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Question by vividhelix · Jul 09, 2015 at 10:48 PM · lerpifshader writing

Shader - avoiding if via lerp

I have a shader where I'm computing four different values and then using the minimum of them, something like:

 float lerpFactor1 = ...;
 float lerpFactor2 = ...;
 float lerpFactor3 = ...;
 float lerpFactor4 = ...;

However, I have some additional info which may make it pointless to compute them (for example, if radius1==0 there's no point in calculating lerpFactor1 - it ends up being 1).

I was thinking of adding if's but I recall those may be bad for performance. How could I rewrite this?

In non-shader pseudo-code, I would write this as:

 float lerpFactor = 1;
 
 if (radius1>0)
  lerpFactor = min(lerpFactor, computeLerpFactor1Here);
 if (radius2>0)
  lerpFactor = min(lerpFactor, computeLerpFactor2Here);
 if (radius3>0)
  lerpFactor = min(lerpFactor, computeLerpFactor3Here);
 if (radius4>0)
  lerpFactor = min(lerpFactor, computeLerpFactor4Here);


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avatar image _Gkxd · Jul 10, 2015 at 12:32 AM 0
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I don't think the difference between using an if statement versus just calculating the $$anonymous$$imum is significant enough to justify being concerned about their relative performance.

If you really care though, you can try rendering a very high number of polygons (over 1 million) using both versions and see which one is faster. In a normal game, it's unlikely that you'll ever have a situation where this makes a difference. And even if you do, there are most likely other bottlenecks that improving would result in a larger increase in performance.

avatar image vividhelix · Jul 10, 2015 at 02:17 AM 0
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This is in a pixel shader that runs for the entire screen. Thanks for your opinion, I'm definitely against premature optimization but this was more about curiosity on how to write it so I can benchmark an if version version a lerp-ed version.

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