Importance Sampling the wavelength for glasses in spectral multiplexed mode
When crossing a glass in spectral multiplexed rendering, only one wavelength (one spectral bin) is kept. Currently the sampling for this wavelength is uniform which is inefficient for colored glass.
This branch replaces the uniform sampling with importance sampling based on the per-wavelength transmittance of the material. That way, wavelengths which are likely to contribute the most are kept more often.
When the material is only slightly colored and does not present many variations in its transmittance, uniform sampling is still used to avoid computing, storing and using a CDF and PDF on the GPU.
Comparison in term of mean variance, time, and efficiency (= 1/(Variance*Time), higher is better) with 46500 ray per pixel (128x128 pixels):
| Sampling | Mean Variance | Time | Efficiency |
|---|---|---|---|
| Uniform | 0.000024 | 379.18s | 107.73 |
| IS | 0.000012 | 372.15s | 231.73 |
To reproduce: ./malia -log_variance -samples 46500 -wr 380:750:4 -wpp 93 -logging 3 -scene glass_sphere.msf
Scenes :
- data_IS.zip : glass_sphere has one rough glass (the red one), glass_sphere_512 all smooth ones.
- mat_prob.zip
To compare either use the master branch with kappa = 0 and using only extinction parameter, or IS must be disabled manually in microfacet.cu, ask me if needed.