UE4 Open World AI Navigation

And how did we get here

Recast by Mikko Mononen

Best thing that happened to game AI navigation since sliced bread!• Lightweight, fast and open-source! • De facto industry standard• Resurrected and available on github

https://github.com/memononen/recastnavigation

UE4 Navigation parts

• Navigation bounds volumes• Navigation relevant actors – geometry & other data (links, areas)

• Navigation Octree– stores navigation-relevant data– speeds up gathering relevant data for

specific navmesh tile’s generation

So what’s the problem?

This doesn’t scale all that well

GDC Demo Map, 256 km2

Naive approach

Navigation Octree ~10.3GB

NavMesh ~500MB

Plus ~30 minute navmesh generation (8 core machine)And requires loading all levels for navmesh generation purposesThis, of course, is useless.

Why it was so much?

• Empty NavMesh tile - 176 bytes• 10000 tiles, ~50kB each, 16000uu x

16000uu • Every piece of navigation-relevant

data kept in Navigation Octree

Solution ideas

Dump the navigation octree!– Static navmesh of 500MB is acceptable– No work required, supported out of the

box– NO GO, we wanted to show dynamic

navmesh generation!

Solution ideas

Custom Landscape-specific navigation graph–Main navoctree

occupant– NO GO, more then

just lanscapes in the demo

Solution ideas

High level navigation mesh– Huge voxels, very

low precision– Low memory, fast

generation

– NO GO, would require a custom local-navigation solution

No worries!

Low-hanging fruit solution• We don’t need all of navmesh all the time• We need only the nav octree information

required by tiles we need

Applied to UE4 Navigation System

• Navigation bound stay the same• Navigation octree to store only the

data relevant “now”• Introduce a way to communicate to

Navigation System where navmesh is required

Navigation Octree

• Lazy geometry gathering– outside of game thread

• Landscape slicing– never stored in octree– constant cost, outside of game thread

Navigation Invokers

• Internal and external radii• Updated on a regular basis• Only “new” tiles require work

GDC Demo Map, 256 km2

Naive approach At GDC

Navigation Octree ~10.3GB 572MB

NavMesh ~500MB 31MB

The end result still not ideal But we’re not done yet! :D

Future work

• Don’t store anything in nav octree unless it’s necessarry– Currently only geometry is treated the smart way

• Moveable navigation bounds • Give lazily gathered octree data a lifetime

limit• Multithreaded lazy geometry gathering

Hit me up mieszko.zielinski@epicgames.comMieszkoZ