View Full Version : Transform Feedback and Random Pixels

07-29-2011, 05:22 PM
Hi OpenGL forums, this is my first post here! This place has helped me quite a bit in the past thanks to google searches that come here, so thank you. I hope someone can help with this one. The big picture is that I'm implementing Chris Wyman's Adaptive Caustic Mapping algorithm (his page here (http://www.cs.uiowa.edu/~cwyman/pubs.html)) as part of a larger project, which will combine it with some real-time global illumination algorithms.

Right now when I run my program I get random pixels around the entire screen that turn random colors, and some of them even change color over time. This is true for pixels both inside AND outside the actual running program window (i.e. pixels in firefox and even on the desktop itself change). The pixels stay until I refresh the window where they show up. The exact line of code where this happens has this call:

glGetQueryObjectiv(queryID, GL_QUERY_RESULT_AVAILABLE, &yesorno);

This is in a loop that runs until yesorno is true, after which I get the value I'm looking for with:

glGetQueryObjectiv(queryID, GL_QUERY_RESULT, &primCount);

If I comment out the loop and the checking for whether primCount (which holds the number of vertices written out with transform feedback) is available, that second line still causes the random pixels to show up. The transform feedback code section, a few lines above the glGetQueryObjectiv line is this:

// Go ahead and draw
glDrawArrays( GL_POINTS, 0, inputPrims );

inputPrims is a VBO that holds a set of somewhat random x and y values between 0 and 1. It is created with this code:

glGenBuffers(1, &genericTraversalStartBuffer);
int resolution = 64;
float *causticStartPoints = (float *)malloc( resolution * resolution * 4 * sizeof( float ) );
for (int i = 0; i < resolution * resolution; i++)
int x = (i % resolution);
int y = (i / resolution);
causticStartPoints[4*i+0] = x/(float)resolution;
causticStartPoints[4*i+1] = y/(float)resolution;
causticStartPoints[4*i+2] = causticStartPoints[4*i+3] = 0;
glBindBuffer( GL_ARRAY_BUFFER, genericTraversalStartBuffer);
glBufferData( GL_ARRAY_BUFFER, resolution*resolution*4*sizeof( float ), causticStartPoints, GL_STATIC_DRAW );
glBindBuffer( GL_ARRAY_BUFFER, 0 );

That code is taken straight from the Wyman's code download. inputPrims above is set to the ID of genericTraversalStartBuffer in its function - they are the same.

primCount does contain a value, which is wrong for what I want, but it is at least something. Perhaps if I can figure out the random pixels it will also solve the problem of incorrect counts. Some possibly important information about my rig, which is a nearly-new 17 inch macbook pro:

OS: Windows 7
Video Card: AMD Radeon HD 6750M
OpenGL version according to glGetString(GL_VERSION): 4.1.10428 Compatibility Profile Context
GLSL version found with the same method: 4.10
Dev Environment: Visual C++ 2005 Express
Windowing is done with SDL

Thanks for any help!

07-29-2011, 09:06 PM
primCount does contain a value, which is wrong for what I want

From my experience with using TF, if the primCount is not equal to the inputPrims, this tells me that TF is not setup properly. You need to recheck the shaders to make sure that you are writing to the TF output attribute from the shader.

07-30-2011, 08:39 AM
Hi mobeen, thank you for the reply.

I actually had more info written about the value of primCount, but I cut it out because the post would have been even longer. So here it is!

With Adaptive Caustic Mapping, primCount will actually very rarely be the same as inputPrims. What is supposed to happen is that the vertices in inputPrims are checked against a texture containing the normals of a refractive object in the scene. Some vertices (i.e. photons) in inputPrims will "hit" the refractive object, and some will not. This is all done in the geometry shader that is attached when glDrawArrays() is run. The ones that don't hit (interact with the refractive object) are discarded. This goes through a loop on the various mipmap levels of that texture containing the normals, so you are adaptively sampling the photons that hit the refractive objects in the scene. This garners a very nice speed boost because you are not wasting processing time on photons that don't matter. Older caustic mapping algorithms always processed all the photons in the scene, whether they are refracted or not.

So, in regular operation, as long as the refractive object does not take up the entire view of the light source being used, primCount should be less than inputPrims.