Hi Bob,
Hi all, I’m trying to figure out the best method to pass per vertex
“attributes” to shaders using osg and I can’t seem to find many examples
of attribute use. My guess is there’s a reason for that and I should
probably code up a uniform array of some kind instead.
There's no reason other than no example has been made yet.
Mike Weiblen's osgtoy-data repository has a compactdisc.osg file that
shows that attributes work. It's a single .osg file that contains the
model and the shaders to show a compact disc with the color diffraction
effect (different colors visible depending on the viewing angle).
There's also a compactdisc-texcoord.osg file that does the same thing
using one of the texcoord arrays. That's always an option, btw, you can
just co-opt one of the unused arrays from the fixed pipeline (like
texcoord for texture unit 1 if your model uses a single texture on unit 0).
Of course having a data file that has attributes won't show you how to
use them in your program, and how to update them when you need to, but
at least it proves that they're usable.
I need to pass
“thickness” as an “attribute” and that varies with the geometry /
osg::Geode.
Does it vary based on the geode, or on the geometry's vertices? The
sentence above is very ambiguous. If you need just one value per geode,
you can perfectly well use a single uniform. Attributes are for when
things vary per vertex.
I’ve studied the osg::Drawable and its attribute types, and
the osg::Program and its addBindAttribLocation, but I can’t find any
examples of the callback or other mechanism to actually get the data set
into the attributes from the scenegraph.
Well, that's because it's kind of up to you. Once you've set your
attribute array in the geometry object, and called addBindAttribLocation
to tell the program that a given attribute location potentially has data
in it, you can fill in the data however you like. If your attributes
don't change you can set them once at startup. If they vary per frame
then you'll want to either put an update callback somewhere or break up
your frame loop and then do your calculations there. (I personally
prefer update callbacks, that way a given node can do all the work it
has to do without needing to modify any other parts of the program, but
that's just a preference)
Hope this helps,
J-S
--
______________________________________________________
Jean-Sebastien Guay [email protected]
http://www.cm-labs.com/
http://whitestar02.webhop.org/
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