In my experience, authors almost never deliberately delete a navmesh, unless they've completely reworked a cell from scratch - and that's fairly rare. Instead, what I see is one of two things:

 

1. The author makes edits to an existing navmesh in the CK. When they finalize the navmesh, the CK deletes the original navmesh and saves the edits as a completely new mesh.
2. The author makes edits to an existing navmesh in the CK. When they finalize the navmesh, the CK merges the mesh with a nearby vanilla mesh and deletes the original.
3. Both - the CK merges two adjacent meshes and deletes both originals.

 

In theory, within an isolated case of just the mod itself and the vanilla masters, a deleted navmesh shouldn't cause major problems. However, the more complex the setup becomes, the more likely it is that the deletion will wipe out changes to the original mesh made by other mods. In the case of highly modded areas, this can happen very fast. Additionally, the removal and replacement of the old mesh will result in edge link changes to adjacent navmeshes, creating pseudo-ITM records that can also wipe out changes from other mods. This can quickly escalate from a nuissance situation involving NPCs taking "the long way around" or refusing to move into a crash-to-desktop situation if, for example, door links reference a deleted navmesh or pathfinding ends up relying on a deleted mesh. 

 

Long story short, deleted navmesh getting into a mod isn't the author's fault. It is however a deficiency if the author doesn't fix it before releasing the mod to the public. This kind of issue is a major defect that we need to fix when integrating the mod into a complex load order.

Here's our 15 deleted navmeshes in the plugin, listed in the xEdit cleaning report:

 

 

To assess each mesh, we'll start by holding down the control key and double-clicking on the navmesh form ID within this report. That will take us to the navmesh record.

 

 

Next, we'll control-left click on the SexyBanditCaptives.esp column header on the left. This will switch the SBC instance of the form ID, which should reveal the new SBC navmesh on the left.

 

 

Well, that's interesting. If you recall from above, we should be seeing an SBC form ID navmesh here. This might in fact be a genuine player-initiated navmesh deletion... but before we jump to that rather disappointing conclusion, let's examine this further and see what's going on. The original has a single edge link to another mesh, so maybe that'll reveal a replacement mesh. Keep in mind the vertices we see above - if we see the same vertices on a linked mesh to that other ref, then we might have found a rather odd example of a Type 2 navmesh deletion. 

 

 

Hmmm. That's concerning. It looks like this other mesh - F0673 - contains no edge links... they're all deleted in the override.

 

 

Something funky is going on here. We better open this up in the CK and take a look within the GUI. By the way, this is a great case of a situation where you must use the CK if you want good results. There are many other functions where tools like xEdit or external script compilers or whatever do the job better, but positioning objects in 3D and working with landscape and navmesh are all CK activities, for better or for worse.

 

 

The green shaded area that I circled in yellow is the deleted navmesh. I don't see any reason for this deletion so when we get back we'll just undelete it entirely. But what about that adjoining mesh with no edge links?

 

 

That's it there, the inside of the green square. It goes without saying that those edge links were removed in error - we can clearly see that its meant to interface with other adjacent cells. Fortunately, if we click "finalize navmesh" for this cell with no other edits and then save it, the CK should automatically repopulate the links.

 

 

Here's what we see when we go back into xEdit:

 

 

So its an easy fix, but now we've got another red flag... beyond just repairing the deleted meshes, we'll want to look at all navmesh edits in this mod to make sure there aren't similar defects.

 

Now here's the next deletion, below:

 

 

This is another straight-up deletion with many edge links to other meshes. Let's examine it in the CK.

 

 

So that's the deleted navmesh. This, again, is a major problem but its pretty easy to fix by just undeleting the original navmesh.

So that's the first 2 (+1 not deleted but defective) of 15 deletions. I hope they're not all like this. Let's review the remaining navmeshes and see what we find.

 

[NAVM:000E9049] - Deleted with no replacement. Had edge links. Linked mesh also deleted
[NAVM:00108270] - Deleted with no replacement. *Note - cell has a new EDID from this mod, so presumably this is an active content location for this mod. Had edge links to multiple cells
[NAVM:00108BE8] - DNR
[NAVM:000F065B] - DNR 
[NAVM:001020F3] - Type 1
[NAVM:00105F89] - DNR
[NAVM:000FFDB3] - Probable Type 2
[NAVM:000FFDB1] - Probable Type 2
[NAVM:000FFDAA] - Probable Type 2
[NAVM:000E8404] - DNR
[NAVM:000ED011] - DNR
[NAVM:000ED01E] - DNR
[NAVM:00016630] - DNR

 

So we're looking at 73% of the navmesh deletions being actual deletions rather than CK replacement issues. While these are almost always easily fixed by undeleting the mesh, it's a needless pain in the neck and you as the end user are still going to want to look at the cells to make sure the author hasn't made a major change to them that the vanilla mesh will cause problems interacting with.

 

We do however have examples of Type 1 and Type 2 navmeshes, and for educational purposes let's review them.

 

The sole Type 1 deletion we have here involves Tamriel -31, -4, which based on its SBC Editor ID is a quest location for this mod. Note how the deleted navmesh's first vertex was at -124096, -16256, -4600. A quick look at the new SBC navmesh (04002411) reveals that it covers the same area.

 

 

This is pretty easy to fix. We'll copy the original navmesh down, then delete the deleted record in xEdit. 

 

 

With that done, now we'll go into the SBC navmesh and rename it to the deleted one.

 

 

Now the form ID deletion is resolved.

 

 

Out of curiosity, let's see what the difference is between vanilla an SBC.

 

 

Groan. Edge Link deletions. Based on what we've seen before, I don't know if I really trust this. Let's open up this navmesh in the CK and see if its a legit deletion. So, check it out below, here's the first triangle with a removed edge link. The yellow seam is the edge of the navmesh. You tell me, do you think it was correct to remove the edge link from this cell?

 

 

I think at this point we have to make a change to today's discussion plan.

I was really hoping that this blog could offer more educational value, but I feel like instead I'm just beating on this mod. Fortunately, the Type 2 deletion seems to be less problematic, so we can get some good demonstration content out it.

 

In this case, the three deleted navmeshes appear to be the interior of one of the Riften watchtowers, and they've been merged into the main cell for that area. That main mesh does appear to actually change some of the vertex coordinates, so it might be a legitimate type 2, but I think we need to determine what this changes before we decide if its worth putting effort into fixing.

 

  

Let's overlay the two meshes and see how they look in the CK. First, the green highlights show the new mesh from SBC.

 

 

Now compare to the original - it looks like they are basically the same but of course it doesn't include the deleted areas. And those areas do in fact appear to be navmeshes that don't offer a lot of value being separate, or even opportunities for conflicts. 

 

 

To fix a Type 2 deletion, we need to do two things. First, we need to identify the new "winning" navmesh, which we've already done. Second, we need to undelete the losing navmeshes in xEdit. Once that's done, and those meshes are again visible in the CK, we're going to delete everything from them except for triangle zero, and then we drop that last triangle well below the Z plane of the cell. 

 

 

 

With all those done, we can hit "finalize navmesh" and save the file.

 

 

This will allow the mesh parts to remain undeleted without duplicating triangles in the merged mesh, and it'll make it immediately clear to us in the future if some future mod tries to edit this mesh - that'll ensure we know to go in and pass off those changes to the "winner" mesh.

So to recap, we reviewed three different ways to repair navmesh. In some cases, when the mesh was deleted in error, we just undelete it; we might also have to go back into the CK and finalize it to restore deleted edge links, though that's really more of a conflict resolution & dirty edit cleanup type deal than strict deleted navmesh repair. Second, we might hit a Type 1 deletion, where we need to replace the deleted record with the edited version that the CK spun off into a new record. Finally, we might see a Type 2 deletion where we need to trim down superseded navmesh and then hide it deep underneath the terrain. We didn't see a Type 3 in this case study, but the solution is to just do the Type 1 fix and then the Type 2 fix (rename the "winner", undelete the rest and then trim & drop). 

 

After that, the next step would be to rebuild the Navmesh Information Map for the main mod. To do this, just delete the NAVI record from the plugin, load it up in the CK, and save. Then, you'll almost certainly be revisiting these navmeshes during conflict resolution... but that's a topic for a different blog.