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.
