This example demonstrates how to modify a VisionOS RealityKit model using a custom shader graph built using Reality Compose Pro, use it in a project, and then animate the parameters of the shader graph in Swift.
The goal is to render a clipped sphere in vision OS and animate the clipping of the sphere. See the example below:
Doing this by animating the sphere's geometry at runtime would be inefficient. A better way is to use shaders. However, on VisionOS, shaders are not supported due to valid security concerns (thanks Facebook). However, RealityKit on visionOS does provide the ability to create MaterialX shaders using Reality Compose Pro. This example demonstrates how to create a custom shader graph using Reality Compose Pro, expose shader parameters to Swift, and then animate them.
This simple shader graph controls the output opacity of the geometry's pixels based on the distance from the model's center. I take the geometry's texture coordinate value (see the Texcoord node in the example), extract the Y value from it (the Swizzle_1 node), then compare it (ifgreater and ifgreater1) to user-defined parameters MIN_Y, MAX_Y to decide if the pixel should be visible or not. The output of the shader graph is an unlit surface (UnlitSurface material).
The output color of the unlit surface is hard-coded to magenta. To use this shader graph in production, you'll want to expose the color as a parameter. You may also need to create variants of this graph for different material types.
It's possible that this graph could be streamlined, but I wanted to keep it simple for this example.
The Swift code is relatively simple. You can modify a material's shader graph parameters using the following code:
try material.setParameter(name: "MIN_Y", value: .float(value))Accessing the model entities and the materials upon them is a bit of a pain, but I've been using my RealityKitSupport helper library to make it easier.
Unfortunately, RealityKit's (rather convoluted) explicit animation functionality does not support animating shader parameters. However, you can write your own timer-based animation loop to animate the parameters yourself. The code implements a SwiftUI TimelineView to animate the parameters of every frame (via RealityView's update handler). This example is straightforward, but you may want to use a more sophisticated animation implementation for an actual project.
FB13645019: RealityKit's animation system cannot animate ShaderGraphMaterial custom parameters