Metal alpha blending is not working

Question

For whatever reason I am having issues with alpha blending in metal. I am drawing to a MTKView and for every pipeline that I create I do the following:

descriptor.colorAttachments[0].blendingEnabled = YES;
descriptor.colorAttachments[0].rgbBlendOperation = MTLBlendOperationAdd;
descriptor.colorAttachments[0].alphaBlendOperation = MTLBlendOperationAdd;
descriptor.colorAttachments[0].sourceRGBBlendFactor = MTLBlendFactorSourceAlpha;
descriptor.colorAttachments[0].sourceAlphaBlendFactor = MTLBlendFactorSourceAlpha;
descriptor.colorAttachments[0].destinationRGBBlendFactor = MTLBlendFactorOneMinusSourceAlpha;
descriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactorOneMinusSourceAlpha;

However for whatever reason that is not causing alpha testing to happen. You can even check in the frame debugger and you will see vertices with an alpha of 0 that are being drawn black rather than transparent.

One thought I had is that some geometry ends up on the exact same z plane so if alpha blending does not work on the same z plane that might cause an issue. But I dont think that is a thing.

Why is alpha blending not working?

I am hoping to blend as if they were transparent glass. Think like this.

Answer 1

Alpha blending is an order-dependent transparency technique. This means that the (semi-)transparent objects cannot be rendered in any arbitrary order as is the case for (more expensive) order-independent transparency techniques.

1. Make sure your transparent 2D objects (e.g., circle, rectangle, etc.) have different depth values. (This way you can define the draw ordering yourself. Otherwise the draw ordering depends on the implementation of the sorting algorithm and the initial ordering before sorting.)
2. Sort these 2D objects based on their depth value from back to front.
3. Draw the 2D objects from back to front (painter's algorithm) using alpha blending. (Of course, your 2D objects need an alpha value < 1 to actually see some blending.)

Your blend state for alpha blending is correct:

// The blend formula is defined as:
// (source.rgb * sourceRGBBlendFactor  )   rgbBlendOperation (destination.rgb * destinationRGBBlendFactor  ) 
// (source.a   * sourceAlphaBlendFactor) alphaBlendOperation (destination.a   * destinationAlphaBlendFactor)
// <=>
// (source.rgba * source.a) + (destination.rgba * (1-source.a))

descriptor.colorAttachments[0].blendingEnabled             = YES;
descriptor.colorAttachments[0].rgbBlendOperation           = MTLBlendOperationAdd;
descriptor.colorAttachments[0].alphaBlendOperation         = MTLBlendOperationAdd;
descriptor.colorAttachments[0].sourceRGBBlendFactor        = MTLBlendFactorSourceAlpha;
descriptor.colorAttachments[0].sourceAlphaBlendFactor      = MTLBlendFactorSourceAlpha;
descriptor.colorAttachments[0].destinationRGBBlendFactor   = MTLBlendFactorOneMinusSourceAlpha;
descriptor.colorAttachments[0].destinationAlphaBlendFactor = MTLBlendFactorOneMinusSourceAlpha;