[M4IF Technotes] Chroma components in inter4v mode

[Gary Sullivan]

+> -----Original Message-----
+> From: Christoph Lampert [mailto:chl   math.uni-bonn.de] 
+> as far as I see from the docs, in 4 motion vectors per MB 
+> mode, chroma
+> components are compensation using the average vector of the four
+> motion vectors.
Something like that.
+> Why was it chosen like that? It sound like that's exactly the wrong
+> position, because that might be wrong for _all_ of the four blocks. 
+> Also, it makes motion search much more difficult.
Most people don't use chroma at all in the motion search.
(Although they would get better quality if they did, given
sufficient processing power.)
+> If would have been logical to either add a fifth vector, or 
+> 2 bits to 
+> signalize which of the blocks' vector to use, or something similar. 
That might not be worth the extra bits it would cost to send the
vector or indication.  And it would take extra processing power to
determine the values to put in those extra bits.
Chroma doesn't usually consume very many bits overall
relative to luma and isn't as visually important as
luma, so often it is treated as not being very important
in a codec design.  Chroma tends to take about 10-20% of
the total bit rate, so even if you could cut that in half
with some better processing method you wouldn't save that
much in the overall bit rate.
Best Regards,
Gary