[FFmpeg-devel] [PATCH/RFC] H.264 FMO+ASO decoding

[Jason Garrett-Glaser]

On Wed, Jul 21, 2010 at 1:20 PM, Stefan Gehrer <stefan.gehrer at gmx.de> wrote:
> Hi,
>
> attached is a patch that implements the header stuff for FMO decoding
> in H.264 baseline streams. It decodes the slice group map and also
> provides means that at the start of a slice the actual x/y position
> of the first macroblock can be determined from first_mb_in_slice.
> If this is done, slices can be decoded out of order, i.e.
> first_mb_in_slice does not have to increase for slices of the same
> picture (aka ASO).
> Let's take as example a tiny video of 4x3 macroblocks which
> has a slice group map with two slice groups like this:
>
> 1 ?1 ?1 ?1
> 1 ?0 ?0 ?1
> 1 ?1 ?1 ?1
>
> Each slice group itself (0 or 1) is decoded in raster-scan order,
> so that the macroblock addresses are as follows:
>
> 2 ?3 ?4 ?5
> 6 ?0 ?1 ?7
> 8 ?9 10 11
>
> So if a slice comes along with first_mb_in_slice equal to 7 we need
> to start decoding at MB position x=3 and y=1.
>
> Unfortunately, the real challenge starts here. A lot of neighbor
> context handling (i4x4 modes, non-zero counts, MVs) has to be
> handled differently when the assumption of raster-scan order of
> macroblocks is not true anymore. Also, deblocking can only be done
> after the picture has been fully decoded. This is because deblocking
> goes across slice boundaries and slice group boundaries and the
> neighbor MB might just happen to be the last to be decoded in the
> picture.

Not if disable_deblock_idc == 2.

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