Hi Ethan,
Thanks, that makes sense that it would just be rendering a quad and that the
original scene geometry would be lost. However, the GLSL geometry shader only
accepts primitives of the type point, line, or triangle->is it perhaps
rendering two triangles to the geometry shader to make up the quad? How would I
even go about determining since there's no debugging available?
But back to what I'm trying to do, I'm trying to use a geometry shader to
calculate the min, max, mean, std dev, and histogram of an RTT texture. Fellow
osg forum member Aurelius has advised me that he has working code that does
this using geometry shaders and pointed me to:
http://http.developer.nvidia.com/GPUGems3/gpugems3_ch41.html
and
This seems rather complicated for a starter. Also it requires feedback
buffers which I never got working in OSG, so there might be some more
obstacles here.
http://developer.amd.com/wordpress/media/2012/10/GPUHistogramGeneration_I3D07.pdf
but the first example only has Cg code and not GLSL, and the second example is
a paper that describers an algorithm but doesn't have any src. These are both
potentially great resources, but I'm struggling to just get a basic
pass-through geometry shader working to get some sort of starting point.
Geometry shaders are really simple. You simply need to know your input
primitives.
A very basic geometry shader looks like this:
#version 400
layout(triangles) in; //we receive triangles
layout(triangle_strip, max_vertices = 3) out;
void main()
{
for (int i = 0; i < 3; ++i)
{
vertex = gl_in[i].gl_Position;
gl_Position = vertex;
EmitVertex();
}
EndPrimitive();
}
But seriously, there are examples in OSG touching geometry shaders and
there are plenty of tutorials about glsl and shaders.
cheers
Sebastian
As a side note, I am also considering using a compute shader since this would be the more
natual fit for this type of algorithm while the geometry shader method is more of a
"hack" that goes against the original intention of the geometry shader, but I'd
be happy using either method, I'm just trying to get some traction on either of them.
-Ethan
SMesserschmidt wrote:
Am 18.11.2013 15:32, schrieb Ethan Fahy:
Hello,
Preface: I realize this question comes about because I've never really learned
OpenGL/GLSL from the ground up and am likely missing some simple concepts, but
I mostly have been coasting by at the osg middleware level and have been doing
OK so far.
If I want to do some simple post-processing I can create a render-to-texture camera and
render to the framebuffer. I can attach a texture to the framebuffer and then create
another "screen" camera to render that texture to the screen. I can add a GLSL
shader program to this texture so that before the texture gets rendered to the screen it
gets an effect added to it using the shaders.
When I use shaders attached to 3-D model nodes in the scene itself, the meaning of
the vertex and frag shaders is easy to understand->the vertices of the 3-D
model are the vertices referenced in the vertex shader. However, When I render my
scene to a texture and then do a simple pass-through vertex and frag shader combo,
what is the meaning of the vertices in this scenario? I had assumed that once you
render your scene to a texture, all knowledge of the original scene's geometry and
vertex locations has been lost, is this true? If so, then what vertices am I
dealing with? It's easy enough to follow along with examples and to use a simple
pass-through vertex shader, but I'd like to understand this better because I now
want to insert a geometry shader in between the vertex and frag shaders and again
I'm not sure whether to use point, line, or triangle in my geometry shader as the
primitive type because I thought that the geometry and primitives of the original
scene wou
ld
b
e lost after rendering to texture.
Usually when doing the post-processing pass you will be rendering to a
fullscreen quad. So the vertices you are dealing with are those of the
quad you are rendering too.
And yes, If you don't any further actions, rendering to texture will not
preserve the information on your orignal vertices etc.
The question is what you want to achieve. A geometry shader inbetween
your postprocessing pass will work on the quads vertices.
Maybe you should elaborate which kind of post processing you want to
achieve, so we can help you here.
What am I missing here? Any clarification is most welcome.
-Ethan
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