Don't cull upper face of waving liquids below solid nodes (#17302)

src/client/content_mapblock.cpp

@@ -450,14 +450,44 @@ void MapblockMeshGenerator::drawSolidNode()
 			continue;
 		if (n2 == CONTENT_IGNORE)
 			continue;
+		// For a waving liquid source, keep the top face even when a solid node
+		// is directly above: wave animation can pull the surface down and expose
+		// a gap where the face was culled. Also keep backface culling off so the
+		// face is visible from below e.g. looking up from underwater.
+		// Submerged solids surrounded by liquid or other solid nodes on all sides are excluded.
+		bool liquid_needs_top_face = face == 0
+			&& cur_node.f->drawtype == NDT_LIQUID
+			&& cur_node.f->waving == 3
+			&& data->m_enable_waving_water;
+		if (liquid_needs_top_face) {
+			liquid_needs_top_face = false;
+			static const v3s16 h_dirs[4] = {
+				v3s16(1,0,0), v3s16(-1,0,0), v3s16(0,0,1), v3s16(0,0,-1)
+			};
+			for (const v3s16 &d : h_dirs) {
+				const ContentFeatures &f_side = nodedef->get(data->m_vmanip.getNodeNoEx(p2 + d));
+
+				bool side_is_translucent = !(f_side.visuals->solidness || f_side.visuals->visual_solidness);
+				bool side_is_same_flowing_liquid =
+					f_side.drawtype == NDT_FLOWINGLIQUID && cur_node.f->sameLiquidRender(f_side);
+
+				// Draw the top face as soon there's a translucent node diagonally above to
+				// avoid visual gaps in the liquid surface
+				if (side_is_translucent && !side_is_same_flowing_liquid) {
+					liquid_needs_top_face = true;
+					break;
+				}
+			}
+		}
 		if (n2 != CONTENT_AIR) {
 			const ContentFeatures &f2 = nodedef->get(n2);
-			if (f2.visuals->solidness == 2)
+			if (f2.visuals->solidness == 2 && !liquid_needs_top_face)
 				continue;
 			if (cur_node.f->drawtype == NDT_LIQUID) {
 				if (cur_node.f->sameLiquidRender(f2))
 					continue;
-				backface_culling = f2.visuals->solidness || f2.visuals->visual_solidness;
+				backface_culling =
+					!liquid_needs_top_face && (f2.visuals->solidness || f2.visuals->visual_solidness);
 			}
 		}
 		faces |= 1 << face;

src/client/mapblock_mesh.h

@@ -48,6 +48,7 @@ struct MeshMakeData
 	bool m_generate_minimap = false;
 	bool m_smooth_lighting = false;
 	bool m_enable_water_reflections = false;
+	bool m_enable_waving_water = false;
 
 	const NodeDefManager *m_nodedef;
 

src/client/mesh_generator_thread.cpp

@@ -91,6 +91,7 @@ MeshUpdateQueue::MeshUpdateQueue(Client *client):
 {
 	m_cache_smooth_lighting = g_settings->getBool("smooth_lighting");
 	m_cache_enable_water_reflections = g_settings->getBool("enable_water_reflections");
+	m_cache_enable_waving_water = g_settings->getBool("enable_waving_water");
 }
 
 MeshUpdateQueue::~MeshUpdateQueue()
@@ -239,6 +240,7 @@ void MeshUpdateQueue::fillDataFromMapBlocks(QueuedMeshUpdate *q)
 	data->m_generate_minimap = !!m_client->getMinimap();
 	data->m_smooth_lighting = m_cache_smooth_lighting;
 	data->m_enable_water_reflections = m_cache_enable_water_reflections;
+	data->m_enable_waving_water = m_cache_enable_waving_water;
 }
 
 /*

src/client/mesh_generator_thread.h

@@ -106,6 +106,7 @@ private:
 	// TODO: Add callback to update these when g_settings changes, and update all meshes
 	bool m_cache_smooth_lighting;
 	bool m_cache_enable_water_reflections;
+	bool m_cache_enable_waving_water;
 
 	void fillDataFromMapBlocks(QueuedMeshUpdate *q);
 };