[M4IF Technotes] Error resilience -- what does it mean to discard a MB?

[Gaurav Aggarwal]

Sorry for being a little naive and not searching through archives closely
to understand the issue. My query relates to error resilience tools of
MPEG-4 SP/ASP.
I am having trouble with the term "discard a macroblock" based on the
four stragies desribed in the Annex E of the Visual Spec (part 2). In short,
for an I-VOP, the DC coefficients are together and the AC separate.
Suppose, the AC coefficients have a problem for a few macroblocks.
Now, for the corresponding macroblock, should I display the "DC"
uniform 16x16 patch, or should I use the macroblock at the corresponding
location in the previous picture?
Also, should I use this erroneous macroblock for DC/AC coeff prediction
and for motion prediction? I guess, do not use for either.
Finally, the Annex E says that for an I-VOP, do not display it if the
four strategies say that its fine to use a macroblock. Doesn't this
imply that reversible VLCs have NO meaning for an I-VOP, because each
macroblock will be intra, and hence no need bothering to decode the
AC coefficients in reverse direction because anyway an intra-MB will
not be decoded...
Would really appreciate if you can point me to some reference
implementation or other relevant sources.
And BTW, is handling of errors using the 4 strategies "normative" or
informative. Do I need to definitely support RVLC in backward direction
and Data Partitioning to be truly MPEG-4 SP compliant?
Thanks,
Gaurav
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