How come there are almost no post on lightmaps on this site?
Because everything that needs to be said about them has been. Besides, it’s just a texture; how much is there to discuss?
per pixel shading using shaders produces overhead and a huge load on the engine, and why calculate something real time when it can be precalculated and stored in textures?!!
Because it can’t be precalculated and stored in textures.
Lightmaps are a hack (or, less hyperbolically, an approximation). As such, they have limitations. The most serious of which is shadowing.
Lightmaps only provide shadows for objects that were in the scene in that position when the light map was generated. If all you’re doing is rendering static terrain, that’s fine. But what happens when you want to do better? What if you want to move the terrain (open doors, destroy boxes, etc)? Or even just render something else that moves?
Well, in lightmapped engines, you don’t get shadows from those characters. You can try to approximate it, of course, but that’s about the limit of what you’re capable of doing.
Furthermore, once you use lightmaps, you’ve limited yourself to mostly static lighting. That might work when you’re indoors in some corridor-based shooter, but it has no meaning for reality when you step outside and the sun rises and sets.
Personally, I don’t think lightmaps are worthwhile in the general case. In certain limited, specific cases, perhaps. But in general, the cost of fragment programs has reached the point where actual lighting is performant. And since actual lighting allows for so much more, there’s really no point in using lightmaps.