[osg-users] Multiple Shaders

[J. Brent Spears]

Hi,

What happens if I attach a program shader to a group node and then a different program shader to the child node? Are both shaders executed? Just one? What order? Precedence, etc?

Thank you!

Cheers,
GB

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http://forum.openscenegraph.org/viewtopic.php?p=41831#41831

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[Glenn Waldron]

GB, 

The child's shader program completely replaces the parent's shader program.

Glenn Waldron / Pelican Mapping / @glennwaldron

[Jean-Sébastien Guay]

Hi Brent,

> What happens if I attach a program shader to a group node and then a different program shader to the child node? Are both shaders executed? Just one? What order? Precedence, etc?

The same rule as for any other state applies here: a state attribute on
a child is inherited from the parent, unless the child specifies its own
attribute.

However, shaders are not executed while traversing the graph, they are
only executed when something is drawn. Only one state attribute can be
active on a given state set at a time, so no, shaders cannot be combined
like that. So the question is which program is active on a given
drawable, and that is determined by the path down to the drawable and
which program was last encountered on that path.

Say you have

root <-- programA
/ \
/ \
no program --> Node1 Node2 <-- programB

Well, if Node1 has any drawables under it, programA will be active when
those drawables are drawn, and if Node2 has any drawables under it,
programB will be active for them.

Hope that helps,

J-S
--
______________________________________________________
Jean-Sebastien Guay jean-seba...@cm-labs.com
http://www.cm-labs.com/
http://whitestar02.dyndns-web.com/

[Stephen Haith]

Very timely thread - I am looking at doing the same thing.

How can you apply a shader effect to the entire scene, AND another effect to only a part of the tree?

Thanks,

Stephen

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Read this topic online here:

http://forum.openscenegraph.org/viewtopic.php?p=41851#41851

[Glenn Waldron]

Stephen,

You need some kind of shader composition. Personally, I prefer the VirtualProgram approach taken in the osgvirtualprogram example. IIRC, Robert did not consider this flexible enough to be a general-purpose shader comp solution, so it's not in the core OSG. However it suited our needs quite well so we adopted and extended it for use in the osgEarth project.

http://github.com/gwaldron/osgearth/blob/master/src/osgEarth/ShaderComposition

Glenn Waldron / Pelican Mapping / @glennwaldron

[hait...@gdls.com]

Glenn,
Thanks for the response. Do you know where I can find the
osgvirtualprogram example?
The program I have worked on has recently moved from osg 2.2 to 2.8.3 (dark
ages, I know). I do not see osgvirtualprogram in the osg examples
directories for 2.2, 2.8.2, or 2.8.3.

I am adding precipitation effects (and dust effects) to a project that
currently uses shaders produce a rendered scene that represents sensor
returns - the render target is read back by the main program,
where the data is consumed. For the app/channel that has the sensor, the
program/shader is added to a node near the top of the tree. I have added
the precip effect program/shader to a node slightly below this one - at the
same level as the top terrain and entities node. The sensor shader
basically does range calculation, applies some error, and does some
processing for alpha fragments. For the visual channel, that has no higher
shaders, this works fine. However, for the sensor channel, the precip
particles don't get seen by sensor shader.

My implementation of rain/snow is based on a slightly modified
osgPrecipitationEffect. Perhaps shader is not the way to go for the precip
effect, but I thought it would provide the best efficiency. Nevertheless,
I can definitely see the need in the near future to have multiple,
accumulated, non-overriding effects. Currently, I would like to be able to
have the results from osgPrecipitationEffect like shader be seen by the
higher sensor shader.

Any thoughts?

Stephen Haithcock
General Dynamics Land Systems
38500 Mound Rd.
Sterling Heights, MI 48310
MZ 436-40-15
(586) 825-8573

Glenn Waldron
<gwaldron@gmail.c
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Stephen,

http://github.com/gwaldron/osgearth/blob/master/src/osgEarth/ShaderComposition

Thanks,

Stephen

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[Glenn Waldron]

Stephen,

That example is in the main distro; you need to enable the examples in CMake to see it. The source is here

http://www.openscenegraph.org/projects/osg/browser/OpenSceneGraph/trunk/examples/osgvirtualprogram

Yes, yours is a case for shader composition. PrecipitationEffect is self-contained so you would need to modify it to make it work in a composition framework.

BTW here is a write-up on the shader composition approach we use in osgEarth. Like I said, we started with VirtualProgram and extended it to support user function injection.

http://osgearth.org/wiki/OeDevelShaderComposition

Glenn Waldron / Pelican Mapping / @glennwaldron