gltfpack: Use strip-optimized index encoding

This makes index buffer size more or less the same before zlib, but
noticeably smaller after zlib, and additionally improves vertex
compression ratio before and after zlib, sometimes dramatically.

Because the resulting index order is less efficient from ACMR
perspective, this is only done when high compression option (-cc) is
chosen.

tools/gltfpack.cpp

@@ -123,6 +123,7 @@ struct Settings
 	int texture_quality;
 
 	bool compress;
+	bool compressmore;
 	bool fallback;
 
 	int verbose;
@@ -1151,11 +1152,14 @@ void simplifyMesh(Mesh& mesh, float threshold, bool aggressive)
 	}
 }
 
-void optimizeMesh(Mesh& mesh)
+void optimizeMesh(Mesh& mesh, bool compressmore)
 {
 	size_t vertex_count = mesh.streams[0].data.size();
 
-	meshopt_optimizeVertexCache(&mesh.indices[0], &mesh.indices[0], mesh.indices.size(), vertex_count);
+	if (compressmore)
+		meshopt_optimizeVertexCacheStrip(&mesh.indices[0], &mesh.indices[0], mesh.indices.size(), vertex_count);
+	else
+		meshopt_optimizeVertexCache(&mesh.indices[0], &mesh.indices[0], mesh.indices.size(), vertex_count);
 
 	std::vector<unsigned int> remap(vertex_count);
 	size_t unique_vertices = meshopt_optimizeVertexFetchRemap(&remap[0], &mesh.indices[0], mesh.indices.size(), vertex_count);
@@ -1349,7 +1353,7 @@ void processMesh(Mesh& mesh, const Settings& settings)
 		reindexMesh(mesh);
 		filterTriangles(mesh);
 		simplifyMesh(mesh, settings.simplify_threshold, settings.simplify_aggressive);
-		optimizeMesh(mesh);
+		optimizeMesh(mesh, settings.compressmore);
 		break;
 
 	default:
@@ -4598,6 +4602,11 @@ int main(int argc, char** argv)
 		{
 			settings.compress = true;
 		}
+		else if (strcmp(arg, "-cc") == 0)
+		{
+			settings.compress = true;
+			settings.compressmore = true;
+		}
 		else if (strcmp(arg, "-cf") == 0)
 		{
 			settings.compress = true;
@@ -4667,6 +4676,7 @@ int main(int argc, char** argv)
 		fprintf(stderr, "-tc: convert all textures to KTX2 with BasisU supercompression (using basisu executable)\n");
 		fprintf(stderr, "-tq N: set texture encoding quality (default: 50; N should be between 1 and 100\n");
 		fprintf(stderr, "-c: produce compressed gltf/glb files\n");
+		fprintf(stderr, "-cc: produce compressed gltf/glb files with higher compression ratio\n");
 		fprintf(stderr, "-cf: produce compressed gltf/glb files with fallback for loaders that don't support compression\n");
 		fprintf(stderr, "-v: verbose output (print version when used without other options)\n");
 		fprintf(stderr, "-h: display this help and exit\n");