Add unmix/inverse lerp

Author: Sparsh-N

Diff for: src/doc/stdlib.md

@@ -166,6 +166,12 @@ the computations are performed component-by-component (separately for `x`,
     $ x*(1-\alpha) + y*(\alpha) $
 
 
+*`type`* **`unmix`** (*`type`* `x`, *`type`* `y`, *`type`* `alpha`) <br> *`type`* **`unmix`** (*`type`* `x`, *`type`* `y, float alpha`)
+  : The `unmix` function returns a inverse of `mix`, similar to inverse linear interpolation:
+    $ (x - (\alpha)) / (y - x) $
+    The x and y value can't be the same, as we can't divide by zero.
+
+
 *`type`* **`select`** (*`type`* `x`, *`type`* `y`, *`type`* `cond`) <br> *`type`* **`select`** (*`type`* `x`, *`type`* `y, float cond`) <br> *`type`* **`select`** (*`type`* `x`, *`type`* `y, int cond`)
   : The `select` function returns `x` if `cond` is zero, or `y`
     if `cond` is nonzero. This is roughly equivalent to `(cond ? y : x)`,

Diff for: src/shaders/stdosl.h

@@ -164,6 +164,32 @@ float  mix (float  x, float  y, float  a) BUILTIN;
 closure color mix (closure color x, closure color y, float a) { return x*(1-a) + y*a; }
 closure color mix (closure color x, closure color y, color a) { return x*(1-a) + y*a; }
 
+#if 0
+normal unmix (normal x, normal y, normal a) { return (a - x) / (y - x); }
+normal unmix (normal x, normal y, float  a) { return (a - x) / (y - x); }
+vector unmix (vector x, vector y, vector a) { return (a - x) / (y - x); }
+vector unmix (vector x, vector y, float  a) { return (a - x) / (y - x); }
+point  unmix (point  x, point  y, point  a) { return (a - x) / (y - x); }
+point  unmix (point  x, point  y, float  a) { return (a - x) / (y - x); }
+color  unmix (color  x, color  y, color  a) { return (a - x) / (y - x); }
+color  unmix (color  x, color  y, float  a) { return (a - x) / (y - x); }
+float  unmix (float  x, float  y, float  a) { return (a - x) / (y - x); }
+#else
+normal unmix (normal x, normal y, normal a) BUILTIN;
+normal unmix (normal x, normal y, float  a) BUILTIN;
+vector unmix (vector x, vector y, vector a) BUILTIN;
+vector unmix (vector x, vector y, float  a) BUILTIN;
+point  unmix (point  x, point  y, point  a) BUILTIN;
+point  unmix (point  x, point  y, float  a) BUILTIN;
+color  unmix (color  x, color  y, color  a) BUILTIN;
+color  unmix (color  x, color  y, float  a) BUILTIN;
+float  unmix (float  x, float  y, float  a) BUILTIN;
+#endif
+closure color unmix (closure color x, closure color y, float a) { return (a - x) / (y - x); }
+closure color unmix (closure color x, closure color y, color a) { return (a - x) / (y - x); }
+
+// TODO: ADD REMAP FROM MIX AND UNMIX
+
 normal select (normal x, normal y, normal cond) BUILTIN;
 vector select (vector x, vector y, vector cond) BUILTIN;
 point  select (point  x, point  y, point  cond) BUILTIN;