[In this reprinted #altdevblogaday in-depth piece, game programer Simon Yeung shares an overview on how to bake light maps of indirect lighting data by sampling from the photon map.] Continuing from my previous post, this article will describe how light maps are calculated from the photon map. My light map stores incoming radiance of indirect lighting on a surface which are projected into Spherical Harmonics (SH) basis. Four SH coefficients is used for each color channels. So three textures are used for RGB channels (total 12 coefficients).

Baking the light map

To bake the light map, the scene must have a set of unique, non-overlapping texture coordinates (UV) that correspond to a unique world space position so that the incoming radiance at a world position can be represented. This set of UV can be generated inside a modeling package or using UVAtlas. In my simple case, this UV is mapped manually. To generate the light map, given a mesh with unique UV and the light map resolution, we need to rasterize the mesh (using scan-line or half-space rasterization) into the texture space with interpolated world space position across the triangles. So we can associate a world space position to a light map texel. Then for each texel, we can sample the photon map at the corresponding world space position by performing a final gather step just like in the previous post for offline rendering. So the incoming radiance at that world space position, hence the texel in the light map, can be calculated. Then the data is projected into SH coefficients, stored in three 16-bits floating point textures. Below is a light map that extracts the dominant light color from SH coefficients:

Using the light map

After baking the light map, during run-time, the direct lighting is rendering the usual way, a point light is used to approximate the area light in the ray traced version, and the difference is more noticeable at the shadow edges.

direct lighting only, real time version

direct lighting only, ray traced version

Then we sample the SH coefficients from the light map to calculate the indirect lighting:

indirect lighting only, real time version

indirect lighting only, ray traced version

Combining the direct and indirect lighting, the final result becomes:

direct + indirect lighting, real time version

direct + indirect lighting, ray traced version

As we store the light map in SH, we can apply a normal map to the mesh to change the reflected radiance.

Rendered with normal map

Indirect lighting with normal map

We can also apply some tessellation, adding some ambient occlusion (AO) to make the result more interesting:

Rendered with light map, normal map, tessellation and AO

Rendered with light map, normal map, tessellation and AO