Question How can I replace this "if" in my shader?

Depending on "_Color.a" I want to lerp the output variable "_InviWabble".
But I need no simple linear output value.

So the output value should first start from 0 and rise. Then reaching a maximum, and then decreasing again to a low end value.
In other words, a kind of lerp() with 3 key points.

What I want is:
If _Color.a == 1, then the output (_InviWabble) should be 0.
If _Color.a == 0.25, then the output (_InviWabble) should be 0.0225.
If _Color.a == 0, then the output (_InviWabble) should be _WabbleAtInvisible (is approximately at 0.006).

Therefore I have this code in shader frag():

if(_Color.a > 0.25)

     _InviWabble = lerp(0.03, 0, _Color.a);

else

    _InviWabble = lerp(_WabbleAtInvisible, 0.0225, _Color.a*4);

How can I do the same without the slow "if/else" ?

Thanks for reading and for every answer

 

This should work just fine. Conditionals are not slow themselves. And in this case, as I understand _Color is a uniform (it comes from a property, right?), so there will be no branch divergence. And even if there were, executing two lerps is not a big deal.
If you need more control over the output value you can instead sample a gradient texture using _Color.a as uv coordinate.

 

You could use lerp and step function to get rid of that if, but I have no real knowledge if it would be any faster, and like Oxeren said, it will most likely nowadays work just fine. And that if-else is much more readable...

Anyway, something like this:
(I just simplified your code to an example that visualises the result, using uv x value to drive the test. But you see that the result should be identical.)

With if:

Code (HLSL):

1. float valueA = i.uv.x;
2. float valueB = 0.006;
3. fixed4 result = 0;
4.  
5. if(valueA > 0.25) {
6.     result = lerp(0.03, 0, valueA);
7. }
8. else {
9.     result = lerp(valueB, 0.0225, valueA * 4);
10. }

Without if:

Code (HLSL):

1. result = lerp(
2.                 lerp(0.03, 0, valueA),
3.                 lerp(valueB, 0.0225, valueA * 4),
4.                 step(valueA, 0.25)
5.              );

 

Step is still a hidden if, high level language and all ...

 

Yeah, @Olmi 's example is basically just adding another lerp, but there's still a conditional. It's the equivalent of doing:

Code (csharp):

1. float extraLerpBlend = 0.0;
2. if(_Color.a > 0.25)
3.     extraLerpBlend = 1.0;
4.  
5. _InviWabble = lerp(
6.     lerp(_WabbleAtInvisible, 0.0225, _Color.a*4),
7.     lerp(0.03, 0, _Color.a),
8.     extraLerpValue);

Basically, it's slower.*

What you're doing is already going to be the fastest option. If statements don't mean branches most of the time. It'll be a ternary operator on any modern hardware, which is very fast. Basically it'll do this:

Code (csharp):

1. float tempA = lerp(0.03, 0, _Color.a);
2. float tempB = lerp(_WabbleAtInvisible, 0.0225, _Color.a*4);
3. _InviWabble = _Color.a > 0.25 ? tempA : tempB;

On most platforms your original code and that example will compile to identical shaders. And that's fine because there's not really a way to do it faster.


* except on some old and/or buggy mobile hardware which have really bad shader compilers which prefer step() over ternary operators, even though they should compile to identical code.

 

Thank you all very much