Visibility optimization is used to speed up Fallout 4’s rendering performance. Instead of the rendering subsystem having to determine each piece of geometry individually, the subsystem merges like geometries into precombined meshes. The previs then calculates and builds a graph for the rendering subsystem to use based on those geometries.
A common analogy is a Lego house. All the pieces are individual until they are snapped together to form what looks like a singular piece. Unlike a Lego house, the optimization system is designed to turn off if it detects a change to the cell to avoid errors since a change to a cell would invalidate any meshes anyway. Modders must then rebuild the precombined references and recalculate the previs to optimize their mod and reduce game instability. This presents a load order challenge for players, especially for those on Xbox.
What types of mods are designed to disable the precombines and turn off the optimization?
- Mods that replace trees, buildings, other static objects with Havok® Physics’ collision
- Mods that alter landscape terrain and height replacements
- Mods that contain ‘dirty’ cell edits
- Mods that contain ‘wild’ cell edits
- Mods that promote performance and enhancement by deleting cell references
- Mods from a new modder or porter that do not know that you cannot delete references in BGS games without detrimental effects
- Mods that extend the scrapping functionality of the settlement system
A competently-made mod that adds to — but does not replace, ‘set initially disabled’, delete or undelete — should not turn off the system.
If these types of mods have not rebuilt the optimization system, then they should be considered dangerous and not downloaded. The end-user does not have to say anything to the modder or porter. Simply do not download.
However, many of the newer mods do rebuild the optimization system, and with that comes another problem: Load Order. Once the precombines have been rebuilt, adding another mod below it that disables it will disable it for the load order. Edits from another mod won’t show up if they are above the mod that builds the precombines. It requires careful management and careful modding.
Precombined meshes are very large. A load order with a large amount of cell edits could have anywhere from 150mb to 8 gb of space used. Plan your playthrough and Build your Load Order Framework.
Deleted and “Dirty” References
While not rebuilding the optimization system is considered dangerous, mods that contain deleted references are considered too dangerous for use and should be avoided. However, if the description of a mod says ‘deleted’, it does not necessarily mean that. There is a method to set a reference to ‘initially disabled’ that will not cause CTD upon load when another mod references it.
There are new modders and porters that have not yet learned that it is a best modding practice to clean mods and remove any deletions manually. (Even though cleaning mods has been considered a best modding practice since The Elder Scroll V: Oblivion, there are many contradictory tutorials and sources still available).
If you have a question about a specific mod, and it has a PC version, I can look it up for you and tell you if it has dirty edits. It is preferable that you ask about specific mods on the Parent Thread or Discord. PC-users should use xEdit to view the mod and check.
There is no reason at all to have dirty edits in a mod; but xEdit will not catch ‘wild’ edits so manual cleaning is required after every modding session. Auto Quick Clean will not rebuild the precombines, so the mods that have deleted cell references should be avoided altogether.
Thank you to DAmanding for sharing their method of mod cleaning with me on Discord. It is far more efficient than the way I had been doing it prior to speaking with them.
A Vault Dweller's Load Order Survival Guide: How to Improve Performance by Returning Game Previs to Vanilla
A Vault Dweller's Load Order Survival Guide: Optimization Best Practices