如何从SCNGeometry元素中提取三角形索引

Question

我已经尝试过此方法，但似乎无法正常工作，我正在尝试使用现有节点的顶点和三角形索引创建自定义几何。

void extractInfoFromGeoElement(NSString* scenePath){
    NSURL *url = [NSURL fileURLWithPath:scenePath];
    SCNScene *scene = [SCNScene sceneWithURL:url options:nil error:nil];
    SCNGeometry *geo = scene.rootNode.childNodes.firstObject.geometry;
    SCNGeometryElement *elem = geo.geometryElements.firstObject;
    NSInteger componentOfPrimitive = (elem.primitiveType == SCNGeometryPrimitiveTypeTriangles) ? 3 : 0;
    if (!componentOfPrimitive) {//TODO: Code deals with triangle primitive only
        return;
    }
    for (int i=0; i<elem.primitiveCount; i++) {
        void *idxsPtr = NULL;
        int stride = 3*i;
        if (elem.bytesPerIndex == 2) {
            short *idxsShort = malloc(sizeof(short)*3);
            idxsPtr = idxsShort;
        }else if (elem.bytesPerIndex == 4){
            int *idxsInt = malloc(sizeof(int)*3);
            idxsPtr = idxsInt;
        }else{
            NSLog(@"unknow index type");
            return;
        }
        [elem.data getBytes:idxsPtr range:NSMakeRange(stride*elem.bytesPerIndex, elem.bytesPerIndex*3)];
        if (elem.bytesPerIndex == 2) {
            NSLog(@"triangle %d : %d, %d, %d\n",i,*(short*)idxsPtr,*((short*)idxsPtr+1),*((short*)idxsPtr+2));
        }else{
            NSLog(@"triangle %d : %d, %d, %d\n",i,*(int*)idxsPtr,*((int*)idxsPtr+1),*((int*)idxsPtr+2));
        }
        //Free
        free(idxsPtr);
    }
}