Custom geometries

[Roger H. <rog...@mi...>]

With QuickDraw3D, Apple were gradually opening up the extensibility.
First we had custom attributes, then custom renderers, then custom
groups. Then they dropped the technology and we started work on
Quesa.

I have been thinking about custom geometries. I had a look at the code
of one of the geometries which Apple themselves added, the Torus.
Its meta-handler just overrides six methods, kQ3XMethodTypeObjectNew,
kQ3XMethodTypeObjectDelete,  kQ3XMethodTypeObjectDuplicate,
kQ3XMethodTypeGeomCacheNew, kQ3XMethodTypeGeomGetAttribute
and kQ3XMethodTypeGeomUsesSubdivision.

It is possible to register a custom class which inherits from geometry, 
and
to override the first three of the methods. The next three are private 
to
Quesa, or at least they are in E3Globals.h, which is not supposed to be
a public API.

The 'get geometry attribute' method seems a fairly straightforward thing
to do. In fact I'm surprised it is not built into the geometry class 
itself,
but there may be some gotcha that makes this difficult. Anyway I
don't see too much of a problem if we were to make the constant
public by moving it to Quesa.h.

'uses subdivision' is not even a real method, it merely returns
true or false. Again I personally don't see this as much of an issue.

The kQ3XMethodTypeGeomCacheNew method is actually what
gets called when we call Q3Geometry_GetDecomposed, so in
retrospect the name could maybe have been
kQ3XMethodTypeGeomDecompose. To me this seems a
rather better name to make public. Another alternative I guess
would be to open up the submit methods for render, write,
pick and calculate bounds modes. This would of course be
a bit more complicated.

Your thoughts please on :
The whole concept of custom geometries, are we ready for them, the pros 
and cons.
The way (if found beneficial) of opening up the API to make this 
possible.

Roger Holmes.