Finally, after a lot of efforts and a lot of hardwork, lighter2 is able to produce caustics. Also a lot of bugfixes have been made in the original code, the major one being the one with lighting produced with the photons. The power of photons had to be scaled to match the illumination of raytracer, there is no need for that now.
Ok, enough of talking here is something pleasant for your eyes.
(This image was generated without any scaling of photons, and with only photon mapping enabled for direct as well as indirect lighting)
Now, regarding the intricacies of getting photon mapping working.
1.) Added a tag into the world file regarding the material, about whether it can produce caustics or not, also if it can produce caustic, what is the refractive index of the material. (For air the refractive index is considered to be 1, also if it is not mentioned for a caustic producing material, it is assumed to be 1. So, the caustics might not be that accurate, if one doesn't mention the refractive index.)
2.) While parsing the scene, the positions (Bounding spheres) of all the meshes which contain the material which can produce caustics are stored in a list. This list is used in the photon emission stage. The point light is considered to be like a spotlight which emits photons only in the direction of these meshes while emitting caustic photons.
3.) The photon tracing is facilitated with refracting these caustic photons, these photons are traced till the time they hit a diffuse surface, and instead of reflecting these photons in random direction they are refracted using simple laws of refraction (this is the point where refractive index is required).
4.) Finally in the final gathering stage it is not just the normal photon map which is used but also the caustic photon map.
(A lot of debugging and reworking also went into this process, which took all this time)
As for the future, I am trying to gt support for direction and spotlight lights for photon mapping and caustics, and also area lighting. I plan to get these working before the submission date for GSoC, and then it would be time for optimizing lighter2, in my opinion the memory optimization should be of higher priority than time optimization.