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我从 iOS 上的 LUT 文件创建了一个 3D 纹理，如下所示：

    let dim = 33
    let textureDescriptor = MTLTextureDescriptor()
    textureDescriptor.textureType = .type3D
    textureDescriptor.pixelFormat = .rgba32Float
    textureDescriptor.width = dim
    textureDescriptor.height = dim
    textureDescriptor.depth = dim
    textureDescriptor.usage = .shaderRead

    let texture = device.makeTexture(descriptor: textureDescriptor)

    texture!.replace(region: MTLRegionMake3D(0, 0, 0, dim, dim, dim),
                    mipmapLevel:0,
                    slice:0,
                    withBytes:values!,
                    bytesPerRow:dim * MemoryLayout<Float>.size * 4,
                    bytesPerImage:dim * dim * MemoryLayout<Float>.size * 4)

然后我尝试在片段着色器中使用这个LUT，如下所示：

 fragment half4 fragmentShader ( MappedVertex in [[ stage_in ]],
                              texture2d<float, access::sample> inputTexture [[ texture(0) ]],
                              texture3d<float, access::sample> lutTexture [[ texture(1) ]]
                              )
{
   constexpr sampler s(s_address::clamp_to_edge, t_address::clamp_to_edge, min_filter::linear, mag_filter::linear);
   float3 rgb = inputTexture.sample(s, in.textureCoordinate).rgb;

   float3 lookupColor = lutTexture.sample(s, rgb).rgb;
   return half4(half3(lookupColor), 1.h);
}

恐怕我没有得到正确的结果。代码中的一切都完美吗？我是否正确采样了 3d 纹理？

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