[Plib-devel] ssg Leaf call backs

[Curtis L. O. <cu...@in...>]

Steve,

Here's another one for you.

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.

Assuming you are doing a depth first traversal of the scene graph, it
would be nice if the predraw callback would be called when a branch is
first decended to, and the postdraw callback would be called after all
the "kids" have been processed (last thing on the way back out.)

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.

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.

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.

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.

Regards,

Curt.
-- 
Curtis Olson   University of MN, ME Dept.       Flight Gear Project
Twin Cities    cu...@me...                  cu...@fl...
Minnesota      http://www.menet.umn.edu/~curt   http://www.flightgear.org