Apr 9, 2010

Spherical Harmonics Lighting

Hello everyone!

It's time for another post on another rendering method! :) Currently, I am working on a light pre-pass renderer. I have just finished to include SH lighting inside.

There exist several methods to compute the lighting solution of a virtual environment. Some methods are fully dynamic (IdTech4, CryEngine) or fully/partially static (idTech3, UnrealEngine). For fully dynamic methods, since the lighting solution is often unified, there is no problem to compute the lighting of dynamic objects. However, for static methods, there can be several problems.

Let's take the case of an environment having its lighting solution stored in a static lightmap. When you add dynamic objects in the virtual environment, you have to compute their lighting solution. But how can we compute the light that reach each dynamic object using only their position/orientation and the surrounding environment?

Common methods are using probe or light volume. For instance, in Quake3, a light volume is define for the entire environment and each Voxel stores an ambient color plus a directional colored source (Quake3 map Specs). Another solution is to used probes positioned by artists in the virtual environment (Source engine). At each of this location, irradiance can be computed and stored in several formats (SH, directional light, Source basis, etc).

For my light pre-pass renderer, I decided to use a light volume having each Voxel containing a 2 bands spherical harmonics as visible on the next screen-shots.


A quake3 map with it's corresponding SH volume.


My test map with its corresponding SH volume.

For each dynamic object, the SH volume is sampled on the CPU using tri-linear interpolation (using object's position). The final SH contribution is added during the final pass of the light pre-pass pipeline according to the normals stored in the g-buffer (And in my renderer, each surface is rendered using relief mapping).



A dynamic object lit only by dynamic lights (left) and
using the SH volume (right).
Note that the lighting information stored in the lightmap now
affects the object final look.

The SH volume is computed as a pre-process. For each Voxel, I render the virtual environment in a cube map as in my previous demo. Finally, the surrounding colored environment stored in this cube map is transformed into the SH basis and stored in its corresponding SH volume Voxel.

I plan to store only global illumination in the lightmap of my environments so the SH volume will only contains global illumination contribution. Then, direct lighting will be computed on static/dynamic objects using standard unified light rendering and shadowing. I also plan to add Crytek's light propagation volume later...

I will continue to work on this engine in parallel to the writing of my PhD thesis, other little demos I have in my pipeline and future job research.

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