On Wed, Aug 31, 2011 at 3:48 PM, Nicolas Rougier <Nicolas.Rougier@inria.fr> wrote:

Hi folks,

I've been playing with gaussian texture filtering using shaders and I obtained a strange "grid" effect on a completely uniform texture. Since all fragments get the same processing and same neighbouring values,I wonder if the grid is the result of some kind of artifact with floating points or the way texture coordinates are interpolated. Or I missed something completely obvious…

I attached the code below. It is self-contained and you should see the grid I'm talking about.

It feels like the texture coordinates as you calculate them in interpolate(...), and the offsets in filter(...) are to blame.  I assume you're trying to do something like a "manual" minification filter using a gauss kernel.  Without knowing more about the algorithm, I can't say.  

My recommendation is to try gamedev.net in the OpenGL forum, as there are a large number of people there who can help.