GLSL strange behaviour

Discussion created by fetti on Jan 5, 2011
Latest reply on Jan 11, 2011 by frali
Position of variables matters where it shouldnt...

Hi there!

I have written a GLSL fragment shader and pretty quickly found out that if (performance_needed) branching_is_something_to_avoid

So I wanted to be smart. My code is a flow simulation, reading inputs from sampler2D's. Among other data that are saved in those textures, there is a float value named "obst" that is 0.0 for "no obstacle" and 1.0 for "obstacle":

read = texture2D(Texture3, gl_TexCoord[0].st);
float obst;
obst = read.x;

Now, further down in main() there is an if-clause:

fOut = fIn - omega * (fIn-fEq);     // works perfectly,
if (obst==1.0) fOut
= fIn[opp];        // but is slow!

So, in order to avoid that if-clause, I made it a sum, abusing the "obst" value to switch of the term to be neglegted:

fOut = (1.0-obst)*(fIn - omega * (fIn-fEq)) + fIn[opp]*obst;

And here comes the catch: the above line does _not_ work. I had a hard time finding out that I have to switch the position of "obst":

... + fIn[opp]*obst;       // original code, wont work, is equal to ... + 0.0
... + obst*fIn[opp
];       // works very well

Is this by design or is it a feature?
I am on Ubuntu 10.04, Intel Core 2 Duo, ATI Mobility HD 2600