Revised shader performance question

OpenGL
1

looks something is wrong when I post!
Hi, all
I had a fragment shader like this:
…
void main(void)
{
vec4 color(0.0);
if(A)
{
// convolution to texture_0
…
color = …
}
else
{
// convolution to texture_1 & texture_0
…
color =…
}
gl_FragColor = color;
}

It works OK.
the interesting thing is: the performance is decreased a lot( at least half) with above if /else branch. following are my testing results (for else part):

  1. comment out the code in if branch( just have color = vec4(1.0, 0.0, 0.0, 1.0)
    the performance is improved

  2. separate the if /else into two functions, then call these two function in main() like "
    void main(void)
    {
    vec4 color = vec4(0.0);
    if(A)
    color = caltexture_0(texture_0);
    else
    color = caltexture_1(texture_1);
    gl_FragColor = color;
    }
    "
    the performance is improved too.

  3. separate the if/else into two shaders. the performance is improved even better.

this only happens for complicated convolution, for simple convolution, there are no noticeable difference between above testing.

------------------thought it is related to the compiler ( I am working on linux, NV Quadro graphic card)-------------- anybody has a proven explanation for it?

many many thanks

2

Many cards don’t support real dynamic branches in fragment shaders. They usualy execute the whole code (so the ‘then’ and the ‘else’ part) and choose the result with a conditional write.

Still, I am puzzled that your 2) option provides better performance. Are you sure that functions perform the same calculations as the above if() … else code?

3

thanks.
how they choose the result with a conditional write?
works like"
void main(void)
{
vec4 color0 = vec4(0.0);
vec4 color1 = vec4(0.0);
color0 = //convolution…calculation
color1 = //convolution…calculation
if(A)
gl_FragColor = color0;
else
gl_FragColor = color1:
}
(how to check my card works in that way?)

for option 2, I am pretty sure it performs the same calculations.

4

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