SceneKit Rigged Character Animation increase performance

Question

I have *.DAE files for characters each has 45-70 bones, I want to have about 100 animated characters on the screen.

However when I have ~60 Characters the animations takes ~13ms of my update loop which is very costly, and leaves me almost no room for other tasks.

I am setting the animations "CAAnimationGroup" to the Mesh SCNNode when I want to swap animations I am removing the previous animations with fadeOut set to 0.2 and adding the new Animation with FadeIn set to 0.2 as well. -> Is it bad ? Should I just pause a previous animation and play a new one ? or is it even worse?

Is there better ways to animate rigged characters in SceneKit maybe using GPU or something ?

Please get me started to the right direction to decrease the animations overhead in my update loop.

Update After Contacting Apple via Bug radar I received this issue via E-Mail:

This issue is being worked on to be fixed in a future update, we will let you know as soon as we have a beta build you can test and verify this issue.

Thank you for your patience.

So lets wait and see how far Apple's Engineers will enhance it :).

Answer 1

SceneKit does the skeletal animation on the GPU if your vertices have less than 4 influences. From the docs, reproduced below:

---

SceneKit performs skeletal animation on the GPU only if the componentsPerVector count in this geometry source is 4 or less. Larger vectors result in CPU-based animation and drastically reduced rendering performance.

---

I have used the following code to detect if the animation is done on the GPU:

- (void)checkGPUSkinningForInScene:(SCNScene*)character
                          forNodes:(NSArray*)skinnedNodes {
  for (NSString* nodeName in skinnedNodes) {
    SCNNode* skinnedNode =
        [character.rootNode childNodeWithName:nodeName recursively:YES];
    SCNSkinner* skinner = skinnedNode.skinner;
    NSLog(@"******** Skinner for node %@ is %@ with skeleton: %@",
          skinnedNode.name, skinner, skinner.skeleton);
    if (skinner) {
      SCNGeometrySource* boneIndices = skinner.boneIndices;
      SCNGeometrySource* boneWeights = skinner.boneWeights;
      NSInteger influences = boneWeights.componentsPerVector;
      if (influences <= 4) {
        NSLog(@" This node %@ with %lu influences is skinned on the GPU",
              skinnedNode.name, influences);
      } else {
        NSLog(@" This node %@ with %lu influences is skinned on the CPU",
              skinnedNode.name, influences);
      }
    }
  }
}

You pass the SCNScene and the names of nodes which have SCNSkinner attached to check if the animation is done on the GPU or the CPU.

However, there is one other hidden piece of information about animation on the GPU which is that if your skeleton has more than 60 bones, it won't be executed on the GPU. The trick to know that is to print the default vertex shader, by attaching an invalid shader modifier entry as explained in this post.

The vertex shader contains the following skinning related code:

#ifdef USE_SKINNING
uniform vec4 u_skinningJointMatrices[60];

....

    #ifdef USE_SKINNING
  {
    vec3 pos = vec3(0.);
    #ifdef USE_NORMAL
    vec3 nrm = vec3(0.);
    #endif
  #if defined(USE_TANGENT) || defined(USE_BITANGENT)
    vec3 tgt = vec3(0.);
    #endif
    for (int i = 0; i < MAX_BONE_INFLUENCES; ++i) {
#if MAX_BONE_INFLUENCES == 1
        float weight = 1.0;
#else
        float weight = a_skinningWeights[i];
#endif
      int idx = int(a_skinningJoints[i]) * 3;
      mat4 jointMatrix = mat4(u_skinningJointMatrices[idx], u_skinningJointMatrices[idx+1], u_skinningJointMatrices[idx+2], vec4(0., 0., 0., 1.));
            pos += (_geometry.position * jointMatrix).xyz * weight;
      #ifdef USE_NORMAL
            nrm += _geometry.normal * mat3(jointMatrix) * weight;
      #endif
      #if defined(USE_TANGENT) || defined(USE_BITANGENT)
            tgt += _geometry.tangent.xyz * mat3(jointMatrix) * weight;
      #endif
    }
    _geometry.position.xyz = pos;

which clearly implies that your skeleton should be restricted to 60 bones.

If all your characters have the same skeleton, then I would suggest just check if the animation is executed on CPU or GPU using the above tips. Otherwise you may have to fix your character skeleton to have less than 60 bones and not more than 4 influences per vertex.