case TILE_MATERIAL_ALPHA:
+		case TILE_MATERIAL_LIQUID_TRANSPARENT:
+		case TILE_MATERIAL_WAVING_LIQUID_TRANSPARENT:
+			return true;
+		}
+		return false;
+	}
+
 	// Ordered for size, please do not reorder
 
 	video::ITexture *texture = nullptr;
@@ -308,7 +320,8 @@ struct TileSpec
 		for (int layer = 0; layer < MAX_TILE_LAYERS; layer++) {
 			if (layers[layer] != other.layers[layer])
 				return false;
-			if (!layers[layer].isTileable())
+			// Only non-transparent tiles can be merged into fast faces
+			if (layers[layer].isTransparent() || !layers[layer].isTileable())
 				return false;
 		}
 		return rotation == 0

src/defaultsettings.cpp

@@ -244,6 +244,7 @@ void set_default_settings()
 	settings->setDefault("enable_particles", "true");
 	settings->setDefault("arm_inertia", "true");
 	settings->setDefault("show_nametag_backgrounds", "true");
+	settings->setDefault("transparency_sorting_distance", "16");
 
 	settings->setDefault("enable_minimap", "true");
 	settings->setDefault("minimap_shape_round", "true");