如何在SceneKit中使用SCNBufferBindingBlock？请求使用示例

Great question! I’ve worked with this exact API before, so let’s walk through a complete, working example of using handleBinding(ofBufferNamed:frequency:handler:) to use Metal shaders with SCNNode without relying on SCNTechniques.

First, you’re right—this method lets you directly bind dynamic data buffers to your SCNProgram, skipping the need for SCNTechniques. It’s perfect for passing per-frame or per-node data to your Metal shaders.

Step 1: Create Your Metal Shader #

First, write a simple Metal shader file (save it as CustomShader.metal in your project) that expects a custom buffer:

#include <metal_stdlib>
#include <SceneKit/scn_metal>

// Define the structure of our custom buffer data
struct CustomBuffer {
    float4 tintColor;
    float elapsedTime;
};

// Vertex shader: Pass buffer data to the fragment shader
vertex SCNVertexCustom customVertex(SCNVertexInput inVertex [[stage_in]],
                                   constant CustomBuffer& buffer [[buffer(0)]]) {
    SCNVertexCustom outVertex;
    outVertex.position = inVertex.position;
    outVertex.normal = inVertex.normal;
    outVertex.texcoord0 = inVertex.texcoord0;
    // Pass the tint color to the fragment shader via vertex output
    outVertex.color = buffer.tintColor;
    return outVertex;
}

// Fragment shader: Use buffer data to modify the final color
fragment half4 customFragment(SCNVertexCustom inVertex [[stage_in]],
                             constant CustomBuffer& buffer [[buffer(0)]]) {
    half4 baseColor = half4(inVertex.color.rgb, 1.0);
    // Animate the red channel using elapsed time
    baseColor.r += sin(buffer.elapsedTime) * 0.3;
    return baseColor;
}

Step 2: Set Up the SCNProgram and Binding in Swift #

Now, let’s wire this up in Swift. Your original code was missing a buffer name and actual data writing—here’s the complete version:

import SceneKit
import QuartzCore

// Define a matching Swift struct for our Metal buffer data
struct CustomBuffer {
    var tintColor: simd_float4
    var elapsedTime: Float
}

// 1. Initialize the SCNProgram
let customProgram = SCNProgram()

// 2. Load the Metal shader code from your file
guard let shaderURL = Bundle.main.url(forResource: "CustomShader", withExtension: "metal") else {
    fatalError("Couldn't find Metal shader file")
}
do {
    let shaderSource = try String(contentsOf: shaderURL)
    customProgram.vertexShader = shaderSource
    customProgram.fragmentShader = shaderSource
} catch {
    fatalError("Failed to load shader: \(error)")
}

// 3. Define the buffer name (must match what we use in the Metal shader)
let bufferName = "CustomBuffer"

// 4. Set up the binding handler (per-frame update)
var bufferData = CustomBuffer(tintColor: simd_float4(0.2, 0.7, 0.9, 1.0), elapsedTime: 0.0)
customProgram.handleBinding(ofBufferNamed: bufferName, frequency: .perFrame) { (stream, node, shadable, renderer) in
    // Update the elapsed time with the current system time
    bufferData.elapsedTime = Float(CACurrentMediaTime())
    
    // Write the updated data to the buffer stream
    stream.writeBytes(&bufferData, length: MemoryLayout<CustomBuffer>.stride)
}

// 5. Link the buffer to the shader's semantic
customProgram.setSemantic(SCNGeometrySource.Semantic.custom.rawValue, for: .vertex, options: ["buffer": bufferName])

// 6. Attach the program to a material and node
let boxGeometry = SCNBox(width: 1, height: 1, length: 1, chamferRadius: 0.1)
let customMaterial = SCNMaterial()
customMaterial.program = customProgram
boxGeometry.materials = [customMaterial]

let boxNode = SCNNode(geometry: boxGeometry)

// Add the node to your scene (assuming you have a SCNView set up)
let scene = SCNScene()
scene.rootNode.addChildNode(boxNode)

Key Notes for Your Original Code #

• Buffer Name: You left the buffer name empty—this needs to match the identifier you use in your Metal shader (we used "CustomBuffer" here).
• Data Writing: Your callback was empty—you need to write actual data to the stream parameter so the shader can access it.
• Semantic Link: Using setSemantic tells SceneKit how to map your buffer to the shader’s input.

Using Per-Node Binding #

If you want to pass node-specific data instead of per-frame, just change the frequency to .perNode and adjust the callback to use the node parameter:

customProgram.handleBinding(ofBufferNamed: bufferName, frequency: .perNode) { (stream, node, shadable, renderer) in
    // Example: Use the node's position to modify the tint color
    let nodePosition = node.position
    bufferData.tintColor = simd_float4(Float(nodePosition.x), Float(nodePosition.y), Float(nodePosition.z), 1.0)
    bufferData.elapsedTime = Float(CACurrentMediaTime())
    
    stream.writeBytes(&bufferData, length: MemoryLayout<CustomBuffer>.stride)
}

This will update the buffer data for each node individually, letting you have unique shader behavior per node without SCNTechniques.

内容的提问来源于stack exchange，提问作者Chris