SceneKit Metal Shader使用clip_distance裁剪球体边界不平滑的解决问询

Absolutely, you can fix that jagged clipping edge and get a smooth, anti-aliased result! The issue with your current code is that you're using a hard vertex-based clip check—each triangle is either fully clipped or fully rendered, which leads to the polygonal look since your sphere is made up of triangular faces.

Here's how to implement smooth clipping: #

Instead of clipping entire triangles at the vertex stage, we'll calculate the distance from each fragment (pixel) to your clipping plane in the fragment shader, then blend the opacity of the fragment based on how close it is to the plane. This creates a soft, smooth transition between rendered and clipped areas.

Step 1: Update the Vertex Shader

We no longer need the clip_distance attribute—instead, we'll pass the model-space position of each vertex to the fragment shader so we can calculate the per-fragment distance.

#include <metal_stdlib>
using namespace metal;
#include <SceneKit/scn_metal>

struct NodeBuffer {
    float4x4 modelTransform;
    float4x4 modelViewProjectionTransform;
    float4x4 modelViewTransform;
    float4x4 normalTransform;
    float2x3 boundingBox;
};

struct VertexInput {
    float3 position [[attribute(SCNVertexSemanticPosition)]];
    float2 uv [[attribute(SCNVertexSemanticTexcoord0)]];
};

struct VertexOut {
    float4 position [[position]];
    float2 uv;
    float3 modelPosition; // Pass model-space position to fragment shader
};

vertex VertexOut textureSamplerVertex(VertexInput in [[ stage_in ]], constant NodeBuffer& scn_node [[buffer(1)]]) {
    VertexOut out;
    out.position = scn_node.modelViewProjectionTransform * float4(in.position, 1.0);
    out.uv = in.uv;
    out.modelPosition = in.position; // Send original model-space position
    return out;
}

Step 2: Modify the Fragment Shader

In the fragment shader, we'll calculate how far each pixel is from your clipping plane (z = 1.5). We'll use smoothstep to create a soft transition zone—any fragment within a small range around the plane will fade out smoothly.

fragment float4 textureSamplerFragment(VertexOut out [[ stage_in ]], texture2d<float, access::sample> customTexture [[texture(0)]]) {
    constexpr sampler textureSampler(coord::normalized, filter::linear, address::repeat);
    float4 color = customTexture.sample(textureSampler, out.uv );
    
    // Define our clipping plane and transition range
    float clipZ = 1.5;
    float transitionWidth = 0.1; // Adjust this to make the transition softer/harder
    
    // Calculate distance from fragment to clipping plane
    float distanceToClip = out.modelPosition.z - clipZ;
    
    // Use smoothstep to create a smooth alpha transition
    // Fragments behind the plane (z <= 1.5) are fully opaque; fragments in the transition zone fade out
    float alpha = smoothstep(transitionWidth, 0.0, distanceToClip);
    
    // Apply the alpha to the color
    color.a = alpha;
    
    return color;
}

Step 3: Enable Alpha Blending in SceneKit

To make the smooth fade work, you need to ensure your SCNMaterial is set up to handle transparency:

let material = SCNMaterial()
material.shaderModifiers = [.surface: "// Your shader code here"]
material.transparencyMode = .singleLayer
material.writesToDepthBuffer = true // Optional, depending on your scene setup
material.blendMode = .alpha

Why this works: #

• Instead of clipping entire triangles, we're adjusting the opacity of individual pixels based on their distance to the clipping plane.
• The smoothstep function creates a gradual transition between 0 and 1 over the transitionWidth range, which eliminates the jagged polygonal edge.
• You can tweak transitionWidth to make the edge softer (larger value) or sharper (smaller value).

内容的提问来源于stack exchange，提问作者Mark Lau