Commits on Aug 21, 2024

1. docs: Fix specification clarity and behavior of smoothstep …

   Swap the confusing edge0/edge1 nomenclature of the smoothstep
   declaration to the more intuitive low/high.
   
   It was pointed out on Slack by Arnon Marcus that the spec's
   description of smoothstep was ambiguous about the behavior when
   low==high: does it return 0 or 1 if x is the same value? The
   documentation is unclear.
   
   The implementation returned 1, which I think is the "correct" behavior
   in the sense that it matches the results of step() function with that
   edge. So update the documentation to match.
   
   Also Arnon pointed out that things are especially weird if low > high,
   it's non-monotonic.  This seems to be fixed by simply reversing the
   relative order of the `if x <= low` and `if x >= high` tests:
   basically, it also makes it match step(x, high) and be monotonic.
   
   This is a cleaner formal definition of what smoothstep should do,
   namely:
   
       if (x >= high) {
           return 1.0f;
       } else if (x <= low) {
           return 0.0f;
       } else {
           float t = (x - low) / (high - low);
           return (3.0f-2.0f*t)*(t*t);
       }
   
   Signed-off-by: Larry Gritz <[email protected]>

   

   lgritz committed Aug 21, 2024
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