Some Further Lighting Development Possibilities

***Warning, more technical lighting talk ahead***


Like many of you I got around to seeing Avatar over the holidays, which naturally lead me to reading some behind the scenes interviews. One particular sentence jumped out at me in this AWN Article:

We came up with a system based primarily on image-based lights but then converted the whole system to spherical harmonics," Letteri explains. "What that meant was we could pre-compute all the lighting contributions in a scene and then put the characters and everything in with the lighting and the TDs could move the lights around.


This got me very curious. Our current solution, called Ambient Occlusion Environment Sampling, uses a very similar method to get that nice animation friendly indirect light that is one of the secretes behind great lighting. But for every rendered frame our method has to recalculate the environment light contribution, whereas the method used with Avatar does not. That is ton of wasted render time for us.

So my question was, what is this magical "spherical harmonics" and how could it fit in our production. What I found through some research is the technique also can be achieved through what is known as a Diffuse convolution envmap. It's a bit technical of a read, but I know exactly how to implement it for us. Our lighting work flow would look something like this:

1. Figure out time of day and sun direction for film.
2. Bake Physical Sky enviornment hdri with this method and apply the correct filter to the image.
3. Attach "spherical harmonics" environment to ambient slot to a white lambert for testing.
4. Light the scene with other lights as we see fit, such as a rim lights and possibly fill lights.
5. Pipe the the "spherical harmonics" into the render passes and tweak further in composting.

Ta Da!! We just potentially shaved off minutes of every frame rendered at no sacrifice in quality!!!

So to conclude, we can implement a similar lighting approach used with Avatar that can save us a ton of time. Don't know about you, but for me, that is a very exciting prospect.