[Mp4-tech] Colocated Params in H.264

[Tourapis, Alexis]

Dear Harshit,
One issue you should not ignore is that different slices within a
picture may use different references (i.e. due to reordering) which
would basically affect the collocated parameters. Of course, I have to
admit that this code was written more in terms of convenience since it
would be most likely better, in the scenario of multiple slices, to only
compute the co-located parameters for only areas covered by that slice.
Currently this function computes this information for all macroblocks
regardless of the slice coverage. Of course, you should not take the JM
as a reference of how this should be implemented. The implementation of
this was one of the trickiest issues we encountered in the JM because of
various dependencies that existed in the software and this can (and at
some point will) be cleaned up significantly. For reference, you could
lets say immediately access the motion data directly from the list
buffers and compute the exact values only when you need them (i.e. when
you encounter a direct mode).
The same is also true for direct_8x8_inference_flag. 
Best regards,
Alexis
________________________________
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Harshit Tiwari
Sent: Friday, April 25, 2024 8:05 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] Colocated Params in H.264
Hello Folks,
I am trying to understand the compute_colocated function in JM 13.2
reference software for H.264. Some of the things are not clear to me: 
1. The field mv & ref_idx information can be computed within this
function itself. Is it too much of a computational overhead to do it
every slice rather than once per decoded image ? It is also dependent on
how frequently FRM-FLD or FLD-FRM referencing is used.
2. The flow for direct_8x8_inference_flag is not that clear in the
function. Can it not be done after computing the colocated params? in a
separate function for better clarity may be ?
Please let me know if I have misunderstood something.
Thanks in advance
-Harshit
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