Encoding visual objects

Is it possible to encode differently visual objects using Xvid ?

I mean:

- I've got a series of original images, e.g. showing one or two walking men
- I've got a series greylevel image with black, grey and white pixels, 
e.g. obtained in some way from the original images

I want that Xvid encodes a MPEG-4 video in which the pixels of the 
original images corresponding to the white pixels are encoded with a 
higher bitrate than the pixels corresponding to grey pixels, and the 
pixels corresponding to black pixels should have the lowest bitrate.

This should be part of the MPEG-4 core profile, and I think that MoMuSys 
somehow managed this.

Thank you,
  Marco

-- 
| Ing. Marco Bertini - http://viplab.dsi.unifi.it/~bertini
| Dipartimento Sistemi & Informatica
| Universita' di Firenze - Ingegneria elettronica