Render a 3D model (hair) with semi-transparent texture in SceneKit?

Question

I'm trying to render a 3D model in SceneKit but it looks incorrect. For example this model (it's an SCN file with texture and you can reproduce it in your Xcode):

In Xcode Scene Editor it is rendered like this:

Transparency -> Mode -> Dual Layer
Double Sided = true

If I turn off the "Write depth" option, it will look like this: But there are also some issues because it I see only "the lowest layer" of haircut.

I think this should be possible. How to do it right?

Answer 1

The reason that in your 3D model some strands of hair popped out when viewed from different angles is quite usual for SceneKit: your model has semi-transparent material that SceneKit can't render properly due to some inner engine rendering techniques (time 49:35) applied to depth buffer.

In order to deal with this problem there are two solutions:

Solution 1:

Your 3D model must have a completely opaque texture (without semi-transparent parts at all). In that case use .dualLayer property.

let scene = SCNScene(named: "art.scnassets/Hair.scn")!
let hair = scene.rootNode.childNode(withName: "MDL_OBJ", recursively: true)!

hair.geometry?.firstMaterial?.transparencyMode = SCNTransparencyMode.dualLayer

Solution 2:

Strands of hair mustn't be a mono-geometry but must be a compound geometry (consisted of several geometry layers unified in one group).

hair.geometry?.firstMaterial?.colorBufferWriteMask = SCNColorMask.all
hair.geometry?.firstMaterial?.readsFromDepthBuffer = false
hair.geometry?.firstMaterial?.writesToDepthBuffer = false
hair.geometry?.firstMaterial?.blendMode = SCNBlendMode.alpha