Fix missing faceforward() implementation. (#759)

Author: brechtvl

src/shaders/stdosl.h

@@ -202,8 +202,14 @@ float distance (point a, point b, point q)
 }
 normal normalize (normal v) BUILTIN;
 vector normalize (vector v) BUILTIN;
-vector faceforward (vector N, vector I, vector Nref) BUILTIN;
-vector faceforward (vector N, vector I) BUILTIN;
+vector faceforward (vector N, vector I, vector Nref)
+{
+    return (dot(I, Nref) > 0) ? -N : N;
+}
+vector faceforward (vector N, vector I)
+{
+    return faceforward(N, I, Ng);
+}
 vector reflect (vector I, vector N) { return I - 2*dot(N,I)*N; }
 vector refract (vector I, vector N, float eta) {
     float IdotN = dot (I, N);

testsuite/geomath/ref/out.txt

@@ -3,6 +3,8 @@ Compiled test.osl -> test.oso
    hypot (3, 4, 5) = 7.07107
    reflect (0.447214 -0.894427 0, 0 1 0) =  0.447214 0.894427 0
    reflect (0.447214 -0.894427 0, -1 0 0) =  -0.447214 -0.894427 0
+   faceforward (0 0 1, 0 -0.707107 -0.707107) =  0 0 1
+   faceforward (0 0 1, 0 -0.707107 -0.707107, 0 0 -1) =  0 0 -1
  testing total-internal reflection:
    refract (0.707107 -0.707107 0, 0 1 0, 1.42) =  0 0 0
    frensel:  Kr= 1  Kt = 0  R = 0.707107 0.707107 0  T = 0 0 0
@@ -23,6 +25,8 @@ Compiled test.osl -> test.oso
    hypot (3, 4, 5) = 7.07107
    reflect (0.447214 -0.894427 0, 0 1 0) =  0.447214 0.894427 0
    reflect (0.447214 -0.894427 0, -1 0 0) =  -0.447214 -0.894427 0
+   faceforward (0 0 1, 0 -0.707107 -0.707107) =  0 0 1
+   faceforward (0 0 1, 0 -0.707107 -0.707107, 0 0 -1) =  0 0 -1
  testing total-internal reflection:
    refract (0.707107 -0.707107 0, 0 1 0, 1.42) =  0 0 0
    frensel:  Kr= 1  Kt = 0  R = 0.707107 0.707107 0  T = 0 0 0
@@ -43,6 +47,8 @@ Compiled test.osl -> test.oso
    hypot (3, 4, 5) = 7.07107
    reflect (0.447214 -0.894427 0, 0 1 0) =  0.447214 0.894427 0
    reflect (0.447214 -0.894427 0, -1 0 0) =  -0.447214 -0.894427 0
+   faceforward (0 0 1, 0 -0.707107 -0.707107) =  0 0 1
+   faceforward (0 0 1, 0 -0.707107 -0.707107, 0 0 -1) =  0 0 -1
  testing total-internal reflection:
    refract (0.707107 -0.707107 0, 0 1 0, 1.42) =  0 0 0
    frensel:  Kr= 1  Kt = 0  R = 0.707107 0.707107 0  T = 0 0 0
@@ -63,6 +69,8 @@ Compiled test.osl -> test.oso
    hypot (3, 4, 5) = 7.07107
    reflect (0.447214 -0.894427 0, 0 1 0) =  0.447214 0.894427 0
    reflect (0.447214 -0.894427 0, -1 0 0) =  -0.447214 -0.894427 0
+   faceforward (0 0 1, 0 -0.707107 -0.707107) =  0 0 1
+   faceforward (0 0 1, 0 -0.707107 -0.707107, 0 0 -1) =  0 0 -1
  testing total-internal reflection:
    refract (0.707107 -0.707107 0, 0 1 0, 1.42) =  0 0 0
    frensel:  Kr= 1  Kt = 0  R = 0.707107 0.707107 0  T = 0 0 0

testsuite/geomath/test.osl

@@ -16,6 +16,14 @@ test ()
         printf ("   reflect (%g, %g) =  %g\n", vI, vN, pretty(reflect (vI, vN)));
         vN = vector(-1, 0, 0);
         printf ("   reflect (%g, %g) =  %g\n", vI, vN, pretty(reflect (vI, vN)));
+        vI = vector(0, -1, -1);
+        vI = normalize(vI);
+        vN = vector(0, 0, 1);
+        printf ("   faceforward (%g, %g) =  %g\n",
+                vN, vI, pretty(faceforward (vN, vI)));
+        vector vNg = vector(0, 0, -1);
+        printf ("   faceforward (%g, %g, %g) =  %g\n",
+                vN, vI, vNg, pretty(faceforward (vN, vI, vNg)));
 
         // test total internal reflection (going from greater to lesser IOR)
         printf (" testing total-internal reflection:\n");