[M4IF Technotes] Object questions

[Rob Koenen]

> 1- What's the maximum number of objects that can exist in a 
> given frame and
> does this number vary with the profile?

Max objects is per scene, and varies with Profile _and_Level_.
In Simple   Level 0 it is 1 single object; goes up to 32 for
the highest levels.
> 2- Are there any pointers on how to decide which parts of an 
> image should be
> treated as objects and which as part of the background? I 
> suppose factors
> could be size of the object, speed of movement and level of 
> contrast between
> it and its surroundings.

One thing should be clear: ALL ELEMENTS IN THE SCENE ARE OBJECTS.
Also the sound, also rectangular video, also any graphics and
text. Your question seems to relate to encoding however. 
One more thing to be clear YOU ARE NOT FORCED TO DO AN ANALYSIS
when you want to encode a sequence. Just can just treat the whole
scene as a singular rectanguler object. When you do do an analysis,
it is completely up to you how you divide the srource material
into objects. This is not normatively described in the standard.
> 
> 3- Can I arbitrarily change which objects I am using (ie 
> whereas object 1
> was a tree in frame 1, in frame 2 I decide that the tree no 
> longer needs to
> be treated as an object and so I allocate object code 1 to a 
> cow instead)?
> Or can I only make a change if the new object allocation occurs in an
> I-frame?

You can arbitrarily change objects and update the scene using BIFS.
If you decide to merge the duck into the background OBJECT, you
need to update that object. If you want to do that using an I-VOP,
fine. If you think you can do it with a P-VOP, that's your choice.
Individual objects can have their own 'object rate'.
> 4- Can objects have holes in them, like a doughnut or are 
> they assumed to be solid?

Objects can have lots of holes. You can actually use an alpha mask
to describe the object, so it can even have fuzzy edges.
> 5- I noted that I can transmit object information with or 
> without motion
> vectors and also with or without prediction errors. What if 
> the shape and
> position hasn't changed. Do I transmit anything at all and 
> assume that the
> decoder will re-draw what it had last time?

This is no different than in 'normal' video coding.
Rob