Theoretically, using six shadow mapped spot lights to piece up a shadow casting point light is not a big deal. Especially if the light geometry calculation for deferred lighting is correct. Well, in my engine, it was not the case. 🙂
There was a hidden calculation error that resulted in a frustum with incorrect corner points that made it bigger than desired. This error caused no problem so far because projected mask texture on spotlights and “discard” codes in fragment shader prevented the extra pixels from being seen. But once I have used spot lights with no mask to render point light, the result was erroneous. It looked so strange and mysterious that it took few hours to find the root of the problem.
Finally, the shadows on point light are working now, and I proudly present some screenshots here.