clusterizer: Fix meshlet construction for an empty index set

An empty submesh should result in 0 generated clusters; this was
previously asserting in kdtreeBuild - the assertion was benign but it
seems better to support this via an early out.

Author: zeux

demo/tests.cpp

@@ -1059,6 +1059,17 @@ static void clusterBoundsDegenerate()
 	assert(bounds2.center[2] - bounds2.radius <= 0 && bounds2.center[2] + bounds2.radius >= 1);
 }
 
+static void meshletsEmpty()
+{
+	const float vbd[4 * 3] = {};
+
+	meshopt_Meshlet ml[1];
+	unsigned int mv[4];
+	unsigned char mt[8];
+	size_t mc = meshopt_buildMeshlets(ml, mv, mt, NULL, 0, vbd, 4, sizeof(float) * 3, 64, 64, 0.f);
+	assert(mc == 0);
+}
+
 static void meshletsDense()
 {
 	const float vbd[4 * 3] = {};
@@ -2187,6 +2198,7 @@ void runTests()
 
 	clusterBoundsDegenerate();
 
+	meshletsEmpty();
 	meshletsDense();
 	meshletsSparse();
 

src/clusterizer.cpp

@@ -615,6 +615,9 @@ size_t meshopt_buildMeshlets(meshopt_Meshlet* meshlets, unsigned int* meshlet_ve
 
 	assert(cone_weight >= 0 && cone_weight <= 1);
 
+	if (index_count == 0)
+		return 0;
+
 	meshopt_Allocator allocator;
 
 	TriangleAdjacency2 adjacency = {};