docs: Fix specification clarity of smoothstep

It was pointed out on Slack by Arnon Marcus that the spec's
description of smoothstep was ambiguous about the behavior when
edge0==edge1==x: does it return 0 or 1 here?

The implementation returned 1, which I think is the "correct" behavior
in the sense that when edge0==edge1, the results should match the
step() function with that edge.

* Fix the descriptions in the spec to match the implementation and
  what I think is the best behavior.

* Also change paremter names edge0/edge1 to low/high, which I think
  is much more clearly self-documenting.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

src/doc/languagespec.tex

@@ -3488,23 +3488,23 @@ \section{Pattern generation}
 performed component-by-component (separately for $x$, $y$, and $z$).
 \apiend
 
-\apiitem{float {\ce linearstep} (float edge0, float edge1, float x) \\
-\emph{type} {\ce linearstep} (\emph{type} edge0, \emph{type} edge1, \emph{type} x)}
+\apiitem{float {\ce linearstep} (float low, float high, float x) \\
+\emph{type} {\ce linearstep} (\emph{type} low, \emph{type} high, \emph{type} x)}
 \indexapi{linearstep()}
-Returns 0 if $x \le {\mathit edge0}$, and 1 if $x \ge {\mathit edge1}$,
+Returns 0 if $x \le {\mathit low}$, and 1 if $x \ge {\mathit high}$,
 and performs a linear
-interpolation between 0 and 1 when ${\mathit edge0} < x < {\mathit edge1}$.
-This is equivalent to {\cf step(edge0, x)} when {\cf edge0 == edge1}.
+interpolation between 0 and 1 when ${\mathit low} < x < {\mathit high}$.
+This is equivalent to {\cf step(low, x)} when {\cf low == high}.
 For \color and \point-like types, the computations are
 performed component-by-component (separately for $x$, $y$, and $z$).
 \apiend
 
-\apiitem{float {\ce smoothstep} (float edge0, float edge1, float x) \\
-\emph{type} {\ce smoothstep} (\emph{type} edge0, \emph{type} edge1, \emph{type} x)}
+\apiitem{float {\ce smoothstep} (float low, float high, float x) \\
+\emph{type} {\ce smoothstep} (\emph{type} low, \emph{type} high, \emph{type} x)}
 \indexapi{smoothstep()}
-Returns 0 if $x \le {\mathit edge0}$, and 1 if $x \ge {\mathit edge1}$,
+Returns 0 if $x < {\mathit low}$, and 1 if $x \ge {\mathit high}$,
 and performs a smooth Hermite
-interpolation between 0 and 1 when ${\mathit edge0} < x < {\mathit edge1}$.
+interpolation between 0 and 1 when ${\mathit low} < x < {\mathit high}$.
 This is useful in cases where you would want a thresholding function
 with a smooth transition.
 
@@ -3513,17 +3513,16 @@ \section{Pattern generation}
 performed component-by-component.
 \apiend
 
-\apiitem{float {\ce smooth_linearstep} (float edge0, float edge1, float x, float eps) \\
-\emph{type} {\ce smooth_linearstep} (\emph{type} edge0, \emph{type} edge1, \emph{type} x, \emph{type} eps)}
+\apiitem{float {\ce smooth_linearstep} (float low, float high, float x, float eps) \\
+\emph{type} {\ce smooth_linearstep} (\emph{type} low, \emph{type} high, \emph{type} x, \emph{type} eps)}
 \indexapi{smooth_linearstep()}
-This function is strictly linear between ${\mathit edge0}+{\mathit eps}$ and ${\mathit edge1}-{\mathit eps}$
-but smoothly ramps to 0 between ${\mathit edge0}-{\mathit eps}$ and ${\mathit edge0}+{\mathit eps}$
-and smoothly ramps to 1 between ${\mathit edge1}-{\mathit eps}$ and ${\mathit edge1}+{\mathit eps}$.
-It is 0 when $x \le {\mathit edge0}-{\mathit eps}$, and 1 if $x \ge {\mathit edge1}+{\mathit eps}$,
-and performs a linear
-interpolation between 0 and 1 when ${\mathit edge0} < x < {\mathit edge1}$.
-For \color and \point-like types, the computations are
-performed component-by-component.
+This function returns 0 if $x < {\mathit low}-{\mathit eps}$, and 1 if
+$x \ge {\mathit high}+{\mathit eps}$, is strictly linear between
+${\mathit low}+{\mathit eps}$ and ${\mathit high}-{\mathit eps}$ but smoothly
+ramps to 0 between ${\mathit low}-{\mathit eps}$ and ${\mathit low}+{\mathit eps}$
+and smoothly ramps to 1 between ${\mathit high}-{\mathit eps}$ and
+${\mathit high}+{\mathit eps}$. For \color and \point-like types, the
+computations are performed component-by-component.
 \apiend
 
 

src/doc/stdlib.md

@@ -458,36 +458,34 @@ the computations are performed component-by-component (separately for `x`,
     performed component-by-component (separately for $x$, $y$, and $z$).
 
 
-`float` **`linearstep`** `(float edge0, float edge1, float x)` <br> *`type`* **`linearstep`** (*`type`* `edge0`, *`type`* `edge1`, *`type`* `x`)
+`float` **`linearstep`** `(float low, float high, float x)` <br> *`type`* **`linearstep`** (*`type`* `low`, *`type`* `high`, *`type`* `x`)
 
-  : Returns 0 if `x` $\le$ `edge0`, and 1 if `x` $\ge$ `edge1`, and performs a
-    linear interpolation between 0 and 1 when `edge0` $<$ `x` $<$ `edge1`.
-    This is equivalent to `step(edge0, x)` when `edge0 == edge1`. For `color`
+  : Returns 0 if `x` $<$ `low`, and 1 if `x` $\ge$ `high`, and performs a
+    linear interpolation between 0 and 1 when `low` $<$ `x` $<$ `high`.
+    This is equivalent to `step(low, x)` when `low == high`. For `color`
     and `point`-like types, the computations are performed
     component-by-component (separately for $x$, $y$, and $z$).
 
 
-`float` **`smoothstep`** `(float edge0, float edge1, float x)` <br> *`type`* **`smoothstep`** (*`type`* `edge0`, *`type`* `edge1`, *`type`* `x`)
+`float` **`smoothstep`** `(float low, float high, float x)` <br> *`type`* **`smoothstep`** (*`type`* `low`, *`type`* `high`, *`type`* `x`)
 
-  : Returns 0 if `x` $\le$ `edge0`, and 1 if `x` $\ge$ `edge1`, and performs a
-    smooth Hermite interpolation between 0 and 1 when `edge0` $<$ `x` $<$
-    `edge1`. This is useful in cases where you would want a thresholding
+  : Returns 0 if `x` $<$ `low`, and 1 if `x` $\ge$ `high`, and performs a
+    smooth Hermite interpolation between 0 and 1 when `low` $<$ `x` $<$
+    `high`. This is useful in cases where you would want a thresholding
     function with a smooth transition.
 
     The *`type`* may be any of of `float`, `color`, `point`, `vector`, or
     `normal`.  For `color` and `point`-like types, the computations are
     performed component-by-component.
 
 
-`float` **`smooth_linearstep`** `(float edge0, float edge1, float x, float eps)` <br> *`type`* **`smooth_linearstep`** (*`type`* `edge0`, *`type`* `edge1`, *`type`* `x`, *`type`* eps)
+`float` **`smooth_linearstep`** `(float low, float high, float x, float eps)` <br> *`type`* **`smooth_linearstep`** (*`type`* `low`, *`type`* `high`, *`type`* `x`, *`type`* eps)
 
-  : This function is strictly linear between `edge0 + eps` and `edge1 - eps`
-    but smoothly ramps to 0 between `edge0 - eps` and `edge0 + eps`
-    and smoothly ramps to 1 between `edge1 - eps` and `edge1 + eps`.
-    It is 0 when `x` $\le$ `edge0-eps,` and 1 if `x` $\ge$ `edge1 + eps`,
-    and performs a linear interpolation between 0 and 1 when
-    `edge0` < x < `edge1`. For `color` and `point`-like types, the
-    computations are performed component-by-component.
+  : This function returns 0 if `x` $<$ `low-eps,` and 1 if `x` $\ge$ `high + eps`,
+    is strictly linear between `low + eps` and `high - eps`, but smoothly
+    ramps to 0 between `low - eps` and `low + eps` and smoothly ramps to 1
+    between `high - eps` and `high + eps`. For `color` and `point`-like types,
+    the computations are performed component-by-component.
 
 
 %## Noise functions

src/shaders/stdosl.h

@@ -319,73 +319,73 @@ point step (point edge, point x) BUILTIN;
 vector step (vector edge, vector x) BUILTIN;
 normal step (normal edge, normal x) BUILTIN;
 float step (float edge, float x) BUILTIN;
-float smoothstep (float edge0, float edge1, float x) BUILTIN;
+float smoothstep (float low, float high, float x) BUILTIN;
 
-color smoothstep (color edge0, color edge1, color x)
+color smoothstep (color low, color high, color x)
 {
-    return color (smoothstep(edge0[0], edge1[0], x[0]),
-                  smoothstep(edge0[1], edge1[1], x[1]),
-                  smoothstep(edge0[2], edge1[2], x[2]));
+    return color (smoothstep(low[0], high[0], x[0]),
+                  smoothstep(low[1], high[1], x[1]),
+                  smoothstep(low[2], high[2], x[2]));
 }
-vector smoothstep (vector edge0, vector edge1, vector x)
+vector smoothstep (vector low, vector high, vector x)
 {
-    return vector (smoothstep(edge0[0], edge1[0], x[0]),
-                   smoothstep(edge0[1], edge1[1], x[1]),
-                   smoothstep(edge0[2], edge1[2], x[2]));
+    return vector (smoothstep(low[0], high[0], x[0]),
+                   smoothstep(low[1], high[1], x[1]),
+                   smoothstep(low[2], high[2], x[2]));
 }
 
-float linearstep (float edge0, float edge1, float x) {
+float linearstep (float low, float high, float x) {
     float result;
-    if (edge0 != edge1) {
-        float xclamped = clamp (x, edge0, edge1);
-        result = (xclamped - edge0) / (edge1 - edge0);
+    if (low != high) {
+        float xclamped = clamp (x, low, high);
+        result = (xclamped - low) / (high - low);
     } else {  // special case: edges coincide
-        result = step (edge0, x);
+        result = step (low, x);
     }
     return result;
 }
-color linearstep (color edge0, color edge1, color x)
+color linearstep (color low, color high, color x)
 {
-    return color (linearstep(edge0[0], edge1[0], x[0]),
-                  linearstep(edge0[1], edge1[1], x[1]),
-                  linearstep(edge0[2], edge1[2], x[2]));
+    return color (linearstep(low[0], high[0], x[0]),
+                  linearstep(low[1], high[1], x[1]),
+                  linearstep(low[2], high[2], x[2]));
 }
-vector linearstep (vector edge0, vector edge1, vector x)
+vector linearstep (vector low, vector high, vector x)
 {
-    return vector (linearstep(edge0[0], edge1[0], x[0]),
-                   linearstep(edge0[1], edge1[1], x[1]),
-                   linearstep(edge0[2], edge1[2], x[2]));
+    return vector (linearstep(low[0], high[0], x[0]),
+                   linearstep(low[1], high[1], x[1]),
+                   linearstep(low[2], high[2], x[2]));
 }
 
-float smooth_linearstep (float edge0, float edge1, float x_, float eps_) {
+float smooth_linearstep (float low, float high, float x_, float eps_) {
     float result;
-    if (edge0 != edge1) {
+    if (low != high) {
         float rampup (float x, float r) { return 0.5/r * x*x; }
-        float width_inv = 1.0 / (edge1 - edge0);
+        float width_inv = 1.0 / (high - low);
         float eps = eps_ * width_inv;
-        float x = (x_ - edge0) * width_inv;
+        float x = (x_ - low) * width_inv;
         if      (x <= -eps)                result = 0;
         else if (x >= eps && x <= 1.0-eps) result = x;
         else if (x >= 1.0+eps)             result = 1;
         else if (x < eps)                  result = rampup (x+eps, 2.0*eps);
         else /* if (x < 1.0+eps) */        result = 1.0 - rampup (1.0+eps - x, 2.0*eps);
     } else {
-        result = step (edge0, x_);
+        result = step (low, x_);
     }
     return result;
 }
 
-color smooth_linearstep (color edge0, color edge1, color x, color eps)
+color smooth_linearstep (color low, color high, color x, color eps)
 {
-    return color (smooth_linearstep(edge0[0], edge1[0], x[0], eps[0]),
-                  smooth_linearstep(edge0[1], edge1[1], x[1], eps[1]),
-                  smooth_linearstep(edge0[2], edge1[2], x[2], eps[2]));
+    return color (smooth_linearstep(low[0], high[0], x[0], eps[0]),
+                  smooth_linearstep(low[1], high[1], x[1], eps[1]),
+                  smooth_linearstep(low[2], high[2], x[2], eps[2]));
 }
-vector smooth_linearstep (vector edge0, vector edge1, vector x, vector eps)
+vector smooth_linearstep (vector low, vector high, vector x, vector eps)
 {
-    return vector (smooth_linearstep(edge0[0], edge1[0], x[0], eps[0]),
-                   smooth_linearstep(edge0[1], edge1[1], x[1], eps[1]),
-                   smooth_linearstep(edge0[2], edge1[2], x[2], eps[2]));
+    return vector (smooth_linearstep(low[0], high[0], x[0], eps[0]),
+                   smooth_linearstep(low[1], high[1], x[1], eps[1]),
+                   smooth_linearstep(low[2], high[2], x[2], eps[2]));
 }
 
 float aastep (float edge, float s, float dedge, float ds) {