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While working on several Chapter 3 quests, we continuously ran into several unexplainable problems with NPC pathing. After some thorough investigation, I've confirmed a bug in Fallout 4 that probably creates all sorts of bugs, particularly in late-game saves.
TLDR:
Interior workshops like Diamond City Player Home, Vault 88, and Chapter 2's HQ impact the NPC pathing of every other interior cell in the game. Things you build in an interior workshop will block pathing in all other cells if the coordinates of things happen to intersect areas NPCs are meant to walk.
For example, if you build a bed in one cell at coordinate 0/0/0, all other interior cells with a navmesh near 0/0/0 will have a bed shaped hole cut in them.
Technical Details:
When building things inside a settlement, the navmesh is dynamically adjusted to cut a hole where that item exists. We call this navcutting, it's generally awesome as it allows NPCs to path somewhat believably around what you build.
It does so by taking the bounding box of the mesh, and forcing the navmesh that intersects it to reform a new shape around that bounding box.
This is a unique feature of locations flagged as having a Workshop. If we dynamically spawn in items not associated with a workshop, or in cells and locations not designated as part of a workshop, the navcutting does not occur. Or at least it's not supposed to.
It turns out that with Interior cells, this rule is sort of broken.
Interior workshops, such as the Diamond City Player home, Vault 88, or our HQ cells all have the workshop tag to allow them to dynamically alter navmesh, and it generally works. The problem is that it appears to impact every single other interior cell, both other workshop cells, and non-workshop cells.
The bounding box for each built item is now cutting navmesh across all interior cells.
Fortunately, Bethesda seems to not always center their interior locations at the 0/0/0 coordinates, so the problem isn't noticeable in every single cell, since oftentimes, those navcutting bounding boxes won't be anywhere near the navmesh in other cells, but in some cases it can be quite brutal - even blocking NPCs from getting to important spots for quests to continue.
If you would like to try replicating this yourself to see it in action:
1. Take a companion with you to Vault 111 back past your spouse's pod and up the short stairs, you should be able to direct your companion to walk around on that small raised area.
2. Now go heavily build up the area around the red door in the Diamond City Player Home, really cover up as much as you can around that section of the place.
3. Return to that spot in Vault 111, and notice your companion probably won't be able to go up those stairs anymore, and definitely won't be able to path up that area as cleanly as they did before the building.
The same can likely be observed all over the game world in various interior locations, but this was the easiest location I've found to clearly witness the behavior.
I've detailed this in a report to Bethesda in hopes it can be addressed for the next-gen update.
HQ's Impact
For SS2, HQ being an interior settlement means we're contributing to this problem quite a lot, and we've observed that HQ itself negatively impacts itself, the more you build in various floors, the more the navmesh breaks on all floors - to where a fully built HQ, the settlers can hardly move.
We're looking into a solution where we move the HQ cell contents to far off coordinates that won't cross space with any other cell for the 3.0.0 patch so that we can eliminate HQ causing this. So spread the word, before you load 3.0.0 (when it comes out), you will likely want a save where your character is NOT in an HQ cell.
TLDR:
Interior workshops like Diamond City Player Home, Vault 88, and Chapter 2's HQ impact the NPC pathing of every other interior cell in the game. Things you build in an interior workshop will block pathing in all other cells if the coordinates of things happen to intersect areas NPCs are meant to walk.
For example, if you build a bed in one cell at coordinate 0/0/0, all other interior cells with a navmesh near 0/0/0 will have a bed shaped hole cut in them.
Technical Details:
When building things inside a settlement, the navmesh is dynamically adjusted to cut a hole where that item exists. We call this navcutting, it's generally awesome as it allows NPCs to path somewhat believably around what you build.
It does so by taking the bounding box of the mesh, and forcing the navmesh that intersects it to reform a new shape around that bounding box.
This is a unique feature of locations flagged as having a Workshop. If we dynamically spawn in items not associated with a workshop, or in cells and locations not designated as part of a workshop, the navcutting does not occur. Or at least it's not supposed to.
It turns out that with Interior cells, this rule is sort of broken.
Interior workshops, such as the Diamond City Player home, Vault 88, or our HQ cells all have the workshop tag to allow them to dynamically alter navmesh, and it generally works. The problem is that it appears to impact every single other interior cell, both other workshop cells, and non-workshop cells.
The bounding box for each built item is now cutting navmesh across all interior cells.
Fortunately, Bethesda seems to not always center their interior locations at the 0/0/0 coordinates, so the problem isn't noticeable in every single cell, since oftentimes, those navcutting bounding boxes won't be anywhere near the navmesh in other cells, but in some cases it can be quite brutal - even blocking NPCs from getting to important spots for quests to continue.
If you would like to try replicating this yourself to see it in action:
1. Take a companion with you to Vault 111 back past your spouse's pod and up the short stairs, you should be able to direct your companion to walk around on that small raised area.
2. Now go heavily build up the area around the red door in the Diamond City Player Home, really cover up as much as you can around that section of the place.
3. Return to that spot in Vault 111, and notice your companion probably won't be able to go up those stairs anymore, and definitely won't be able to path up that area as cleanly as they did before the building.
The same can likely be observed all over the game world in various interior locations, but this was the easiest location I've found to clearly witness the behavior.
I've detailed this in a report to Bethesda in hopes it can be addressed for the next-gen update.
HQ's Impact
For SS2, HQ being an interior settlement means we're contributing to this problem quite a lot, and we've observed that HQ itself negatively impacts itself, the more you build in various floors, the more the navmesh breaks on all floors - to where a fully built HQ, the settlers can hardly move.
We're looking into a solution where we move the HQ cell contents to far off coordinates that won't cross space with any other cell for the 3.0.0 patch so that we can eliminate HQ causing this. So spread the word, before you load 3.0.0 (when it comes out), you will likely want a save where your character is NOT in an HQ cell.
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