Fix weights in sample_wavelengths when using SRF.

src/films/specfilm.cpp

@@ -301,17 +301,12 @@ class SpecFilm final : public Film<Float, Spectrum> {
         SurfaceInteraction3f si = dr::zeros<SurfaceInteraction3f>();
         si.wavelengths = wavelengths;
 
-        // The SRF is not necessarily normalized, cancel out multiplicative factors
-        UnpolarizedSpectrum inv_spec = m_srf->eval(si);
-        inv_spec = dr::select(inv_spec != 0.f, dr::rcp(inv_spec), 1.f);
-        UnpolarizedSpectrum values = spec * inv_spec;
-
         for (size_t j = 0; j < m_srfs.size(); ++j) {
             UnpolarizedSpectrum weights = m_srfs[j]->eval(si);
             aovs[j] = dr::zeros<Float>();
 
             for (size_t i = 0; i<Spectrum::Size; ++i)
-                aovs[j] = dr::fmadd(weights[i], values[i], aovs[j]);
+                aovs[j] = dr::fmadd(weights[i], spec[i], aovs[j]);
 
             aovs[j] *= 1.f / Spectrum::Size;
         }

src/render/sensor.cpp

@@ -101,10 +101,15 @@ Sensor<Float, Spectrum>::sample_wavelengths(const SurfaceInteraction3f& /*si*/,
                                             Mask active) const {
     if constexpr (is_spectral_v<Spectrum>) {
         if (m_srf != nullptr) {
-            return m_srf->sample_spectrum(
-                    dr::zeros<SurfaceInteraction3f>(),
-                    math::sample_shifted<Wavelength>(sample),
-                    active);
+            auto wavelengths = m_srf->sample_spectrum(
+                                dr::zeros<SurfaceInteraction3f>(),
+                                math::sample_shifted<Wavelength>(sample),
+                                active).first;
+            SurfaceInteraction3f si;
+            si.wavelengths = wavelengths;
+            auto pdf = m_srf->pdf_spectrum(si, active);
+            auto weights = dr::select(pdf != 0.f, dr::rcp(pdf), 0.f);
+            return { wavelengths, weights };
         }
     } else {
         DRJIT_MARK_USED(active);