Yeah I've had ISM's on my mind for a while too
The problem is that they're designed for RSM/etc, where you have a lot of little lights with their own little ISM, which all add up together to approximate a large-area bounce light. Because each of these "bounce lights" is made up of, say, 1000 VPLs, each with their own ISM, the amount of error present in each individual ISM isn't noticable.
I haven't tried it, but I have a feeling that if I've got a street with 100 individual streetlamps on it, and I use ISM to efficiently generate shadows for them, then the ISM artefacts will be hugely noticeable... Maybe I could use ISM's for my "backwards" shadowing idea though, where my "visibility probes" are rendered using the ISM technique? That would allow for huge numbers of dynamic probes...
In Killzone, they use SSR to supplement reflection probes -- SSR, local probes and a global sky probe are all merged.
I'm thinking of a similar thing for the "per-pixel depth hemisphere" system -- you'd have a huge number of approximate visibility probes about the place, which give very coarse mid-range blockers -- e.g. if you're inside a room with one window, don't accept light contributions from a spotlight from behind the wall that's opposite that one window. Obviously these probes would have to be quite conservative, with bleeding/tolerance around the same scale as their radius of influence.
You could then supplement this with screen-space data, in the cases where it's possible to get any results.
On SSAO, it really gives me a headache when games do stuff like this (Far Cry 3, why u do this?)... They've got this great lighting and GI system, and they go and shove a bloody outline filter in the middle of it!?
The last SSAO filter I wrote looks like this (just the contact-darkening on the players, the grass is old-school planar projection stuff, and yeah, there's no shadow-mapping/etc), which I personally don't think is as objectionable as the above, (I would call the above a contrast filter) Maybe I'm biased though...
With the screen-space specular masking, at the moment, I'm pretty hopeful I'll be able to use it. It does break down at the edges of the screen, with thin geo, and with complex depth overlaps... but I also do a lot of image based lighting, and there's not a lot of good ways to shadow an IBL probe, and the scene looks a lot better with these shadows fading in, in the areas where the technique works...
Yeah that could work -- you could kinda have a hierarchy of occlusion values, or shells of occlusion hemispheres. If you know the depth range of each "shell" where the assumptions are valid (occluders and closer than light source), then you can use just the valid ones... You probably wouldn't need too many of these layers to get half decent results...How about two level DSSDO. One for small level detail and one for larger. Then picking better value per pixel?[/size]