Clamp BSDL/MTX Fresnel inputs and fix bootstrap Imath link (#2081)

Clamp generalized Schlick F0/F90 to [0, 1] in the MTX Schlick Fresnel
constructor, and clamp the MTX conductor average-Fresnel fit to [0, 1]
to avoid invalid albedo estimates from the approximation.

Also update BSDL bootstrap linking to use Imath::Imath directly instead of
${ARNOLD_IMATH_TARGETS}.

Signed-off-by: Alejandro Conty <aconty@imageworks.com>

Author: aconty

src/libbsdl/bsdl.cmake

@@ -9,7 +9,7 @@ function(ADD_BSDL_LIBRARY NAME)
     # Bootstrap version of BSDL (without luts)
     add_library(BSDL_BOOTSTRAP INTERFACE)
     target_include_directories(BSDL_BOOTSTRAP INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}/${bsdl_SUBDIR}/include)
-    target_link_libraries(BSDL_BOOTSTRAP INTERFACE ${ARNOLD_IMATH_TARGETS})
+    target_link_libraries(BSDL_BOOTSTRAP INTERFACE Imath::Imath)
 
     # LUT generation tool
     set(BSDL_GEN_HEADERS ${CMAKE_CURRENT_BINARY_DIR}/${bsdl_SUBDIR}/geninclude)

src/libbsdl/include/BSDL/MTX/bsdf_conductor_impl.h

@@ -70,7 +70,7 @@ ConductorFresnel::avg() const
                             d = -0.07785134f;
             const float x = IOR[i], y = extinction[i];
             const float p = a + b * x + c * y + d * x * y;
-            return p / (1 + p);
+            return CLAMP(p / (1 + p), 0.0f, 1.0f);
         },
         lambda_0);
 }

src/libbsdl/include/BSDL/MTX/bsdf_schlick_impl.h

@@ -16,8 +16,8 @@ namespace mtx {
 SchlickFresnel::SchlickFresnel(Power F0, Power F90, float exponent, float _eta,
                                bool backfacing)
     : DielectricFresnel(_eta, backfacing)
-    , F0(F0)
-    , F90(F90)
+    , F0(F0.clamped(0, 1))
+    , F90(F90.clamped(0, 1))
     , exponent(exponent)
     , tir_cos(eta >= 1
                   ? 0