Shadows are quite essential in helping perceive depth and distance. Some games even deliberately exaggerate them in order to deliver emotions, intrigues and/or the dramas to the player (e.i Bioshock, Dead Space, etc). Unfortunately, simulating shadow is one of those expensive luxury or but to some is a necessity. Shadow mapping is one of the common technique on shadow rendering. A shadow map is rendered in the perspective of the light's eye, storing the depth value in a texture which then be used on depth test on the shadow projection pass. Simple as it may sound but when it comes to omni-directional lights (imagine a sun, lighting on every direction) it drastically increase its complexity.
One solution is to render shadow maps with 6 primary 3D axis (up, down, left, right, forward and backward). This would mean one must render the light frustum on each axis 6 times, 600% of the time spent on a single light source, not to mention 6 shadow map textures, heavy on speed and memory (EDIT: Mei de Koh, a friend of mine added that its possible to use a virtual cube map shadow map as not to render the scene 6 times... I haven't study this one though). Enter Dual Paraboloid Shadow Mapping.
Dual Paraboloid Shadow Mapping is a form of optimizing omni-light shadows. Instead of 6 maps, it will only use 2. How? Imagine curving the lens up to the point that if you render the scene pointing forward then pointing backward, you will get a close to perfect spherical vista of the scene. This can be used for environmental mapping (or reflections) but this for another time. The image you see above is using this technique. As you may notice, the shadow penumbra differs based from position and distance to the light source. Although based from developers of STALKER article in GPU Gems 2 they avoided this technique as they are using a deferred rendering (very similar to Light Prepass which I am using). I don't really know why they stated that, but it seem to work on my side. Hopefully, this will be just enough for our game requirements.
(Technical clue: It does really pay off when your native space is the View space.)
There is one technical drawback on my implementation though. As I curve the scene when I'm rendering the lights perspective into a paraboloid, some mesh don't bend well (I'm doing this in the vertex shader). Example is the plane that is the image above. I spent a bit of time on trying to solve this. When the plane was bent, a weird black appears on the edges of the plane. My solution was to flash the shadow map with WHITE first, and do a multiplicative blending (src=DestColor; dest=Zero) when you are rendering the shadow depth. Then presto! no more black edges.
Next, I need to optimize my shadow implementation. If you notice, there still some tweaking to do on the shadow edges. Cheers!