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Motion Vectors

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If the picture is not an intra picture (the bPicIntra member of the DXVA_PictureParameters structure is zero), motion vectors are included in the macroblock control command structure. The number of motion vectors that are included in the structure depends upon the type of picture (for example, B picture or P picture). Additionally, if macroblock-based reference-picture selection (as defined in H.263 Annex U) is in use, then a reference-picture selection index for each motion vector is also included in the macroblock control-command structure.

The space reserved for motion vectors in each macroblock control command structure is generally the amount needed for four motion vectors. Each motion vector is specified using a DXVA_MVvalue structure. These usual cases include the two preceding nonintra cases. The remaining cases (not explicitly defined in the dxva.h header file) are as follows:

  • If OBMC is in use (the bPicOBMC member of the DXVA_PictureParameters structure is 1) and the picture is not the B part of a PB picture (the bPicBinPB member of this structure is zero), space for 10 motion vectors, plus any additional space needed to align to a 16-byte boundary, is included.

  • If OBMC is in use (the bPicOBMC member of the DXVA_PictureParameters structure is 1) and the picture is the B part of a PB picture (the bPicBinPB member of this structure is 1), space for 11 motion vectors, plus any additional space needed to align to a 16-byte boundary, is included.

  • If OBMC is not in use (the bPicOBMC member of the DXVA_PictureParameters structure is zero), the picture is the B part of a PB picture (the bPicBinPB member of this structure is 1), and four motion vectors per macroblock are allowed (the bPic4Mvallowed member of this structure is 1), the space for five motion vectors, plus any additional space needed to align to a 16-byte boundary, is included.