How does the UHD cache work?

[ginter]

I was wondering what the difference is between the UHD cache and other biased approches like photon maps. The documentation claims it avoids splotches but is pretty cryptic about what it actually does. Is it essentially some sort of very conservative irradiance caching?

[maru]

Not a very technical explanation: it stores GI in quite a splotchy way, and is used to speed up path tracing. Path tracing is running on top of it, so the splotches are not visible anyway.
As far as I understand from this paper, it is actually a smart kind of radiance caching.

Update: it is actually a "hybrid" solution, that takes advantage of both worlds of radiance and irradiance caching. It can't be really categorized and explained easily.

[cecofuli]

Photon mapping, like in old V-Ray school method (IM+PM or QMC+PM before LC appeared on VRay) traces rays from the light source inside the scene.
It means that you can calculate the PM only one time and you can move the camera where you want, without to calculate again the PM. PM is view independent.
But it's complicated to setup and slow (you must to set, for each light, how much photons you want to shoot) and it's very slow with high settings.

LC or UHD has replaced the PM. They are view dependent and rays are fired from camera to the scene, it's faster than PM  and it has less problems, especially near the corners.
Both are splotchy , but UHD much less splotchy than PM. Now, nobody use PM.
IM has nothing to do with PM pr LM. IM is another story.

[ginter]

Great answers, thanks a lot! I'll def. read the thesis. What are the main differences w/ vray's light cache? From skimming it appears to be directional while IIRC the LC isn't?

[cecofuli]

LC and UHD Cache are almost the same.
IM is for direct light computation (interpolation method), similar to QMC or Brute Force (precise method).