From Valve Developer Community
A Navigation Mesh represents the "walkable areas" of a map. This data is required by Bots and Hostages in Counter-Strike: Source and Counter-Strike: Global Offensive, AI Bots and the Horseless Headless Horsemann in Team Fortress 2, and Bots and Infected in Left 4 Dead and Left 4 Dead 2, allowing them to "know" how to move around in the environment.
Navigation Mesh data is stored in a NAV file corresponding to the map file (.bsp) for which it is used. For example, the nav mesh for
cstrike\maps\de_dust.bsp is stored in
To support the many community-created maps, CS:S and CS:GO include an automatic mesh generation system. The first time you attempt to play a custom map with bots, the generation system will build a
.nav file for that map. Depending on the size and complexity of the map, this may take a few minutes to a few hours. Most maps take about 5 minutes to auto-generate a .nav file. Once the generation is complete, a .nav file is saved to your hard drive for future use.
For games that don't support automatic mesh generation, a navigation mesh can still be generated by a game client using the console's map command to load a map, then using sv_cheats 1, followed by nav_generate. You can then upload the created .nav file to your server.
It is recommended that map creators create navigation meshes before releasing their map to the public.
What is computed during generation
The following steps occur during Navigation Mesh generation:
- Starting at a player spawn point, walkable space is sampled by "flood-filling" outwards from that spot, searching for adjacent walkable points
- Rectangular Navigation Areas are constructed from the sampled data
- Hiding and Sniper spots are computed
- Encounter Spots and Approach Points are computed (this can take awhile)
- Initial Encounter areas are computed
Manually starting the generation process
To start the generation process manually, type
nav_generate from the console.
To see the resulting Navigation Mesh, type
nav_edit 1. To turn the mesh editor off, type
nav_edit 0. You will need to have
sv_cheats 1 to be able to view the navigation mesh.
Explicitly marking walkable areas (i.e: "There's no mesh upstairs")
Some maps that have very steep stairs, ramps, or hills may confuse the generation system, resulting in the Navigation Mesh not covering the entire map. To solve this, you can manually place additional walkable markers that tell the generator to search outwards from that point during its sampling phase.
To add one or more walkable markers, first aim your view towards a known walkable spot in your map, then type
nav_mark_walkable in the console. When you are finished placing markers, type
nav_generate to re-generate the mesh.
You can remove all previously placed walkable markers using the
Quickly generating meshes for testing
The analysis phase of the generator can be quite time consuming. To skip it, type
nav_quicksave 1 in the console.
- Note:"Quicksaving" skips the analysis phase, and the resultant nav mesh will be missing important data required by the CS Bots and Hostages higher-level planning and attention systems. While useful for testing, editing, and tuning a nav mesh, be sure to do a full analyze (ie:
nav_quicksave 0) before publishing your nav file to others.
However, careful hand-tuning of the Navigation Mesh can markedly improve the performance of Bots. In addition, manually decorating the mesh with Place Names allows the Bots to tell others where events occur, enhances the Counter-Strike radar and includes Place Names in all radio messages.
To enable the editor, type
nav_edit 1 in the console (type
nav_edit 0 to disable it).
- The white cross in the center of your view is the cursor.
- The Navigation Area you are pointing at is the selected area and is shown with a yellow outline.
- Navigation Areas that can be reached from the selected area are shown in red.
- The white line in the selected area denotes where the area will be split if the
nav_splitcommand is executed.
Using mesh editing commands
Although all of the mesh editing commands can be typed in the console, in practice it is far easier to bind these commands to keys. For example,
bind del nav_delete binds the delete key to the command nav_delete. After this binding, pressing the delete key will delete the nav area under your cursor (if you are nav editing mode).
You can temporarily bind many commands at once by using a CFG file. This is useful for bot navigation editing, because there are many commands, and you will probably not want them to be permanently attached to your keys. See Editing Bot Navigation CFG File for a pre-made file and instructions on its use.
See Navigation Mesh Console Command Reference for a list of all commands related to bot navigation.
Basic mesh editing commands
Currently, there is no "undo" command in the Navigation Mesh editing system. Therefore, it is very important to save your work often via the
To split an area into two adjacent, inter-connected areas, aim your cursor at an area and execute the
To merge two adjacent areas:
- Aim your cursor at the first area and execute
- Aim your cursor at the second area and execute
Note that both areas must be the same size along the edge to be merged.
The resulting area will retain all of the connections of the original two areas, and inherit properties (i.e: Place Names, etc) from the marked area.
Splicing creates a new area between two existing areas. Splicing is a powerful tool, and can be used to easily create sloped areas, as shown in the example below.
To splice a new area in between two existing areas:
- Aim your cursor at the first area and execute
- Aim your cursor at the second area and execute
To delete an area, aim your cursor at it and execute
Creating a new area
To manually create a new area:
- Place your cursor at where a corner of the new area should be
- Move your cursor to drag out the area's extent
Adding and removing connections between areas
The connections between areas define whether an agent can move from one area to another. Usually these connections are bidirectional, meaning agents can move back and forth between two areas at will. However, sometimes the connection is one way, as in the case of high ledges that can be jumped down, but not back up.
To make a one-way connection from area A to area B (meaning you can move from A to B):
- Place your cursor on area A
- Mark area A with
- Aim your cursor at area B
To make a bidirectional connection, repeat the above steps reversing the roles of A and B.
To remove all connections from areas A and B:
- Aim at area A
- Mark area A with
- Aim at area B
Adding place names
Once you've generated a nav mesh, Place Names can be assigned to nav areas to label that region of the map. For instance, if there is a prominent house in the map, all of the nav areas in the house can be tagged with the "House" Place Name.
Place Names are used by the CS Bot chatter system to announce where they are, by the radar to show the current location of the player, and they are appended to the player's name when that player uses text chat or radio commands. For example:
ChicagoTed @ Apartment (RADIO): Fire in the hole!
To add or change the name of a location, you need to turn on the place name editor. If you haven't already done so, you'll want to first make sure
nav_edit is set to "1", then type
nav_toggle_place_mode. When the place name editor is enabled (assuming no areas have been named) all the nav squares should turn red.
To select which Place Name you want to assign to an area, type
nav_use_place (name), for example
nav_use_place Bedroom. Next, aim your cursor at an area you want to label with that Place Name and turn on "Place Painting Mode" by executing
nav_toggle_place_painting. While "place painting" is active, each area you point at will be tagged with the current Place Name you selected and will turn green in color. You can change the place name you are painting with by typing
nav_use_place (newname), notice that after changing the place name, all areas already painted with a different name will turn blue and areas painted with the name you just changed to will turn green. When you are finished using "Place Painting Mode", you can exit by executing
For a list of all available Place Names, execute
nav_place_list. Or, if you are working on a CSS, or CS:GO map, you can find a list of available names in the following articles: Standard Place Names for CSS, Standard Place Names for CS:GO .
There are several additional commands that provide tools for picking, flood-filling, replacing, and so on. For a complete reference, see Place Name Commands.
Common mesh problems and solutions
Extraneous areas on stairs and ramps
The automatic generation system tends to build areas that hang off the side of stairs and ramps. These should be removed to force the bots to take the stairs or ramp straight-on.
Railings can be a problem for the current bot navigation system, this includes jump-down links that must pass over railings, they should be avoided unless necessary. The bots will eventually make these jumps, but they are not terribly elegant while doing so.
Preventing excessive jumping
Mark an area with
nav_mark and use the
nav_no_jump command to flag the area to discourage jumping.
Dealing with rotating doors
Clip off the nav area to leave an empty space in the nav mesh to allow for the rotating door. You may have to remake the nav square if deleting part of an existing one doesn't work.
Bots will sometimes try to walk through the opened door brush from one nav area to another and get stuck. Either removing the adjacent nav area or configuring the areas and connections so the bots have to walk around the opened door should prevent this.
Advanced editing techniques
Ladders can be manually built against climbable surfaces. Your editing cursor will turn green when pointing at a climbable surface.
The easy way to build a ladder is to simply point at a climbable surface, and execute
nav_build_ladder isn't what you need, you can manually create a ladder:
- Place your cursor at where a corner of the new ladder should be
- Move your cursor to drag out the ladder's extent
This can be useful to get bots to start going up the ladder, and jump off halfway up, onto the crate:
Sometimes a ladder will be created facing the wrong way. If a ladder is drawn in red, you are looking at it from the back. The ladder is drawn in green if it is facing you. To change the orientation of a ladder, simply point at the ladder and use
Obstacle avoidance and precision movement
nav_stop to disable local obstacle avoidance behaviors and control bot movement.
Marking areas to avoid
nav_avoid to tell bots to avoid the area unless the normal route has become too dangerous.
Controlling where bots Can hide
nav_no_hide to prevent bots from hiding in that area.
This can be used to force the creation of hiding spots during the analysis phase.
Setting up sniper spots
Mark an area using
nav_walk and execute
nav_make_sniper_spots. This will break the area into small sub-areas which are used internally by the navigation mesh to encourage sniping.
Lining up tricky jumps and narrow doorways
Arranging nav areas to force specific paths.
- If the bots are getting hung up on the sides of a door or window, you can carve out a narrow path through the middle of the opening.
- The highest a bot will try to jump is 58 units. If the top of a jump is 59 or more units above the bottom, bots will give up as soon as they jump. If this happens in a spot that the bots can actually jump up to, use
nav_corner_lowerto lower the upper nav area slightly.
Useful console commands
bind mouse4 noclip bind mouse5 nav_toggle_place_painting mat_fullbright 1 r_drawentities 0 nav_select_radius 9000 nav_draw_limit 100 nav_area_bgcolor 0 0 0 255
- Navigation Mesh Console Command Reference
- Left 4 Dead Navigation Meshes
- L4D Level Design/Nav Meshes
- L4D Level Design/Nav Flow
- L4D Level Design/Advanced Nav Editing