demo: Remove single cluster group special casing

meshopt partitioner currently will not merge completely disconnected
clusters, which in some meshes can create a set of early clusters that
are not simplified further. METIS will merge these instead, which alters
the high level DAG construction flow.

It's not clear why single cluster groups should be special: a group with
merged disconnected clusters is just effectively synchronizing the LOD
transition decisions between different clusters.

In the future, meshopt will likely start merging clusters together, but
for now it's cleaner to remove this special case.

Author: zeux

demo/nanite.cpp

@@ -584,7 +584,6 @@ void nanite(const std::vector<Vertex>& vertices, const std::vector<unsigned int>
 
 		size_t triangles = 0;
 		size_t stuck_triangles = 0;
-		int single_clusters = 0;
 		int stuck_clusters = 0;
 		int full_clusters = 0;
 		size_t components_lod = 0;
@@ -603,22 +602,6 @@ void nanite(const std::vector<Vertex>& vertices, const std::vector<unsigned int>
 			if (groups[i].empty())
 				continue; // metis shortcut
 
-			if (groups[i].size() == 1)
-			{
-#if TRACE
-				printf("stuck cluster: singleton with %d triangles\n", int(clusters[groups[i][0]].indices.size() / 3));
-#endif
-
-				if (dump && depth == atoi(dump))
-					dumpObj("cluster", clusters[groups[i][0]].indices);
-
-				single_clusters++;
-				stuck_clusters++;
-				stuck_triangles += clusters[groups[i][0]].indices.size() / 3;
-				retry.push_back(groups[i][0]);
-				continue;
-			}
-
 			std::vector<unsigned int> merged;
 			for (size_t j = 0; j < groups[i].size(); ++j)
 				merged.insert(merged.end(), clusters[groups[i][j]].indices.begin(), clusters[groups[i][j]].indices.end());
@@ -636,7 +619,7 @@ void nanite(const std::vector<Vertex>& vertices, const std::vector<unsigned int>
 
 			float error = 0.f;
 			std::vector<unsigned int> simplified = simplify(vertices, merged, kUseLocks ? &locks : NULL, target_size, &error);
-			if (simplified.size() > merged.size() * kSimplifyThreshold || simplified.size() / (kClusterSize * 3) >= merged.size() / (kClusterSize * 3))
+			if (simplified.size() > merged.size() * kSimplifyThreshold)
 			{
 #if TRACE
 				printf("stuck cluster: simplified %d => %d over threshold\n", int(merged.size() / 3), int(simplified.size() / 3));
@@ -693,7 +676,7 @@ void nanite(const std::vector<Vertex>& vertices, const std::vector<unsigned int>
 		    double(full_clusters) * inv_clusters * 100, double(triangles) * inv_clusters, double(xformed_lod) * inv_clusters, double(components_lod) * inv_clusters, double(boundary_lod) * inv_clusters, avg_group,
 		    int(triangles));
 		if (stuck_clusters)
-			printf("; stuck %d clusters (%d single, %d triangles)", stuck_clusters, single_clusters, int(stuck_triangles));
+			printf("; stuck %d clusters (%d triangles)", stuck_clusters, int(stuck_triangles));
 		printf("\n");
 
 		if (kUseRetry)