[XviD-devel] help me please (about setedges() and others)

[Christoph Lampert]

On Wed, 29 Oct 2003, Nicola Bicocchi wrote:
>   i'm playing with xvid in my thesis and i don't understand some things:

Hi,
what's the thesis about? 

> 1) in encoder creation: 
>   pEnc->mbParam.edged_width = 16 * pEnc->mbParam.mb_width + 2 * EDGE_SIZE;
>   pEnc->mbParam.edged_height = 16 * pEnc->mbParam.mb_height + 2 * EDGE_SIZE;
> 
> why edged_width and height are so big ?

It's not very big, it's just 2*EDGE_SIZE (1 left, 1 right) larger than the
original frame. mb_width is the width in "number of macroblocks"! Those
are 16 pixels wide each. 

> which is the function of the extra memory allocated ?
> i don't understand the function image_setedges() called in FrameCodeP. what does it do ?

A general rule in MPEG-4 is the following: If a position (e.g. a
vector) points to outside the (visible) frame, the pixel value at the edge is 
used instead of the nonavailable one. 
Since it would be too slow to always check for outside boundaries and 
decide to use edges pixel instead, we just add a region of (at
least) 16 pixels to every edge, which is identically filled with this edge
pixel value. Then e.g. a macroblock lying half inside and half outside the
normal frame can be just be treated the same as one completely inside the
frame, so we get rid of all checks and special cases. 


> 2) i would like to store videos recorded by a fixed camera.
> in these videos background stands still and moving objects are quite little.
> in your opinion are there big problems if i would like to code not the whole frame
> but just a part of it (a rectangular region) ??

Yes. The big problem is, that it wouldn't work with XviD, and you would
have to leave MPEG-4 Advanced Simple Profile. There is support for
encoding arbitrary objects, but that's only in Main Profile. XviD is far
from that. You can't just encode a part of the frame, that's not possible
with the current routines. 
If you knew in advance, that some parts of the image do not move and will
never move, then you can crop the input. But with moving objects, the
changing reagion of course changes over time. 

But that it not much of a problem: If a part of the frame doesn't change
at all, it is usually (for P- and B-VOPs) encoded in SKIP mode which
means, each 16x16 macroblock takes just a single bit in the bitstream. 

gruel