[Mp4-tech] h.264 codec(decoding of 4x4 intraprediction mode) JM9.2(FRExt)

[Alexis Michael Tourapis]

Dear Suraj,
Karsten answered the same question a few days back. pix_a corresponds to pixels on the left, while pix_b, pix_c, pix_d correspond to different pixel positions above your current one. In MBAFF mode it is possible for a macroblock to be in either field or frame mode. This creates an "issue" at macroblock edges for intra prediction since it is possible for the adjacent blocks to be in a different coding mode than the current one (i.e. current is frame while adjacent is in field and vice versa). The basic idea behind this is that pixels need to be predicted horizontally from exactly the same "parity" adjacent pixels and not from opposite parity ones.
Although this is not really an issue for prediction pixels coming from MBs above your current position (only one block line needs to be read which corresponds to the same field parity), you would need to properly select the positions of the prediction pixels coming from the left depending on the coding modes of the current and left neighboring MBs. For example if current is frame and left is in field, pix_a[0] and pix[2] should correspond to even parity fields and therefore should be taken from the top MB of the adjacent MB pair. pix_a[1] and pix[3] however correspond to odd parity and therefore should be taken from the bottom MB instead. Somehow similar rules exist if current is in field mode and adjacent is frame. I think you should probably check section 6.4.9.2 for more information.
Best regards,
Alexis
> hello experts
> 
> could anyone give a clear explanation on the following
> variables and why array is been used only for pix_a
> PixelPos pix_a[4];
> PixelPos pix_b, pix_c, pix_d;
> 
> best regards 
> suraj kumar
> 
> 
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