BufferGeometryUtils: Let `toTrianglesDrawMode()`modify geometries in-place. (#33878)

examples/jsm/loaders/GLTFLoader.js

@@ -3776,6 +3776,18 @@ class GLTFParser {
 
 				}
 
+				// Convert strip/fan primitives to triangles
+
+				if ( primitive.mode === WEBGL_CONSTANTS.TRIANGLE_STRIP ) {
+
+					geometryPromise = geometryPromise.then( geometry => toTrianglesDrawMode( geometry, TriangleStripDrawMode ) );
+
+				} else if ( primitive.mode === WEBGL_CONSTANTS.TRIANGLE_FAN ) {
+
+					geometryPromise = geometryPromise.then( geometry => toTrianglesDrawMode( geometry, TriangleFanDrawMode ) );
+
+				}
+
 				// Cache this geometry
 				cache[ cacheKey ] = { primitive: primitive, promise: geometryPromise };
 
@@ -3854,16 +3866,6 @@ class GLTFParser {
 
 					}
 
-					if ( primitive.mode === WEBGL_CONSTANTS.TRIANGLE_STRIP ) {
-
-						mesh.geometry = toTrianglesDrawMode( mesh.geometry, TriangleStripDrawMode );
-
-					} else if ( primitive.mode === WEBGL_CONSTANTS.TRIANGLE_FAN ) {
-
-						mesh.geometry = toTrianglesDrawMode( mesh.geometry, TriangleFanDrawMode );
-
-					}
-
 				} else if ( primitive.mode === WEBGL_CONSTANTS.LINES ) {
 
 					mesh = new LineSegments( geometry, material );

examples/jsm/utils/BufferGeometryUtils.js

@@ -799,12 +799,13 @@ function mergeVertices( geometry, tolerance = 1e-4 ) {
 }
 
 /**
- * Returns a new indexed geometry based on `TrianglesDrawMode` draw mode.
- * This mode corresponds to the `gl.TRIANGLES` primitive in WebGL.
+ * Converts the given geometry to the `TrianglesDrawMode` draw mode, which
+ * corresponds to the `gl.TRIANGLES` primitive in WebGL. The conversion only
+ * rewrites the index, so the geometry is modified in place and returned.
  *
  * @param {BufferGeometry} geometry - The geometry to convert.
  * @param {number} drawMode - The current draw mode.
- * @return {BufferGeometry} The new geometry using `TrianglesDrawMode`.
+ * @return {BufferGeometry} The converted geometry using `TrianglesDrawMode`.
  */
 function toTrianglesDrawMode( geometry, drawMode ) {
 
@@ -894,13 +895,12 @@ function toTrianglesDrawMode( geometry, drawMode ) {
 
 		}
 
-		// build final geometry
+		// updated indices
 
-		const newGeometry = geometry.clone();
-		newGeometry.setIndex( newIndices );
-		newGeometry.clearGroups();
+		geometry.setIndex( newIndices );
+		geometry.clearGroups();
 
-		return newGeometry;
+		return geometry;
 
 	} else {