The basic idea behind SSR is clear to me. Although there is little useful information around the basic idea is to ismply march along a ray in either view or screen space. Personally I do it in screen space as I think this is better but that I can't say for sure for the lack of information around.
Whatever the case the common approach seems to be to do a linear stepping along the ray and then doing a bisecting search to refine the result. The bisecting search is clear and depending on the step size is around 5-6 steps for a large screen and a ray running across a large part of the screen. The problematic part is the step size.
I made tests with a step size of 20 (not counting the refinement). In this case for a large screen (1680x1050 as an example) this gives for a moderately long ray bouncing from one side of the screen to the other of lets say 1000 pixel length a step size of 1000/20 = 50 pixels. This is quite large and steps right scross thiner geometry like for example the edges of the boxes in the test-bed I put together attached below (smaller than 1680x1050 as it's from the editor). Furthermore it leads to incorrect sampling as seen on the right side.
Now I've seen other people claiming they do (on the same large screen or larger) 16 samples only even for long rays running across the screen. 16 Samples is even less than the 20 I used in the test which already misses geometry a large deal. Nobody ever stated though how these under-sampling issues work out with such a low sampling count. In my tests I required 80-100 samples to keep these undersampling issues somewhat at bay (speed is gruesome).
So the question is:
1) how can 16 samples for the linear search possibly work without these undersampling issues?
Another issue is stuff like a chair or table resting on the ground. All rays passing underneath would work with an exhaustive search across the entire ray. With the linear test though the test goes into the bisecting phase at the first step the ray crosses geometry like the table or chair. The bisecting test then finds no solution and thus leaks the env-map through. Some others seem to not be affected by this problem but what happens there? Do they continue steping along the ray if the bisecting fails? This though would increase the sample count beyond 20+6 and kills the worst case. So another question is:
2) with rays passing underneath geometry at the first linear search hit and bisecting fails to return a result, what do you do? continue on the ray with worse worst case sample count or fail out?
3) how to detect these cases properly to fade out? bluring or more intelligent?