Re:sketch implementation

First, thanks Aitor for this work :)

> The first draft sketch has a major problem of too
> much OO as I see it.

Well, we're going to argue between large and fine
grained OO.  As I see it, things that should certainly
be classes (thinking about a decoder here are:
  An image (your Frame class)
  The decoder itself
  The loop filtering

Below this, it becomes more risky to use OO.

One problem with using virtual functions to allow
different classes to implement MMX, SSE, etc. is that
two function indirections (thanks to virtual function
table) are needed when a function is called.  This
could be costly, I'm not sure.  Codecs like xvid use
only one function pointer indirection (e.g. when I do
a fast iDCT, I jump to the function pointed to by a
funcion pointer whose value was set at
initilialization when my processor was detected).  If
this overhead of the double indirection is not
significant, then we certainly should use virtual
functions to implement different optimizations.

The other reason for not using too much OO is it
allows people who know C but not C++ to use the code.




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