Enabling Alpha Channels On Full-Screen Back Buffers

Enabling Alpha Channels On Full-Screen Back Buffers

In the DirectDraw DDI, the creation of a primary flipping chain has no intrinsic pixel format. Consequently, surfaces in this chain take on the pixel format of the display mode. For example, a primary flipping chain created in a 32bpp mode takes on a D3DFMT_X8R8G8B8 format.

Such a chain is created for many full-screen applications. Because the back buffer of the chain has no alpha channel, the D3DRS_ALPHABLENDENABLE render state and the associated blend-render states for destination surfaces are poorly defined. DirectX 8.1 introduces a new feature that the Direct3D runtime uses to inform a driver of an application's request to create a full-screen flipping chain of surfaces with an alpha channel in the pixel formats of those surfaces.

To indicate support of this feature, the driver must set the D3DCAPS3_ALPHA_FULLSCREEN_FLIP_OR_DISCARD bit (defined in the d3d8caps.h file) in the Caps3 member of the D3DCAPS8 structure. The driver returns a D3DCAPS8 structure in response to a GetDriverInfo2 query as described in Reporting DirectX 8.0 Style Direct3D Capabilities. Support of this query is described in Supporting GetDriverInfo2.

After support of this feature is determined, the driver can receive DdCreateSurface calls with the DDSCAPS2_ENABLEALPHACHANNEL (defined in the ddraw.h file) bit set in the dwCaps2 member of the DDSCAPS2 structure. This bit is only set to create surfaces that are part of a primary flipping chain or that are on stand-alone back buffers.

If the driver detects this bit, the driver determines that the surfaces take on not the display mode's format, but the display mode's format plus alpha. For example, in a 32bpp mode, such surfaces should be given the D3DFMT_A8R8G8B8 format.

This feature is available on Windows XP and later versions and on Windows 2000 operating system versions that have the DirectX 8.1 runtime installed.



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