With some further tweaking I managed to find a set of depth bias values which seems to work well on the few problematic tables I tested, including JF's new test. I'm still kind of unhappy because I don't quite understand the reasons for these troubles, but at least it's a fix for now.
I found the core issue with the two tables that apparently predicated the changes with this depth bias aspect in the first place (between version test5a and test6) that has lead to all the new issues with near table level walls, ramps, and lights residing on either of them (.1-1.2 range). Both Seawitch and Blackout FS versions were about 4 years old and had no FOV / layback settings. I could resolve the inserts / lights issues on both in just a matter of seconds using the previous build (test5a), in which the errors were initially reported, by opening the table and simply entering a value for the FOV moving it from zero to something non-zero. A value of .2 fixed the issues with both tables and yielded no change in it's appearance otherwise.
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Thank you JF, the influence of the FOV parameter is an important discovery.
This whole thing took me hours to figure out, but here's the story: setting FOV to zero should actually switch to orthographic projection (instead of perspective projection). Unfortunately this is not implemented in VP right now, so it fudges it by setting the FOV very low instead (0.01 is the current lower limit). This basically works, but in order to have such a narrow FOV, the camera needs to be extremely far away from the table. Having such small FOV is basically equivalent to having a super-super-telephoto lens on your camera, if you know something about photography. So the camera is moved so far away from the table that floating point accuracy begins to degrade, which means that the Z-buffer precision suffers, and we get Z fighting. This then necessitates an increase in the depth bias value to make lights still show over the table.
All this is compounded by the fact that the routine which is supposed to fit the camera to the table is hopelessly bugged. Have you ever noticed how, if you rotate your table sidewards for a FS view, the centering and scaling of the camera is completely off and you have to adjust it manually by the scale and offset parameters? This is why. Then there's the horrible layback parameter, which was obviously hacked in after the fact and causes all kinds of problems in its own right, one of them being that it's not taken into account by this camera fitting routine. So again you need to do manual fitting of the camera. All of this also produces wrong values for the near and far Z planes, which messes with the Z buffer.
This camera fitting routine will be fixed in VP10, but I can't do this now because it will break the positioning of all old tables.
Sorry for the rant there, but it was kind of painful to trudge through all this. Anyway, to return to the depth bias issue itself: I don't know if you misread my previous post, but there I stated that I found a way to make (seemingly) all tables render correctly, including your recent test for the depth bias issues, so I don't understand why you argue quite so vehemently for the reverting of this change to the Test5a status. Let me be perfectly clear here: depth bias was already in Test5a, just with a different value, and it is absolutely necessary to render lights correctly. There may have been a misunderstanding that this was a new addition to Test6, when in fact I only increased the value (probably excessively so).
So what I propose for now is increasing the minimum allowed FOV in the source code from 0.01 to 1.0 or another more reasonable value. This should mitigate the accuracy problems and allow us to use smaller depth bias values also on these tables. The proper fix would be to implement orthographic projection, which I tried and works well but again breaks compatibility with the camera settings, so has to wait for VP10 unless I find a workaround there.
I did change the depth bias for Barracora, but only on decals, not on lights, and only AFTER Test6. Decals are, by definition, textures which are overlaid over an existing surface at the same height. If the engine can't render them correctly, then it is broken, and moving the decals manually to a higher plane is not a fix, it's a workaround for a broken feature and shouldn't be necessary. So we do have to take this into account. If, however, the FOV was also 0 on Barracora, which I haven't checked yet, then again the FOV increase will help matters and we can relax the depth bias parameter again.
I'm a bit confused about your assertion that rendering glitches on existing tables shouldn't be fixed at the source code level when you yourself have been a major proponent of 100% compatibility with VP9 tables previously. If I had taken the stance you recommend now from the beginning, then fading lights and GI textures still wouldn't work on 90% of VP9 tables and I would be working on VP10 instead. Again to be clear, the new changes I made now should allow compatibility both with old tables like Seawitch and your new depth bias test table, so I hope that everybody can be happy.