Possible to use 1/2 and 1/4 pel?

[TheXung]

This might seem like a dumb question, but is it possible to have one pframe 1/4 pel precision and then the next one only use 1/2?

This is a crazy hunch but it seems that in low motion, the overhead of storing twice the precision motion vectors is less than the gain from the better motion estimation. However, high motion frames will have a larger overhead to overcome and sometimes it is not overcome by the gain from better motion estimation.

Does the mpeg4 spec allow this?

[Koepi]

qpel is just another refinement step. so in most cases a MV will most likely end up in a fullpel or halfpel position, not needing "additional precision".

How would you decide on "halfpel frame" and "qpel frame"?

I think it's impossible and superflous. Either use qpel or leave your hands from it if you don't like the "increasing filesize" (the overhead is minimal...).

Koepi

[sysKin]

Originally posted by TheXung
This might seem like a dumb question, but is it possible to have one pframe 1/4 pel precision and then the next one only use 1/2?

Actually, xvid is ready to do this ;) It stores halfpel and qpel vectors independely, also with their SADs and all other data... There will be no problem with creating a simple loop which will compare qpel results vs halfpel results, and choose one before writing any macroblocks to the bitstream.
I think this was meant this way from the beginning.