如何在USDZ格式中通过USDA材质编辑实现纹理相加/相乘混合运算

可以通过标准USDA材质定义实现该纹理混合需求，USD内置的基础运算节点完全支持你所需的公式逻辑，且兼容iOS原生USDZ预览能力。

你需要的混合公式 (A * mask) + (B * (1 - mask)) 可通过4个USD标准运算节点实现：

• 减法节点：计算1 - 遮罩值得到反向遮罩
• 乘法节点1：计算纹理A与遮罩值的乘积
• 乘法节点2：计算纹理B与反向遮罩的乘积
• 加法节点：将两个乘法结果相加得到最终混合色

以下为修正了原代码中遮罩纹理UV连接笔误后的完整可运行USDA材质定义：

def Material "MixedMaterial"
{
    token outputs:surface.connect = </Object1/MixedMaterial/SurfaceShader.outputs:surface>

    def Shader "SurfaceShader"
    {
        uniform token info:id = "UsdPreviewSurface"
        color3f inputs:diffuseColor.connect = </Object1/MixedMaterial/MixedColorResult.outputs:rgb>
        token outputs:surface
    }

    def Shader "Texture0"
    {
        uniform token info:id = "UsdUVTexture"
        asset inputs:file = @textureA.jpg@
        float2 inputs:st.connect = </Object1/MixedMaterial/Texture0/TexCoordReader0.outputs:result>
        token inputs:wrapS = "repeat"
        token inputs:wrapT = "repeat"
        float outputs:r
        float3 outputs:rgb

        def Shader "TexCoordReader0"
        {
            uniform token info:id = "UsdPrimvarReader_float2"
            token inputs:varname = "UV_Channel_1"
            float2 outputs:result
        }
    }

    def Shader "Texture1"
    {
        uniform token info:id = "UsdUVTexture"
        asset inputs:file = @textureB.jpg@
        float2 inputs:st.connect = </Object1/MixedMaterial/Texture1/TexCoordReader1.outputs:result>
        token inputs:wrapS = "repeat"
        token inputs:wrapT = "repeat"
        float outputs:r
        float3 outputs:rgb

        def Shader "TexCoordReader1"
        {
            uniform token info:id = "UsdPrimvarReader_float2"
            token inputs:varname = "UV_Channel_2"
            float2 outputs:result
        }
    }

    def Shader "Texture2"
    {
        uniform token info:id = "UsdUVTexture"
        asset inputs:file = @Mask.jpg@
        float2 inputs:st.connect = </Object1/MixedMaterial/Texture2/TexCoordReader2.outputs:result>
        token inputs:wrapS = "repeat"
        token inputs:wrapT = "repeat"
        float outputs:r
        float3 outputs:rgb

        def Shader "TexCoordReader2"
        {
            uniform token info:id = "UsdPrimvarReader_float2"
            token inputs:varname = "UV_Channel_3"
            float2 outputs:result
        }
    }
    
    def Shader "MaskSubtract"
    {
        uniform token info:id = "UsdSubtractFloat"
        float inputs:a = 1.0
        float inputs:b.connect = </Object1/MixedMaterial/Texture2.outputs:r>
        float outputs:result
    }

    def Shader "MultiplyAWithMask"
    {
        uniform token info:id = "UsdMultiplyColor3f"
        color3f inputs:a.connect = </Object1/MixedMaterial/Texture0.outputs:rgb>
        float inputs:b.connect = </Object1/MixedMaterial/Texture2.outputs:r>
        color3f outputs:result
    }

    def Shader "MultiplyBWithInverseMask"
    {
        uniform token info:id = "UsdMultiplyColor3f"
        color3f inputs:a.connect = </Object1/MixedMaterial/Texture1.outputs:rgb>
        float inputs:b.connect = </Object1/MixedMaterial/MaskSubtract.outputs:result>
        color3f outputs:result
    }

    def Shader "MixedColorResult"
    {
        uniform token info:id = "UsdAddColor3f"
        color3f inputs:a.connect = </Object1/MixedMaterial/MultiplyAWithMask.outputs:result>
        color3f inputs:b.connect = </Object1/MixedMaterial/MultiplyBWithInverseMask.outputs:result>
        color3f outputs:rgb
    }
}

上述代码用到的所有运算节点均为USD官方定义的标准Shader节点，iOS原生QuickLook预览、ARKit渲染均已原生支持，无需额外自定义材质逻辑。

内容的提问来源于stack exchange，提问作者AGogel