FastExp(): make sure that FastExp(0) == 1

src/pbrt/util/math.h

@@ -411,7 +411,7 @@ inline float FastExp(float x) {
     int i = (int)fxp;
 
     // Evaluate polynomial approximation of $2^f$
-    float twoToF = EvaluatePolynomial(f, 0.999813f, 0.696834f, 0.224131f, 0.0790209f);
+    float twoToF = EvaluatePolynomial(f, 1.f, 0.695556856f, 0.226173572f, 0.0781455737f);
 
     // Scale $2^f$ by $2^i$ and return final result
     int exponent = Exponent(twoToF) + i;

src/pbrt/util/math_test.cpp

@@ -283,6 +283,8 @@ TEST(Math, SumOfProducts) {
 }
 
 TEST(FastExp, Accuracy) {
+    EXPECT_EQ(1, FastExp(0));
+
     Float maxErr = 0;
     RNG rng(6502);
     for (int i = 0; i < 100; ++i) {