Has someone tried this before?

Okay, so I'll be describing a simple approach that I haven't seen being pushed around (I should admit, it bears some pretty obvious similarities to Peter Pike Sloan's et al's Image-Based Proxy Accumulation for Real-Time Soft Global Illumination, though I think my suggested method is a lot simpler. There's been a lot of talk about deferred lighting solutions to renderers. Most of these solutions have one big thing in common: they require a per-pixel screenspace normal map and specular exponent (or some other fancy BRDFish properties) to have been written out in advance. Not a huge limitation, but a real one nonetheless. So what follows removes that limitation:

a) do a standard Z-prepass
b) allocate a 3-deep MRT color buffer, init to black
c) now evaluate all light sources as one normally might for any standard deferred lighting solution, except we add to the color buffers view-space relative Spherical Harmonic representations of the light's irradiance relative to the current pixel.

It's implicit, but to make it explicit and state it outright, you're writing a very simple 3-color 4 coefficient SH into the buffer. Or, alternatively one might choose a hemispherical basis that needs fewer coefficients, but there are good reasons to stick with a spherical one (primarily that you can handle arbitrary reflection vectors).

So, why bother with this? Here are a few interesting "wins".
1) lighting is entirely decoupled from all other properties of the framebuffer - even local normals. Lighting can be evaluated separately when we get to shading using a lighting model that changes relative to the shading.

2) since lighting is typically low frequency in scenes, you can almost certainly get away with constructing the buffer at a resolution lower than the real frame buffer. In fact, since the lighting is independent of things like normal discontinuities, you might even be able to get away with ignoring edge discontinuities. Or you could probably work around this using an ID buffer constructed later, similar to how Inferred lighting works to multisample the SH triplet (thought that seems pretty expensive) and choose the best sample.

3) To me this is really the biggest win of all - because this is decoupled from the need for a G-buffer or any actual properties at the pixel other than its location in space, you could really start doing this work immediately after a prepass has begun! This is becoming more and more important going forward, as getting more and more stuff independent means its easier to break things into separate jobs and distribute across processing elements. In this case you could actually subdivide the work and have the GPU and CPU/SPU split the work in a fairly simple way, and its almost the perfect SPU-type task as you don't need any underlying data from the source pixel other than Z.

4) MSAA can be handled in any number of ways, but at the very least, you can deal with it the same way Inferred Lighting does.

5) There's no reason for specularity to suffer from the typical Light-Prepass problem of color corruption by using diffuse color multiplied by spec intensity to fake specular color. Instead you could just evaluate the SH with the reflection vector. Of course, one does need to consider that given the low frequency of the SH as it applies to specularity...

6) Inferred Lighting evaluates the lighting at low frequency and upscales. Unfortunately, if you have very high frequency normal detail (we generally do), this is bad as this detail is mostly lost as their discontinuity filter only deals with identifying normals at the facet level, and not at the texture level. The suggested method isn't dependant on normals at all as lighting is accumulated independent of them, so it doesn't suffer from that problem.

7) You can start to do a lot of strange stuff with this. For example:
- want to calculate a simple GI approximation? Basically do the standard operating procedure Z based spherical search used in most SSAO solutions, except when a z-texel "passes", accumulate it's SH solution multiplied by its albedo and a transfer factor (to dampen things). Now you've basically got the surrounding lighting...
- want to handle large quantities of particles getting lit without doing a weird forward rendering pass that violates the elegance of your code? Do a 2nd Z-pass, this time picking the nearest values, and render the transparent stuff's Z into a new Z buffer. Now, regenerate the SH buffer using this second nearer Z-set into a new buffer set. You now effectively have a light volume, so when rendering the individual particles, simply lerp the two SH values at the given pixel based on the particle's Z (you could even do this at vertex level sampling the two SH-sets per-pixel seems cost prohibitive). Of course, this assumes you even care about the lighting being different at varying points in the volume, as you could just use the base set.
- if you rearrange the coefficients and place all the 0th coefficients together in one of the SH buffers you can LOD the lighting quality for distant objects by simply extracting that as a loose non-directional ambient factor for greatly simplified shading.
- you can rasterize baked prelighting directly into the solution if your prelighting is in the same or a transformable basis... assuming people still care about that.
- if you construct the SH volume, you could use it to evaluate scattering in some more interesting ways... You could also use this "SH volume" to do a pretty interesting faking of general volumetric lighting. If one were to get very adventurous, you could - instead of using min-z-distance as the top cap, simply use the near plane, and then potentially subdivide along Z if you wanted, writing the lighting into a thin volume texture.

So, the "bad":
- lots of data, as we need 12 coefficients per lit texel. That's a lot of read bandwidth, but really its not any more expensive than every pixel in our scene needing to read the Valve Radiosity Normal Map lighting basis, which we currently eat.
- Dealing with SH's is certainly confusing and complicated. For the most part this only involves adding SH's together which is pretty straightforward. But unfortunately converting lights into SH's is not free. The easiest thing to do is pre-evaluate a directional light basis and simply rotate it to the desired direction. Doable, given we're only dealing with 4 coefficients. Or, directly evaluate the directional light and construct its basis. Once you've a directional light working, you can use it to locally approximate point and spot lights by merely applying their attenuation equations. Of course, if you don't need any of the crazier stuff, you could just use a simpler basis (like the Valve one) where conversion is more straightforward.

Anyway, there we go. If anyone reads this, let me know what you think. It would seem this solution is superior to Inferred Lighting in its handling of lit alpha, as with their solution you can really only "peel" a small number of unique pixels due to the stippling, and the more you peel, the more degradation it causes in the scene to the lighting.

Anyway, for now until I can think of a better name, I'm calling it Immediate Lighting.