ARKit Adding node causes frame drop even after using `prepare` - swift

Question

I am adding a 3D model containing animations to the scene that I previously download from the internet. Before adding this node I use prepare function on it because I wan't to avoid frame drop. But still I get a very short frame drop to about 47 fps. This is caused by executing this prepare function. I also tried using prepare(_:, shouldAbortBlock:) on other dispatch queue, but this still didn't help. Can someone help me resolve this or tell me why there is this happening?
arView.sceneView.prepare([mediaNode]) { [mediaNode, weak self] (success) in
guard let `self` = self else { return }
guard
let currentMediaNode = self.mediaNode as? SCNNode,
currentMediaNode === mediaNode,
!self.mainNode.childNodes.contains(mediaNode)
else { return }
self.mainNode.addChildNode(mediaNode)
}
By the way this is a list of files I'm using to load this model:
https://www.dropbox.com/s/7968fe5wfdcxbyu/Serah-iOS.dae?dl=1
https://www.dropbox.com/s/zqb6b6rxynnvc5e/0001.png?dl=1
https://www.dropbox.com/s/hy9y8qyazkcnvef/0002.tga?dl=1
https://www.dropbox.com/s/fll9jbjud7zjlsq/0004.tga?dl=1
https://www.dropbox.com/s/4niq12mezlvi5oz/0005.png?dl=1
https://www.dropbox.com/s/wikqgd46643327i/0007.png?dl=1
https://www.dropbox.com/s/fioj9bqt90vq70c/0008.tga?dl=1
https://www.dropbox.com/s/4a5jtmccyx413j7/0010.png?dl=1
DAE file is already compiled by Xcode tools so that it can be loaded after being downloaded from the internet. And this is the code I use to load it after it's downloaded:
class func loadModel(fromURL url: URL) -> SCNNode? {
let options = [SCNSceneSource.LoadingOption.animationImportPolicy : SCNSceneSource.AnimationImportPolicy.playRepeatedly]
let sceneSource = SCNSceneSource(url: url, options: options)
let node = sceneSource?.entryWithIdentifier("MDL_Obj", withClass: SCNNode.self)
return node
}

Answer 1

I was experiencing the same issue. My nodes were all taking advantage of physically-based rendering (PBR) and the first time I added a node to the scene, the frame rate dropped significantly, but was fine after that. I could add as many other nodes without a frame rate drop.
I figured out a work around to this issue. What I do is after I create my ARConfiguration and before I call session.run(configuration) I add a test node with PBR to the scene. In order for that node to not appear, I set the node's material's colorBufferWriteMask to an empty array (see this answer: ARKit hide objects behind walls) Then before I add my content I remove that node. Adding and removing this test node does the trick for me.
Here is an example:
var pbrTestNode: SCNNode!
func addPBRTestNode() {
let testGeometrie = SCNBox(width: 0.5, height: 0.5, length: 0.5, chamferRadius: 0)
testGeometrie.materials.first?.diffuse.contents = UIColor.blue
testGeometrie.materials.first?.colorBufferWriteMask = []
testGeometrie.materials.first?.lightingModel = .physicallyBased
pbrTestNode = SCNNode(geometry: testGeometrie)
scene.rootNode.addChildNode(pbrTestNode)
}
func removePBRTestNode() {
pbrTestNode.removeFromParentNode()
}
func startSessionWithPlaneDetection() {
// Create a session configuration
let configuration = ARWorldTrackingConfiguration()
if #available(iOS 11.3, *) {
configuration.planeDetection = [.horizontal, .vertical]
} else {
configuration.planeDetection = .horizontal
}
configuration.isLightEstimationEnabled = true
// this prevents the delay when adding any nodes with PBR later
sceneController.addPBRTestNode()
// Run the view's session
sceneView.session.run(configuration)
}
Call removePBRTestNode() when you add your content to the scene.

Answer 2

Firstly
Get 3D model for AR app with no more than 10K polygons and a texture of 1K x 1K. The best result can be accomplished with 5K...7K polygons per each model. Totally, SceneKit's scene may contain not more than 100K polygons. This recommendation helps you considerably improve rendering performance and, I suppose, you'll have a minimal drop frame.
Secondly
The simplest way to get rid of drop frame in ARKit/SceneKit/AVKit is to use Metal framework. Just imagine: a simple image filter can be more than a hundred times faster to perform on the GPU than an equivalent CPU-based filter. The same things I could say about realtime AV-video and 3D animation – they perform much better on GPU.
For instance, you can read this useful post about using Metal rendering for AVCaptureSession. There's awesome workflow how to use Metal.
P.S. Check your animated object/scene in 3D authoring tool (if it's OK) before writing a code.