Strange shader performance

OpenGL
1

Hello! I’m trying to do a lighting shader and determine which lights are enabled. I made a simple IF to see what lights should be used but it slowed things down instead of speeding things up.

Following code is slower:


for(int i=0;i<5;i++){
  if(gl_LightSource[i].diffuse  != vec4(0.0,0.0,0.0,0.0))
    pointLight(i, n, v, shininess, amb, dif, spe);
}

Follwing code is faster (IF is commented out):


for(int i=0;i<5;i++){
  //if(gl_LightSource[i].diffuse  != vec4(0.0,0.0,0.0,0.0))
    pointLight(i, n, v, shininess, amb, dif, spe);
}

If using IF is a bad thing, how should I determine which lights are turned on? Any help appreciated.

2

How much faster ?
What is your video card ?
Is “pointLight” a costly function ?

Hardware support for conditionals is not always very fast.
Depending on the hardware, both branches are executed, and one discarded. Sometimes there is a fixed cost for IF.

3

In a simple scene which fills the entire screen I gain around 10-20 frames per second.

The pointLight function looks as follows:


void pointLight(in int i, in vec3 N, in vec3 V,
                in float shininess,
                inout vec4 ambient,
                inout vec4 diffuse,
                inout vec4 specular){
  vec3 halfVector;
  vec3 D = gl_LightSource[i].position.xyz - V;
  vec3 L = normalize(D);
  float pf=0.5;
  float nDotL=0.5;
  float dist = 1.0;
  float att = 0.5;
	
  dist = length(D);
  att = (1.0/(gl_LightSource[i].constantAttenuation +
              gl_LightSource[i].linearAttenuation * dist +
              gl_LightSource[i].quadraticAttenuation *
              dist * dist));

  nDotL = dot(N,L);
  pf = pow(max(0.0,dot(N,L)),shininess);	
  ambient  += gl_LightSource[i].ambient * att;	
  diffuse  += gl_LightSource[i].diffuse * nDotL * att;
  specular += gl_LightSource[i].specular * pf * att;
}

My video card is an GeForce 7800 GT.

4

It’s normal. Heard it many times now.
Conditionals are costly. They are worth it if there is a large number of instructions that can be skipped.
If a light is off, either set its properties to zero or write multiple shader versions, one for 1 light, another for 2 lights, etc.

5

You should try to use a boolean “lightEnabled” instead of testing if the diffuse color isn’t pure black. I think it’ll allow the driver to do some optimizations, since the state of a light is a constant for all pixels.

6

Is it really faster to have multiple versions of the same shader with different number of lights, even if I need to switch active shader a few times (3-8) per pass? That sounds like a strange work around.

7

It’s not as strange as it sounds.

Try it, you might like it.

8

This topic was automatically closed 183 days after the last reply. New replies are no longer allowed.