Sets the blend operation used to blend the new alpha value with the previous one written to a render target.
Alpha blending is handled separatedly from colour blending so this can be set differently from the DX11 SET BLEND OPERATION counterpart.
The valid blend operation values map directly to the D3D11_BLEND_OP enumeration described on MSDN.

DX11 SET BLEND OPERATION ALPHA blendState, op, [renderTarget]

blendState

Dword
The blend state to set the blend operation for.

op

Dword
The blend operation to use to blend a new pixel with the data already present on a render target based on the two pixel's alpha values.

[Optional] renderTarget

Dword
An ID of a render target to set this specific blend setting for. The valid range is [0..7]. Defaults to 0 which is the value used if multiple render targets are not used by this state.

This function does not return a value.

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