What is the best currently known and ideally also production-verified approach for sampling environment maps (EM) in a MIS-based uni-directional path tracer and similar types of renderers? I would prefer solutions which are reasonably complicated while reasonably functional to those which provide perfect sampling at cost of super complicated and hard-to-understand implementation.What I know so farThere are some easy ways of sampling EMs. One can sample the needed hemisphere in a cosine-weighted manner,   which ignores both the BSDF and the EM function shapes. As a result,   it doesn't work for dynamic EMs:To improve   the sampling to a usable level, one can sample the luminance of the EM   over the whole sphere. It is relatively easily implemented and the   results are quite good. However, the sampling strategy is still   ignoring the hemispherical visibility information and the cosine   factor (and also the BSDF), resulting in high noise on the surfaces   which are not directly lit by high-intensity areas of the EM:PapersI have found a few papers on the topic, but have not read them yet. Is any of these worth reading and implementing in a forward uni-directional path tracer, or is there something even better?Structured Importance Sampling of Environment Maps (2003) byAgarwal et al.Steerable Importance Sampling (2007) byKartic Subr and Jim Arvo. They claim to present ...an algorithm forefficient stratified importance sampling of environment maps thatgenerates samples in the positive hemi-sphere defined by localorientation of arbitrary surfaces while accounting for cosineweighting.  The paper Importance Sampling Spherical Harmonicscomments on it: They create a triangulated representation of theenvironment map and store the illumination premultiplied by each ofthe first nine spherical harmonic basis functions at every vertex.This forms a steerable basis where the clamped-cosine can beefficiently rotated to any orientation.Practical Product Importance Sampling for Direct Illumination (2008) byPetrik Clarberg and Tomas Akenine-Mller. An algorithm for samplingthe product of environment map lighting and surface reflectance. Useswavelet-based importance sampling.Importance Sampling Spherical Harmonics (2009)by Jarosz, Carr, and Jensenn. The abstract says: ...we present thefirst practical method for importance sampling functions representedas spherical harmonics (SH)...Tone-Mapped Mean-shift Based Environment Map Sampling (2015) by Fenget al. This is pretty new and I have found neither a reference to itnor the paper itself.
Importance Sampling of Environment Maps
rendering;sampling;pathtracing;importance sampling;ibl