如何在Metal管道中同时使用SCNRenderer和CIContext渲染？

Question

我是Metal的新手，尝试在present()和commit()之前在MTLTexture上绘制两次。第一次使用SCNRenderer.render从SCNScene绘制背景，然后第二次使用CIContext.render对具有透明度的用户进行深度过滤。

我的renderPassDescriptor设置为：

    // SCNRenderer
    let viewport = CGRect(x: 0, y: 0, width: CGFloat(currentDrawable.texture.width), height: CGFloat(currentDrawable.texture.width))

    //write to offscreenTexture, clear the texture before rendering using red, store the result
    let renderPassDescriptor = MTLRenderPassDescriptor()
    renderPassDescriptor.colorAttachments[0].texture = currentDrawable.texture
    renderPassDescriptor.colorAttachments[0].loadAction = .clear
    renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(1, 0, 0, 1.0); // red
    renderPassDescriptor.colorAttachments[0].storeAction = .store

    scnRenderer.render(atTime: 0, viewport: viewport, commandBuffer: commandBuffer, passDescriptor: renderPassDescriptor)

    // CIContext render
    ciContext.render(ciImage, to: currentDrawable.texture, commandBuffer: commandBuffer, bounds: ciImage.extent, colorSpace: colorSpace)

如果我切换了scnRenderer.render和ciContext的顺序，则按预期出现在屏幕上。

由于CIContext的第二次绘制使用具有透明度的图像，因此我在此article之后启用了alpha混合。

// Enable alpha Blending
        pipelineDescriptor.colorAttachments[0].isBlendingEnabled           = true
        pipelineDescriptor.colorAttachments[0].rgbBlendOperation           = .add
        pipelineDescriptor.colorAttachments[0].alphaBlendOperation         = .add
        pipelineDescriptor.colorAttachments[0].sourceRGBBlendFactor        = .sourceAlpha
        pipelineDescriptor.colorAttachments[0].sourceAlphaBlendFactor      = .sourceAlpha
        pipelineDescriptor.colorAttachments[0].destinationRGBBlendFactor   = .oneMinusSourceAlpha
        pipelineDescriptor.colorAttachments[0].destinationAlphaBlendFactor = .oneMinusSourceAlpha

这是通过片段着色器的功能：

fragment float4 passThroughFragment(ColorInOut in [[ stage_in ]],
                                texture2d<float> texture [[ texture(0) ]]) {
constexpr sampler colorSampler;
float4 color = texture.sample(colorSampler, in.texCoord);
return color;
}

如何防止CIImage的透明度覆盖为SCNScene绘制的片段颜色？谢谢。