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The alpha blending operation is one of the final operations in the Microsoft® Direct3D® Mobile rendering pipeline. It determines how the rasterizer output pixel value is combined with the existing pixel value in the render target. The alpha blending process is controlled by a number of render states.
The D3DMRS_ALPHABLENDENABLE render state (see D3DMRENDERSTATETYPE) is used to control whether alpha blending is enabled. If alpha blending is disabled, the source input pixel value is passed on as the output of the alpha blend operation. You can set the value for this render state by calling the IDirect3DMobileDevice::SetRenderStatemethod.
The D3DMRS_BLENDOP render state is used to specify the function that Direct3D Mobile should use in the alpha blending operation. The D3DMBLENDOPenumeration identifies the specific functions that are available.
The source and destination pixel values are always multiplied by a blend factor before they are passed to a blending function. The blend factors are controlled by two render states, D3DMRS_SRCBLEND and D3DMRS_DESTBLEND. These factors are applied to all three color channels and the alpha channel.