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Baking direct lighting into light probe

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I have a typical system for rendering light probes, where I render a cube map at the probe position, and then project the cube map onto SH.  I render the lightmapped world geometry into the cube map, which captures indirect lighting from the lights in the lightmap.

 

I'd also like to capture direct lighting from the lightmap lights.  To do this, I was thinking I'd render a sphere with the light's color/radius/position in the cube map for each lightmap light.  Is this the right idea?  Am I missing anything?

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You can do this, and it will work as long as the light is large enough relative to its projection onto the cubemap texels. If it's too small the cubemap rasterization will undersample the light, and you may get poor results due to aliasing.

 

One alternative would be to directly project the light onto SH, and then combine that with the SH from your cubemap. Peter-Pike Sloan gave an analytic formula for computing the zonal harmonics coefficients of a sphere positioned directly above the Z axis in his "Stupid SH Tricks" paper, which can then be easily rotated to their actual world-space direction (I believe that this paper provides the formula for the rotation). One major downside of this approach is that it won't handle occlusion of the light source (or the light source occluding other geometry/lights), unless of course you handle that yourself.

Edited by MJP

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