[Mp4-tech] Conversion Of H.264 stream to H.263 stream

[Sandip Ray]

Dear Chiranjeevi,
In a typical heterogeneous trancoding(i.e. conversion into a bitstream of
different syntax elements), most of the parameters in the bitstream are
mapped from one domain(e.g. H.264 Baseline) to another domain(e.g. H.263),
in such a way that the final reconstructed video has acceptable PSNR and
subjective quality. The mapping reduces the complexity of the transcoder.
The parameters which can't be mapped from one domain to another domain, need
to be decoded completely(or discarded if possible).
H.264 uses CAVLC(Baseline) and CABAC(Main) as entropy coding tool which are
quite different from much simpler VLC used in H.263. Hence variable length
decoding of H.264 bitstream must happen before encoding further. H.264 uses
4x4 integer transform whereas H.263 uses 8x8 DCT for residual pixel values.
I am not aware of any existing algorithm which can map between the two
transform coefficients. Another issue lies in in-loop deblocking filter and
multiple reference frames used in H.264. Deblocking filter modifies pixel
values in all of the reference frames in H.264. Anyway the last two features
can be disabled in H.264 encoder. In short, almost full decoding of H.264
bitstream is required in order get the residual pixel values. Then H.263
encoder can be cascaded to generate H.263 bitstream.
Regards,
Sandip
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