ios - 只需将 Metal frag 着色器设为透明颜色？

Question

我有一个简单的管道描述符

 let p = MTLRenderPipelineDescriptor()
 ...
 p.colorAttachments[0].pixelFormat = .bgra8Unorm

背景是纯蓝色

 let bg = MTLClearColor(red: 0, green: 0, blue: 1, alpha: 1)

 let pass = MTLRenderPassDescriptor()
 pass.colorAttachments[0].loadAction = .clear
 pass.colorAttachments[0].clearColor = bg

我只是在上面画了一个多边形，比如红色的:

fragment half4 fattie_fragment() {
    return half4(1,0,0, 1);
}

或绿色...

fragment half4 fattie_fragment() {
    return half4(0,1,0, 1);
}

在Metal中想要三角形的颜色透明怎么办？

fragment half4 fattie_fragment() {
    return half4(0,1,0, 0.275);
}

(注意 - TBC 我不希望整体 View 是透明的。只是着色器，绘制的三角形。)

Answer 1

当您配置渲染管道描述符时，启用混合并选择一组合适的混合因子和操作。例如:

let p = MTLRenderPipelineDescriptor()
...
p.colorAttachments[0].isBlendingEnabled = true
p.colorAttachments[0].rgbBlendOperation = .add
p.colorAttachments[0].alphaBlendOperation = .add
p.colorAttachments[0].sourceAlphaBlendFactor = .sourceAlpha
p.colorAttachments[0].destinationAlphaBlendFactor = .oneMinusSourceAlpha
p.colorAttachments[0].sourceRGBBlendFactor = .sourceAlpha
p.colorAttachments[0].destinationRGBBlendFactor = .oneMinusSourceAlpha