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Scrapping Mods and Performance Issues

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What are precombines and why do scrap mods hurt performance?
If you have ever used any sort of settlement scrapping mod you've run into an issue with something called precombines (a special sort of combined mesh new to Fallout 4) and therefore had a reduction in your game performance as a result. Precombines were designed by Bethesda to improve performance when rendering 3d items. To see additional aspects of the performance system designed for 3d items see Snarkywriter's thread for details.

Technical explanation of precombines:
"Precombination works by analyzing ref 3D for a cell and merging like geometries across similarly located refs into a different set of NIFs. For example if you had 3 refs to RockWithLichen and 1 ref to RockWithNoLichen tucked together in the corner of a cell, precombination would merge all 4 rock geometries together and all 3 lichen geometries together into a single NIF named for the combination key for that corner of the cell (eg: "Meshes\PreCombined\0000DBBB_01B8656E_OC.NIF"). The cell would store the form IDs for those combined refs so it would know not to load 3D for those NIFs in game since their 3D will already be baked into the combined NIFs." - Ziegfelding (Bethesda staff)

ALL scrap mods that allow you to scrap items not normally scrappable in the vanilla workshop mode that scrap a mesh which is part of a precombined mesh will cause this problem of reduced performance. This great video explains precombines in a fast, user friendly way and I highly recommend it.

Use of the ini setting bUseCombinedObjects=0
The only reason to use this setting ever is if you don't mind a significant performance drop throughout your entire game in order to be able to scrap items at non vanilla locations (whether you're using extended settlement mods, mod created settlements, or a system like Chesko's Camping mod). This is never recommended for low to medium PC hardware. Generally speaking it's just not a good idea to use this setting in general for any reason. Many users of Scrap Everything see it listed at the top of their description page and mistakenly think it's required to use but IT IS NOT REQUIRED FOR SCRAP EVERYTHING.

Why do scrap mods create weird visual effects?
In addition to the performance loss that happens when you break precombines, there is a secondary problem that scrapping certain items may also cause which is breaking previs. For the sake of performance the game's engine reads a special type of data (previs) that was generated with the Creation Kit which says if you're looking at a large item, items that are visually behind it and therefore not visible won't actually be rendered, therefore saving on the cost of rendering additional items. You may have heard of similar terminology related to previs such as preculling, occlusion, or occlusion culling.

Technical explanation of previs:
"Previs is precomputed visibility data and it is calculated using the precombined NIFs. It builds a visibility graph of which non-mobile uncombined refs and combined geometries are visible from different points which we then use in game for preculling the scene. This data is stored for a 3x3 collection of cells in the UVD files (eg: "Vis\Fallout4.esm\0000DBB9.UVD")." - Ziegfelding (Bethesda staff)

Lets say for example that you scrap a large item that normally blocks your view of any items behind it in game. Some examples are the tall hedges in Sanctuary, the movie screen at Starlight Drive-In, and walls without windows or openings. If you delete those items you can experience visual glitches that can fade in and out as you change your camera angle.

Additional problems with using scrap mods
There are problems even beyond the performance drops that come with using scrap mods. Using Sanctuary as an example, if you use a scrap mod to remove the hedges, the building pads (left over after normal vanilla scrapping of destroyed buildings), or the non-destroyed buildings you end up with significant breaks in the navmesh of a settlement. Removing the building pads will result in a section of navmesh that floats above the ground. Removing things like the hedges or regular buildings will result in dead zones where NPCs can't walk at all. Even placing new flooring with navmesh in those locations may not work.

Extended info on Navmesh:
Navmesh is what tells NPCs where they can walk. For some reason Bethesda navmeshed the rooftops of the houses in Sanctuary which is why you can sometimes find your settlers or brahmin up there. When you first load the settlement into memory the game engine randomly assigns your settlers to a navmeshed location.

When Bethesda marked objects as scrappable in vanilla workshop mode they're given a special tag that disables the navmesh underneath that object until it is scrapped. This way you can scrap a car and afterwards NPC's would be able to walk where it used to be. Scrap mods don't have a way to add new navmesh under the items that were not originally meant to be scrapped.

Comments on "safe" scrap mods (will not break precombines):
People usually want to know what mods are "safe" to use. If you want a mod that is safe from harming your performance there isn't one see below.

Raze My Settlement by Whisper (Bionicyardiff) is a SAFE scrap mod that automatically scraps an entire settlement for you. It's safe because it will not scrap any items in a precombine. This means however that it also won't scrap things like leaf and trash piles that most users want to remove. This is available for both PC and Xbox.

Scrap That Settlement (base) by 3lric is an Xbox only mod.

Details on which scrap mods to use (that WILL break precombines):
Spring Cleaning was abandoned more than 2 years ago. It also has some technical issues such as workarounds for vanilla game bugs that no longer exist that make it less than ideal. The team working on Scrap Everything are extremely well versed in the precombines/previs system. While their mod allows you to break that system if you chose to anyway, they're also ideally suited for fixing it when it breaks beyond acceptable levels such as problems that were found in Vault 88 or the Mechanist's Lair.

To my knowledge most scrap mods work the same way as Scrap Everything and are therefore as safe to use as Scrap Everything is (safe meaning it won't break your game, not safe if your goal is to not break precombines). Some mods use alternative ways of doing things such as by scripting and I don't have sufficient technical knowledge of those mods to determine their safe usage (other than Raze My Settlement which is safe on every front).

Of the two most popular precombine breaking mods (Spring Cleaning/Scrap Everything) I recommend using Scrap Everything as long as you don't change the ini setting (bUseCombinedObjects) from default. It is very common for people to misinterpret the statement at the top of the Scrap Everything page about that ini setting. If you are only scrapping in vanilla settlements DO NOT CHANGE THAT SETTING.

For even more technical details, particularly details meant for mod authors please see my older thread on this topic located on Nexus: Fallout 4 Optimization and Performance Systems Explained
 
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