Making a NaN on purpose in WebGL

Question

I have a GLSL shader that's supposed to output NaNs when a condition is met. I'm having trouble actually making that happen.

Basically I want to do this:

float result = condition ? NaN : whatever;

But GLSL doesn't seem to have a constant for NaN, so that doesn't compile. How do I make a NaN?

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I tried making the constant myself:

float NaN = 0.0/0.0; // doesn't work

That works on one of the machines I tested, but not on another. Also it causes warnings when compiling the shader.

Given that the obvious computation didn't work on one of the machines I tried, I get the feeling that doing this correctly is quite tricky and involves knowing a lot of real-world facts about the inconsistencies between various types of GPUs.

Answer 1

Don't use NaNs here.

Section 2.3.4.1 from the OpenGL ES 3.2 Spec states that

The special values Inf and −Inf encode values with magnitudes too large to be represented; the special value NaN encodes “Not A Number” values resulting from undefined arithmetic operations such as 0/0. Implementations are permitted, but not required, to support Inf's and NaN's in their floating-point computations.

So it seems to really depend on implementation. You should be outputing another value instead of NaN