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Fix the spectral majorant of gridvolume and a transmittance underflow #1959

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Fix the spectral majorant of gridvolume and a transmittance underflow #1959

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6f79527
gridvolume: fix the spectral majorant
AuroraRyan0301 Sep 9, 2026
36a8b78
medium: clamp the optical depth in transmittance_eval_pdf()
AuroraRyan0301 Sep 9, 2026
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7 changes: 5 additions & 2 deletions src/render/medium.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -89,8 +89,11 @@ Medium<Float, Spectrum>::transmittance_eval_pdf(const MediumInteraction3f &mi,
Mask active) const {
MI_MASKED_FUNCTION(ProfilerPhase::MediumEvaluate, active);

Float t = dr::minimum(mi.t, si.t) - mi.mint;
UnpolarizedSpectrum tr = dr::exp(-t * mi.combined_extinction);
Float t = dr::minimum(mi.t, si.t) - mi.mint;
// Clamp the optical depth: callers use the ratio tr / pdf, and on a long
// flight under a large majorant both would underflow to zero
UnpolarizedSpectrum tau = dr::minimum(t * mi.combined_extinction, 80.f);
UnpolarizedSpectrum tr = dr::exp(-tau);
UnpolarizedSpectrum pdf = dr::select(si.t < mi.t, tr, tr * mi.combined_extinction);
return { tr, pdf };
}
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15 changes: 13 additions & 2 deletions src/volumes/grid.cpp
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Expand Up @@ -314,8 +314,19 @@ class GridVolume final : public Volume<Float, Spectrum> {

m_texture.update_inplace();

if (!m_fixed_max)
m_max = (float) dr::max_nested(dr::detach(m_texture.value()));
if (!m_fixed_max) {
if (is_spectral_v<Spectrum> && m_texture.channel_count() == 4 && !m_raw) {
// Spectral upsampling stores (c0, c1, c2, scale) per voxel.
// The model spectrum is bounded by 1, so the scale channel
// bounds the volume; the coefficients do not.
using Array = typename TensorXf::Array;
const Array &data = m_texture.tensor().array();
auto idx = dr::arange<dr::uint32_array_t<Array>>(data.size() / 4) * 4 + 3;
m_max = (float) dr::max_nested(dr::gather<Array>(dr::detach(data), idx));
} else {
m_max = (float) dr::max_nested(dr::detach(m_texture.value()));
}
}
}
}

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