Use vertex colors (if present) as attributes during simplification

When importing meshes with vertex colors, we will now supply them to the
simplifier as attributes with weight=1 (in addition to normals). This will
guide the simplification to preserve regions where vertex colors change.

For this change to take full effect, it is also necessary to respect the
full error when selecting LODs; this change does not do that yet, so there
are going to still be cases where vertex colors change abruptly during LOD
switch in a visible manner.

scene/resources/3d/importer_mesh.cpp

@@ -314,6 +314,7 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 		Vector<Vector2> uv2s = surfaces[i].arrays[RS::ARRAY_TEX_UV2];
 		Vector<int> bones = surfaces[i].arrays[RS::ARRAY_BONES];
 		Vector<float> weights = surfaces[i].arrays[RS::ARRAY_WEIGHTS];
+		Vector<Color> colors = surfaces[i].arrays[RS::ARRAY_COLOR];
 
 		unsigned int index_count = indices.size();
 		unsigned int vertex_count = vertices.size();
@@ -368,6 +369,7 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 		const Vector2 *uvs_ptr = uvs.ptr();
 		const Vector2 *uv2s_ptr = uv2s.ptr();
 		const float *tangents_ptr = tangents.ptr();
+		const Color *colors_ptr = colors.ptr();
 
 		for (unsigned int j = 0; j < vertex_count; j++) {
 			const Vector3 &v = vertices_ptr[j];
@@ -385,7 +387,8 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 					bool is_tang_aligned = !tangents_ptr || (tangents_ptr[j * 4 + 3] < 0) == (tangents_ptr[idx.second * 4 + 3] < 0);
 					ERR_FAIL_INDEX(idx.second, normals.size());
 					bool is_normals_close = normals[idx.second].dot(n) > normal_merge_threshold;
-					if (is_uvs_close && is_uv2s_close && is_normals_close && is_tang_aligned) {
+					bool is_col_close = (!colors_ptr || colors_ptr[j].is_equal_approx(colors_ptr[idx.second]));
+					if (is_uvs_close && is_uv2s_close && is_normals_close && is_tang_aligned && is_col_close) {
 						vertex_remap.push_back(idx.first);
 						merged_normals[idx.first] += normals[idx.second];
 						merged_normals_counts[idx.first]++;
@@ -424,23 +427,50 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 		unsigned int merged_vertex_count = merged_vertices.size();
 		const Vector3 *merged_vertices_ptr = merged_vertices.ptr();
 		const int32_t *merged_indices_ptr = merged_indices.ptr();
+		Vector3 *merged_normals_ptr = merged_normals.ptr();
 
 		{
 			const int *counts_ptr = merged_normals_counts.ptr();
-			Vector3 *merged_normals_ptrw = merged_normals.ptr();
 			for (unsigned int j = 0; j < merged_vertex_count; j++) {
-				merged_normals_ptrw[j] /= counts_ptr[j];
+				merged_normals_ptr[j] /= counts_ptr[j];
 			}
 		}
 
-		const float normal_weights[3] = {
-			// Give some weight to normal preservation, may be worth exposing as an import setting
-			2.0f, 2.0f, 2.0f
-		};
-
 		Vector<float> merged_vertices_f32 = vector3_to_float32_array(merged_vertices_ptr, merged_vertex_count);
 		float scale = SurfaceTool::simplify_scale_func(merged_vertices_f32.ptr(), merged_vertex_count, sizeof(float) * 3);
 
+		const size_t attrib_count = 6; // 3 for normal + 3 for color (if present)
+
+		float attrib_weights[attrib_count] = {};
+
+		// Give more weight to normal preservation
+		attrib_weights[0] = attrib_weights[1] = attrib_weights[2] = 2.0f;
+
+		// Give some weight to colors but only if present to avoid redundant computations during simplification
+		if (colors_ptr) {
+			attrib_weights[3] = attrib_weights[4] = attrib_weights[5] = 1.0f;
+		}
+
+		LocalVector<float> merged_attribs;
+		merged_attribs.resize(merged_vertex_count * attrib_count);
+		float *merged_attribs_ptr = merged_attribs.ptr();
+
+		memset(merged_attribs_ptr, 0, merged_attribs.size() * sizeof(float));
+
+		for (unsigned int j = 0; j < merged_vertex_count; ++j) {
+			merged_attribs_ptr[j * attrib_count + 0] = merged_normals_ptr[j].x;
+			merged_attribs_ptr[j * attrib_count + 1] = merged_normals_ptr[j].y;
+			merged_attribs_ptr[j * attrib_count + 2] = merged_normals_ptr[j].z;
+
+			if (colors_ptr) {
+				unsigned int rj = vertex_inverse_remap[j];
+
+				merged_attribs_ptr[j * attrib_count + 3] = colors_ptr[rj].r;
+				merged_attribs_ptr[j * attrib_count + 4] = colors_ptr[rj].g;
+				merged_attribs_ptr[j * attrib_count + 5] = colors_ptr[rj].b;
+			}
+		}
+
 		unsigned int index_target = 12; // Start with the smallest target, 4 triangles
 		unsigned int last_index_count = 0;
 
@@ -451,7 +481,6 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 			PackedInt32Array new_indices;
 			new_indices.resize(index_count);
 
-			Vector<float> merged_normals_f32 = vector3_to_float32_array(merged_normals.ptr(), merged_normals.size());
 			const int simplify_options = SurfaceTool::SIMPLIFY_LOCK_BORDER;
 
 			size_t new_index_count = SurfaceTool::simplify_with_attrib_func(
@@ -459,9 +488,9 @@ void ImporterMesh::generate_lods(float p_normal_merge_angle, Array p_bone_transf
 					(const uint32_t *)merged_indices_ptr, index_count,
 					merged_vertices_f32.ptr(), merged_vertex_count,
 					sizeof(float) * 3, // Vertex stride
-					merged_normals_f32.ptr(),
-					sizeof(float) * 3, // Attribute stride
-					normal_weights, 3,
+					merged_attribs_ptr,
+					sizeof(float) * attrib_count, // Attribute stride
+					attrib_weights, attrib_count,
 					nullptr, // Vertex lock
 					index_target,
 					max_mesh_error,