L4D Level Design/Nav Meshes

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Next we'll create a Navigation Mesh, or "Nav Mesh", for our level. The Nav Mesh represents the "walkable areas" of a map. This allows the Survivor Bots and Infected to "know" how to move around in the environment, and tells the AI 'director' where it can spawn infected.

Create tutorial04.vmf and compile

In Hammer, open the map file tutorial03.vmf that you created in the previous tutorial (if it's not already open). This should be located in the tutorial mapsrc folder, by default:

C:\Program Files\Steam\steamapps\common\left 4 dead\sdk_content\mapsrc\

Now we'll make a new version to work with and compile it:

  • Go to File menu and choose Save As...
  • Change the name to tutorial04.vmf.
  • Press F9 to open the Run Map dialog and click on the OK button to compile it.

Getting Started

When the level loads:

  1. As before, we'll get a "Map is unplayable" dialog because we don't yet have a Nav Mesh. Press Continue to dismiss the dialog.
  2. Bring down your developer console ` (tilde)
  3. Type "director_stop" and press the Enter key.
    This will stop the AI Director from spawning Infected.
  4. Enter the command "nb_delete_all" and press Enter.
    This will remove any Infected and Survivor bots from the current game.
  5. Enter the command "nav_edit 1" and press Enter.
    This will switch the game to Nav Mesh editing mode.
  6. Hide the console again with `.

Generating a new Nav Mesh

Placing a nav_mark_walkable.

With the console closed, look at the floor in the middle of the hallway between the two rooms.

Bring down the console and type "nav_mark_walkable" and press Enter.

Close the console and you should see a pyramid-like shape on the ground where you are pointing.

You have just placed a nav generation marker called a "nav_mark_walkable".

The nav areas automatically generated.

Bring down your console again and type "nav_generate_incremental" and press Enter.

This will generate a set of nav areas around the nav_mark_walkable marker. Nav mesh areas appear as series of connected square blocks. It should generate throughout your entire two room level.

Note:If you have a bigger level, you will need to place more of the nav_mark_walkable markers throughout your level. Each nav_mark_walkable has a radius that it creates areas around. In general, build nav areas for one section at a time to keep the nav mesh clean.
Tip:The nav_generate_incremental automatically saves the Nav Mesh into you map when it is run. If you want to save your progress manually, use can use the "nav_save" command.

Nav Area selection

After a nav_generate_incremental is run, all of the new nav areas it creates stay selected. This comes in handy if you make a mistake and want to run the generation again. To delete the selected nav, enter the command "nav_delete" into the console and it will delete all nav areas that are selected. If you try this, you can recreate the nav areas by simply dropping another nav_mark_walkable marker and running nav_generate_incremental again.

  • To deselect nav, type "nav_clear_selected_set".
  • To select a nav area, look at the area with your cursor and type "nav_toggle_in_selected_set". This will select the nav area you are pointing at.

Nav Config files

As you can tell, editing the Nav Mesh uses a lot of console commands. The most effective way to edit, instead of typing a console command each time, is to bind keys to these commands to use as shortcuts.

A config (.cfg) file has been included that makes it easier to start with.

Bring down your console and type:

bind PGDN "exec nav_mode"

and press Enter.

Now pressing the Page Down key will toggle between "Nav Edit" mode and "Game" mode.

Binding your own keys (advanced)

To set your own keys using the bind command, bring down your console and type:

bind <key> "console command"

For example, if you want to bind the Z key to "nav_toggle_in_selected_set", type this in your console:

bind z "nav_toggle_in_selected_set"

You may want to open the left4dead/cfg/nav_mode.cfg file in a text editor to add or change key bindings.

  • For more information on binding commands to keys, see Bind.
Note:From now on, this tutorial will assume you are using the default nav_mode.cfg. If you're using your own bindings, you'll have to substitute your own key shortcuts for the one given.

Splitting Nav Areas

You will also notice that the Nav Mesh in your level currently has large areas that cover the entire room. Sometimes it's necessary to split those large areas into smaller ones. For example, if you want to add an attribute like where the survivors start and you want the area to be right in front of the weapons table.

You can split up nav areas by using "nav_split".

The white line previews the split that you can make.

In the included nav_mode.cfg file, nav_split is bound to the Insert key.

As you point at the nav areas, you will see a white line that moves either North/South or East/West with your cursor. Position the cursor so that a line draws to the right of the weapon table in the first room you created.

This line shows where the split will happen if you use nav_split.

Performing a nav_split.

Press Insert to perform the split.

Splitting the nav area in the other direction.

Now, position the cursor so that the new area you just split in front of the weapon table is split in half in the other direction.

Press Insert to perform the split and see the result.

Adding Attributes

Nav areas may contain attributes that designate specific purposes. For example, a nav area marked with the attribute "EMPTY" means that a common infected cannot spawn on it. Let's say that you have a kitchen with a counter in it, and you don't want infected to be standing on the counter top when the player gets there because it looks weird. You can mark this area as "EMPTY".

First, enter the "z_debug 1" command to allow the attributes to be viewed.

In our first nav mesh that we're creating, we need to designate where the Survivors are going to start when they appear in the map. Let's mark the area in front of the weapon table that we just split as "PLAYER_START"

Point your cursor at the area in front of the weapon table and use "nav_toggle_in_selected_set" by pressing the Z key.

Selecting a nav area with nav_toggle_in_selected_set.

Now bring down your console and type "mark PLAYER_START" and press Enter.

Marking a nav area with the PLAYER_START attribute.

This will turn the selected area into a Player Start position for the survivors. It should turn a purple color.

Tip:if you mark an area with an attribute and you want to remove it later, select the area and use "clear_attribute <attribute name>" in the console to remove it. For example: clear_attribute PLAYER_START

Left 4 Dead requires certain nav areas in its levels for the Director to work properly. For this simple test level, we will use another attribute called "FINALE".

Walk down to the second room in this level and select the area that occupies by looking at it and pressing the Z key.

Selecting the nav area in the second room.

Bring down the console and type "mark FINALE" and press Enter.

Marking the nav area with the FINALE attribute.

This will turn this room into a finale area and it will turn blue.

Now, the level has all it needs for the Director to work properly. The Director can create a "flow" from the starting point to the destination point.

To make the level a little more playable, let's mark the areas around the Player Start area as "EMPTY" so that the player won't get mobbed right after jumping into the map.

Select all the areas around the Player Start with the Z key.

Selecting the nav areas in the first room except the PLAYER_START area.

Bring down the console and type "mark EMPTY" and press Enter.

Marking the selected areas with the EMPTY attribute.

This will prevent "wandering" common infected from appearing in these areas when the map starts.

Note:It will not prevent mobs from spawning in these areas later. You will need to add the attribute "NO_MOBS" for that.

Save the nav mesh by using "nav_save".

Deleting nav areas

If you make a mistake or if you find that the auto generation of the nav mesh has mistakes, you can delete nav areas with the command "nav_delete". For example, the weapons table doesn't actually need a nav area on top of it. It may cause survivors to jump up onto it, which might not be desirable.

In the config file provided, this command is already bound to the Delete key.

To delete a nav area, point at it, select it with the Z key. Make sure you only have the areas you want to delete selected, then press Delete.

Analyzing the Nav

Your first nav mesh in the game.

Now that we have a basic nav mesh, we need the game to analyze it so that the director knows where it can spawn infected.

Bring down the console and type "nav_analyze" and press Enter.

This might take a few seconds and then the level will reload.

To test spawning infected, use "director_start" in the console and reload the level with "map tutorial04".

Your scene should look something like this now.

Adding Nav to a Ladder

If you open up tutorial03 in the game, and switch to nav_edit 1, you'll see how a ladder looks with nav placed on it.

Creating nav on a ladder.

To create nav on a func_ladder, position your cursor over it in nav_edit mode and use "nav_build_ladder".

Note:The cursor will turn a green color if it is positioned over a func_ladder.

Connecting Nav Areas

While a lot of the nav areas will be connected properly when you use nav_generate_incremental, there are some that you might want to tweak or change. For example, the catwalk we created in tutorial03 might not have connected properly to the areas below it.

Connecting nav areas.

To connect 2 nav areas, select them and use "nav_connect" in the console. The End key is currently bound to this in the nav_mode.cfg file.

This will connect the areas in both directions and form a light blue line as seen in this image. Infected can climb up (they can climb to any area 180 units or lower) and they can drop down. Survivor bots can also drop down.

To connect 2 areas one-way only, you will need to select the one you want to be able to connect to the other and then point to the second with the cursor but don't select it. Then, press End to use "nav_connect".

Make sure in both cases that the two areas you want to connect are not layered on top of one another. This will cause infected to bump their heads when they try to climb up.

Drawing your own Nav areas

Sometimes the nav_generate_incremental leaves out areas that you want to have nav on. In these cases, you will need to draw your own nav area.

Drawing nav areas is done with 2 keys. One key is bound to "nav_begin_area" and other bound to "nav_end_area". The nav.cfg file has nav_begin_area bound to the left mouse button and "nav_end_area" bound to the right mouse button.

Point to the area you want to create an area and click the left mouse button. As you move the mouse around, you can see where the nav area will draw until you press the right mouse button to end drawing and create the area.

Drawing a nav area.

Click the right mouse to activate nav_end_area and see the result.

Note:Creating your own nav areas will not automatically connect them to the rest of the nav mesh. Use nav_connect to connect them to adjacent areas.
Tip:If you accidentally press the left mouse button when you don't want to draw a nav area, just press the left mouse button again and it will cancel the nav creation.

Grid snapping when drawing nav

You might notice that the cursor doesn't snap to a grid when you're in this mode. You can set the snapping properties using "nav_snap_to_grid" in the console.

Tip:In the nav.cfg, the F9 key is bound to "incrementvar nav_snap_to_grid 0 2 1". This will change the grid size when you press the bound key and continue to toggle through the sizes each time you press the F9 key.

It is also possible to draw a ladder using nav_begin_area and nav_end area. Simply point at a corner of the ladder and move the cursor vertically to go to the opposite corner.

L4D Level Design Basics Tutorial
L4D Level Design/Visibility
L4D Level Design/Checkpoints