SceneKit - Crossfade material property textures

The documentation for SCNMaterialProperty.contents states that it is an animatable property and indeed I can perform a crossfade between two colors. However I’m unable to crossfade between two images.

So I’m starting to wonder if this is possible at all or if I need to create a custom shader for this?

I’ve tried an implicit animation, in which case it immediately shows the ‘after’ image:

node.geometry.firstMaterial.diffuse.contents = [UIImage imageNamed:@"before"];
[SCNTransaction begin];
[SCNTransaction setAnimationDuration:5];
node.geometry.firstMaterial.diffuse.contents = [UIImage imageNamed:@"after"];
[SCNTransaction commit];

An explicit animation, which does nothing:

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"contents"];
animation.fromValue = (__bridge id)[UIImage imageNamed:@"before"].CGImage;
animation.toValue = (__bridge id)[UIImage imageNamed:@"after"].CGImage;
animation.duration = 5;
[node.geometry.firstMaterial.diffuse addAnimation:animation forKey:nil];

As well as through a CALayer, which does nothing:

CALayer *textureLayer = [CALayer layer];
textureLayer.frame = CGRectMake(0, 0, 793, 1006);
textureLayer.contents = (__bridge id)[UIImage imageNamed:@"before"].CGImage;
node.geometry.firstMaterial.diffuse.contents = textureLayer;

CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"contents"];
animation.fromValue = (__bridge id)[UIImage imageNamed:@"before"].CGImage;
animation.toValue = (__bridge id)[UIImage imageNamed:@"after"].CGImage;
animation.duration = 5;
[textureLayer addAnimation:animation forKey:nil];

Solution

From my own testing, it doesn't look like this property is actually animatable when texture values are involved (rather than solid color values). Either that's a bug in SceneKit (i.e. it's intended to be animatable but that's not working) or it's a bug in Apple's docs (i.e. it's not intended to be animatable but they say it is). Either way, you should file that bug so you get notified when Apple fixes it.

(It also doesn't look like this is a tvOS-specific issue -- I see it on OS X as well.)

I can understand why animated texture transitions might not be there... from a GL/Metal perspective, that requires binding an extra texture unit and having two texture lookups per pixel (instead of one) during the transition.

I can think of a couple of decent potential workarounds:

Use a shader modifier. Write a GLSL(ish) snippet that looks something like this:
```
uniform sampler2D otherTexture;
uniform float fadeFactor;
#pragma body
vec4 otherTexel = texture2D(otherTexture, _surface.diffuseTexcoord);
_surface.diffuse = mix(_surface.diffuse, otherTexel, fadeFactor);
```
Set it on the material you want to animate using the SCNShaderModifierEntryPointSurface entry point. Then use setValue:forKey: to associate a SCNMaterialProperty with the otherTexture and a CABasicAnimation to animate the fadeFactor from 0 to 1.
Use something more animated (like a SpriteKit scene) as your material property contents, and animate that to perform the transition. (As a bonus, when you do it this way, you can use other transition styles.)