Hi Mike,
I hadn't heard of CUDA, sounds interesting. Though shame it requires Nvidia
hardware.
What was in my mind when I said "part of a shader" was the idea that maybe
there was a way to have a function in a shader that was only executed once
per frame to setup some variables used by the rest of the shader. I guess a
bit like a some kind of lazy initialization method.
Hmmm, now I spelt it out, I just had a thought. Could I implement lazy per
frame initialization within the fragment shader using the value of a pixel
in frame buffer that the shader is writing? i.e. can the shader read pack a
pixel that was written by a previous execution?
Can you explain what you mean by "geometry feedback path"? Not heard of
that.
I'm still a bit puzzled about how to get a shader to execute once per frame,
do I need to apply it to something that only has one vertex or renders to
one pixel?
The reason I say urgg about returning data via information embedded in a
texture is the same reason I don't like any kind of blind type casting. But
you are right, in my case may not be too bad as I probably just want to
return a single float in the red component of a pixel, so I'll give it go.
Cheers.
Chris.
----- Original Message -----
From: "Mike Weiblen" <[EMAIL PROTECTED]>
To: "Chris Denham" <[EMAIL PROTECTED]>; "OpenSceneGraph Users"
<[email protected]>
Sent: Sunday, November 23, 2008 6:49 PM
Subject: Re: [osg-users] Getting a value calculated by a GLSL shader once
per frame.
Hi Chris,
comments inline...
On Sun, Nov 23, 2008 at 9:47 AM, Chris Denham <[EMAIL PROTECTED]>
wrote:
I have been trying to work out how/if you can use a shader to compute a
value (once per frame) and use that value as a constant in the shader for
the rest of the frame.
For example, I want to use a shader to find the minimum value in a
texture
once at the beginning of a each frame, then use that minimum value
(probably
as a uniform) in a fragment shader.
Yes, a uniform would be the right path in.
It seemed to me I have two problems:
1. How do I get a shader (or part of a shader) to only execute once per
frame?
(Not sure what you mean by part of a shader.) But you could use two
passes, with the scene you're analyzing under your special shader, the
other pass would render using the result from the first pass
2. How do I get the minimum value out of the shader in order to get the
application to put it in a uniform?
There are three ways I can think of:
1) the usual way: readback final fragment values written to some
render target, like a texture or FBO.
2) use the geometry feedback path (I have little experience with this
path, so dont know its limitations etc)
3) use CUDA
The only way I could think of to get a value out of a shader was to
render its result to a texture and pick the data out of the texture in
the
application. Urgg.. there must be a better way.
it's not that distasteful is it?
I had the idea that people may be using shaders as a general purpose
'rocket
powered' implementation for all kinds of general algorithms, but I can't
Sure, NV's CUDA does all sorts of general-purpose computing on the
GPU, hiding implementation details behind a parallel compute
abstraction. At NVISION there was a cool demo using CUDA and GL
together to render a scene using both raytracing and rasterization
(used raytracing only on a shiny car, used rasterization for the road
and other background features, a nice approach for quality/performance
tradeoffs)
see how to use them in that way. Does OSG provide a way to use GLSL for
anything other than 'colouring pixels' ;-) ?
afaik OSG doesn't have any direct support for CUDA, but that doesn't
stop you from interpreting a framebuffer result as something other
than colored pixels.
cheers
-- mew
Chris.
- The answers to stupid questions are often more enlightening than the
answers to smart ones.
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--
Mike Weiblen -- Boulder Colorado USA -- http://mew.cx/
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