Optimization of HLSL shaders - optimization

Question

I'm trying to optimize my terrain shader for my XNA game as it seems to consume a lot of ressources. It takes around 10 to 20 FPS on my computer, and my terrain is 512*512 vertices, so the PixelShader is called a lot of times.
I've seen that branching is using some ressources, and I have 3/4 conditions in my shaders.
What could I do to bypass them? Are triadic operators more efficient than conditions?
For instance:
float a = (b == x) ? c : d;
or
float a;
if(b == x)
a = c;
else
c = d;
I'm using also multiple times the functions lerp and clamp, should it be more efficient to use arithmetic operations instead?
Here's the less efficient part of my code:
float fog;
if(FogWaterActivated && input.WorldPosition.y-0.1 < FogWaterHeight)
{
if(!IsUnderWater)
fog = clamp(input.Depth*0.005*(FogWaterHeight - input.WorldPosition.y), 0, 1);
else
fog = clamp(input.Depth*0.02, 0, 1);
return float4(lerp(lerp( output * light, FogColorWater, fog), ShoreColor, shore), 1);
}
else
{
fog = clamp((input.Depth*0.01 - FogStart) / (FogEnd - FogStart), 0, 0.8);
return float4(lerp(lerp( output * light, FogColor, fog), ShoreColor, shore), 1);
}
Thanks!

Answer 1

Any time you can precalculate operations done on shader constants the better. Removing division operations by passing through the inverse into the shader is another useful tip as division is typically slower than multiplication.
In your case, precalculate (1 / (FogEnd - FogStart)), and multiply by that on your second-last line of code.