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Decoding Video

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The Microsoft Direct3D runtime calls the user-mode display driver's DecodeBeginFrame and DecodeEndFrame functions to indicate a time period between these function calls that the user-mode display driver can decode video. Before the user-mode display driver can perform any video decode operations, the Microsoft Direct3D runtime must call the user-mode display driver's SetDecodeRenderTarget function to set the render target surface for those decode operations. However, the call to SetDecodeRenderTarget can occur only outside the begin-frame and end-frame time period.

In protected mode and in the call to DecodeBeginFrame, the Direct3D runtime sets or changes a DirectX VA content key in a variable that the pPVPSetKey member of the D3DDDIARG_DECODEBEGINFRAME structure points to. The decode device uses this key for protected transfers of the compressed DirectX VA buffers for this and subsequent frames.

Note   The Direct3D runtime sets the pPVPSetKey pointer only to change or set the key. To keep the previously set key in use, the runtime sets the pointer to NULL to avoid potentially time consuming reloading of the same key. The driver does not eliminate the redundant settings. A decoder application must avoid redundant settings.

After the render target surface for decode operations is set, the user-mode display driver can receive calls to its DecodeExecute function to perform video decode operations between the begin-frame and end-frame time period.

In calls to DecodeExecute, not all of the buffer types that are specified in the CompressedBufferType members of the DXVADDI_DECODEBUFFERDESC structures of the pCompressedBuffers array of the D3DDDIARG_DECODEEXECUTE structure are used for each decode GUID that the hDecode member of D3DDDIARG_DECODEEXECUTE specifies. For example, the slice-control (D3DDDIFMT_SLICECONTROLDATA), inverse-quantization (D3DDDIFMT_INVERSEQUANTIZATIONDATA), and bit-stream (D3DDDIFMT_BITSTREAMDATA) buffers are required only for variable-length decode (VLD) processing, and the deblocking-control buffer (D3DDDIFMT_DEBLOCKINGDATA) is not used by MPEG-2 at all.

In protected mode, the buffers that were encrypted for a protected transfer with a content key contain a pointer to initial counter values in their buffer descriptors (that is, in variables that the pCipherCounter members of the DXVADDI_DECODEBUFFERDESC structures point to). Each call to the user-mode display driver's DecodeExecute function must perform a protected transfer of such buffers to local video memory before DecodeExecute uses the buffers' data in the decode operation. However, no plans exist to encrypt DirectX VA compressed buffers of types other than residual-difference (D3DDDIFMT_RESIDUALDIFFERENCEDATA) and bit-stream (D3DDDIFMT_BITSTREAMDATA) types.