So yesterday I was attempting to diagnose a bug in my game. The symptom was that when I approached Fort Magia from certain directions, the entire cell in which it is located would fail to load, leaving a big rectangular hole in the landscape with just a few items floating in the air. After some poking with TES4Edit and turning mods on and off, I discovered that the problem went away if I disabled the UOP, which modifies that cell. However, upon further testing, I seem to have discovered that the problem occurs not because of anything specific that the UOP is doing, but because of a general engine bug.

The bug seems to be this: loose non-static items get havoked when the cell they are contained in is loaded. Normally this is no problem, though poorly placed items can sometimes move around. But if the item is placed near a cell border, and if the item havoks its way across the cell border into an as-yet-unloaded neighboring cell, the contents of the neighboring cell will never load.

What led me to discover this was that when the bug occurred with Fort Magia, there was a set of three loose items stacked floating in the air in front of the fort door ( picture http://i86.photobucket.com/albums/k90/in_a_coma_dial_999/Oblivion/before1.jpg ) and a few items right on the border with the neighboring cell. Those items are normally located inside a small ruined tower ( CS pic http://i86.photobucket.com/albums/k90/in_a_coma_dial_999/Oblivion/before0.jpg ). Looking in the CS, I noticed that the border between the Fort Magia exterior cell and its neighboring cell passes right through the center of that tower ( picture http://i86.photobucket.com/albums/k90/in_a_coma_dial_999/Oblivion/cellborder.jpg ). And the items that were floating in front of the Fort Magia door were all ones placed in the neighboring cell, not in the Fort Magia cell itself.

I "fixed" the problem in the CS by moving all of the items into the Fort Magia cell, but still in the little tower ( pic http://i86.photobucket.com/albums/k90/in_a_coma_dial_999/Oblivion/after0.jpg ). That prevents the problem occurring, because the offending items don't get loaded until the cell containing the Fort gets loaded. But... yup, you guessed it: if I approach the area from across the water, the opposite problem can occur, and the cell next to Fort Magia can now fail to load, leaving a big hole there instead.

I decided to make a more general test. I made a trivial mod that places a big rock in Wilderness (17,13) right on the border with (16,13) such that part of the rock extends into (16,13). Then I placed a skull in (16,13) that sits on the protruding part of the rock. Sure enough, if I approach the area from the (16,13) side, the skull rolls into (17,13) and there's a big square hole in the ground. So the problem is definitely reproducible.

My suspicion is that the items getting havoked into a neighboring cell causes the game engine to mark that cell's contents as already loaded. So, when the player moves closer to the cell, the engine doesn't draw the cell contents because it thinks it doesn't need to. There is also some interaction with mods in all of this; it only seems to happen when a mod edits the contents of a cell. I also noticed that any items that are modified by the mod do get drawn -- you'll notice in the pic I linked above that there are a couple of rocks and a tree in the otherwise empty Fort Magia cell -- those items are the ones that the UOP moves. Everything the UOP doesn't touch, doesn't get drawn.

Anyway, I thought I'd share, as I think this may explain some of the weird missing-landscape issues that plague certain areas in the game when mods make changes to cells in those areas.

A very good anolysis of the issue. And just based on your findings, one that's probably spot on. At least for some of them.

Have you tried to see what happens if you use TCL and then go pick up the stuff that's potentially causing issues? For games in which the problem has already happened, perhaps grabbing the visible suspects would prevent it from happening again when you return later.

I would also assume that waiting out a cell reset would fix the issue too if the items causing it have been moved to a location in the cell where they can't havok roll over the cell border.

Yep, I tried exactly that: grabbed all the loose items in the middle of the cell and still on the cell border, and then ran around long enough that the cell was no longer in my cell buffer. Sure enough, the cell appeared fine after that.



Right. That's the rub: if you move the items in the CS, you have to wait for a cell reset if your save traveled there recently, otherwise the problem can still happen.

Well having to wait out respawn once isn't a big problem. It's finding all of the various places this happens that's the problem. We know of a few common problem areas, but given the nature of the problem there could be loads of other lesser known placed.

Plus, there's not a thing that could be done about most mods that drop stuff near cell borders that could roll over.

The place I've seen this most is the cell North-west of the one containing Infested Mine (-35, 12?). One of my mods adds to the cell at the other end of the lake, so not close enough to the one that disappears to be causing it, but of course, that's why I'm in the area to see it. Next time it happens, I'll look around the cell border for a trigger item.

There's a dead goblin lying on a chest almost at the corner of the cell that could be part of the story there. IIRC there's a bottle or two lying around him that could be the offending item.

Yep, looks like another good candidate. Three beer bottles lying on the ground right next to the cell border. Now surrounded with small rocks to keep them from rolling away.

Not much can be done about the goblin and the bottle in his hand though as I'd rather not ruin the scene.

It's going to get to the point where I'm wandering through the game and I'm going to say "Oh, there are more of Arthmoor's rocks"

Heh. Not really. They're all in relatively obscure spots so far.