So had some other code issue that needed attending to. But back to this.
Played around a bit with light probes. Played around with having a lot of them (255 to be exact).
So as i understand, in the current implementation pre computes the normal* 9th coefficient SH light probe into the irradiance map. So each light probe as a spec environment cube map, and the same resolution "premultipiled by normal SH" irradiance cube map.
This then reduces rendering time to sample irrandiance cube, and the correct mip level spectacular map. Making live rendering very fast.
Another way of course is to use just 9 values for a SH of the irradiance light Probe, and then multiply with the normal vector at sample time. Indeed a light Probe could be a matrix for this calculation, (eq 12 of http://graphics.stanford.edu/papers/envmap/envmap.pdf). And as i understand this can be a varying. So the matrix can be constructed per vertex, ie cheaper than per pixel.
The nice thing with that is we know it will work. The math is 100% linear so everything combines linearly no problems. No extra artifacts would be added. Of course sampling each light probe and weighting the result will also give the same result.
Spec is not so nice. It is not linear. However with a dense enough sampling it would "tend to linear". I am still thinking of sampling each env map from parallax adjusted and weight summing the result. However this will produce bizzar artifacts from env maps placed far from each other.
At this stage i will leave cube mapping in place. Moving to octahedron mapping seems like something that fits nicely with moving to nodes.
Now the problem i am mostly trying to solve is triangulation. Not because that is hard. But because in many use cases, in my case even, i won't have or don't need triangles/tetrahedrons. For example a long corridor can be a single probe every now and then with a single edge. Also lots of probes is too slow for precomputing. I realize for pre made maps this isn't a problem. But i need to create probes as part of the "loading" screen. In fact the triangulation/edges/contentedness in my case can be part of probe placement.
Anyway. Random thoughts and just an update of where i am at. Will keep cracking on. feedback always welcome.
Out of interest what is the slow part of calculation the light probes? I assume its the importance sampling step. But i didn't profile it.