Re: [Plib-devel] ssg Leaf call backs

[Steve B. <sjb...@ai...>]

"Curtis L. Olson" wrote:

> It would be convenient to be able to attach pre/post draw call backs
> to branch nodes as well as leaf nodes.  ssg allows me to do this now,
> but it never calls the functions attached to branches, only leaf
> nodes.

The trouble is that leaf nodes are not rendered in order - even though
I don't (yet) sort by material property, I do render translucent leaf
nodes after all the opaque nodes.

Hence, if a branch node were to contain both translucent and opaque
leaves, there would be no good time to call the callbacks.

In future (when I get around to some material sorting), this would be
an even bigger problem.

> The alternative is to depend on the (?non-defined?) behavior that the
> first kid added to a branch is the first one processed during
> ssgCullandDraw().  This doesn't seem very satisfying or robust...
> especially if I later change the leaf nodes and forget to attach the
> call back to the first leaf.

It *definitely* doesn't work - even now - because of the transparency
thing - and even if you are getting away with it right now, I can
promise to break your code sometime in the future.
 
> Or we could be wasteful and attach the callback to every leaf node in
> a sub tree, but I'd rather not do that either, but it would be the
> safest I suppose.

Yep.  I guess that's what you need to do.  :-(
 
> These things would also need to be considered if you ever impliment
> state sorting ... yuk ... the callbacks could get out of sync with the
> leaf nodes being drawn!  Perhaps we could add some sort of marker node
> such that everything below a marker would be state sorted then drawn.
> Then you continue traversing/rendering the rest of the tree as before.

...or call the per-branch callbacks multiple times - once for each batch
of its child nodes.  That would be pretty ugly to implement.
 
> Right now the things I'm interested in doing with callbacks is to set
> blend modes and depth testing.  In some cases here it's just fine to
> set these on a per leaf basis, but in other cases it would be nice to
> be able to set these on a per-subtree basis.

Sounds like I need to dump those things into ssgState so that they can
be done without callbacks.

-- 
Steve Baker                  http://web2.airmail.net/sjbaker1
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