Scene graph design

[kaveh808]

Greetings,

Starting new discussions on some design issues, the first one being our scene graph.

Currently we have the following classes for representing scenes:

scene, group, shape, and transform

Shape has a transform, and its subclasses such as polygon and polyhedron have geometry.

The convention in most 3D systems seems to be to have the transform be a group node and contain one or more group or geometry nodes. I think USD files also have this setup. If so, maybe we should follow the same convention. I can't see a reason not to.

We also need to determine how to encode shape/geometry names (strings or symbols), and scene dag (hierarchy) paths.

In general, perhaps we should follow the example of the USD spec (which I have not read), so we avoid gratuitous conflicts down the road?

Thoughts?

[kaveh808]

I added scene-generics.lisp which contains some functions for navigating scene graphs.

Some design decisions (for now):

• scene-item names are symbols
• scene graph paths (Maya dag paths, USD namespaces) are just lists of names

I watched a video on USD and decided it is too specialized (meant for large-scale production) and complicated for us at this point. Keeping things as simple as they can be is important. It's the top item on my kons-9 manifesto. Yes, I'm working on a manifesto. :)

The video: https://www.youtube.com/watch?v=Yp_TRVD3wjQ

I also decided to keep the transforms as is. I find it simple and flexible, and I've never liked Maya's node duality -- are you working on the node (transform) or shape (geometry).