How To Make a Roguelike: #10 Vision and Fog of War

This article is a guest post from Hexworks. The original can be read here.

The last time around we connected the levels of our dungeon effectively adding a new dimension to our game. This is great, but it gets boring fast, as we can see everything! Let’s improve the situation by adding a vision system and Fog of War to our game!

Augmenting our Entities

Let’s think about how a vision system is supposed to work. First of all each Entity that has vision should have some kind of Attribute which holds their sight radius. There should also be some operation which determines what our Entity can see around it. Finally I think we can agree that most creatures don’t have roentgen sight, so some other Attribute which tells us whether something blocks vision or not would also be useful. Let’s see how we can go about implementing this!

The first thing to add is (you guessed right) an Attribute that we can use to hold the vision radius of our entities:

package com.example.cavesofzircon.attributes

import org.hexworks.amethyst.api.base.BaseAttribute

data class Vision(val radius: Int) : BaseAttribute()

and a flag Attribute which determines whether an Entity blocks vision or not:

package com.example.cavesofzircon.attributes

import org.hexworks.amethyst.api.base.BaseAttribute

object VisionBlocker : BaseAttribute()

Seems straightforward, yes? Now if we think a bit about this, in our game VisionBlocker belongs to the walls only, and Vision is an attribute of the player since fungi don’t have eyes (yet?). Let’s augment EntityFactory with these:

// edit EntityFactory with these changes

import com.example.cavesofzircon.attributes.Vision
import com.example.cavesofzircon.attributes.VisionBlocker

fun newWall() = newGameEntityOfType(Wall) {
        VisionBlocker   // 1

fun newPlayer() = newGameEntityOfType(Player) {
        EntityActions(Dig::class, Attack::class),
            maxHp = 100,
            attackValue = 10,
            defenseValue = 5
        Vision(9)       // 2
    facets(Movable, CameraMover, StairClimber, StairDescender)

Here we:

  1. Add the flag Attribute to the newWall function
  2. And set our player up with a vision radius of 9. This means that the player can see 9 tiles far around them.

Now let’s add a shorthand extension property to AnyGameEntity to be able to tell whether it blocks vision:

// put these to EntityExtensions.kt

import com.example.cavesofzircon.attributes.VisionBlocker

val AnyGameEntity.blocksVision: Boolean
    get() = this.findAttribute(VisionBlocker::class).isPresent

Now we have the data, but there is no behavior yet. What we want to able to do is to tell whether vision is blocked at a given position or not, and also to be able to draw a circle around an entity highlighting the visible tiles. The logical place to put this is World:

import com.example.cavesofzircon.extensions.blocksVision
import com.example.cavesofzircon.attributes.Vision
import org.hexworks.zircon.api.shape.EllipseFactory
import org.hexworks.zircon.api.shape.LineFactory

fun isVisionBlockedAt(pos: Position3D): Boolean {
    return fetchBlockAt(pos).fold(whenEmpty = { false }, whenPresent = {    // 1
        it.entities.any(GameEntity<EntityType>::blocksVision)               // 2

fun findVisiblePositionsFor(entity: GameEntity<EntityType>): Iterable<Position> {
    val centerPos = entity.position.to2DPosition()                  // 3
    return entity.findAttribute(Vision::class).map { (radius) ->    // 4
        EllipseFactory.buildEllipse(                                // 5
            fromPosition = centerPos,
            toPosition = centerPos.withRelativeX(radius).withRelativeY(radius)
            .flatMap { ringPos ->
                val result = mutableListOf<Position>()
                val iter = LineFactory.buildLine(centerPos, ringPos).iterator() // 6
                do {
                    val next =
                } while (iter.hasNext() &&
                    isVisionBlockedAt(Position3D.from2DPosition(next, entity.position.z)).not()
                )                                                               // 7
    }.orElse(listOf())                                                          // 8

EllipseFactory and LineFactory come as part of Zircon but they are abstract enough that we can use them in all kinds of situations like you can see here.

This seems a bit complex so let’s see what happens here!

  1. We use fold which lets us choose what happens when the Maybe is empty and when present.
  2. When present we find out whether any of its entities are blocking vision
  3. Then in findVisiblePositionsFor we start with the position of the Entity
  4. Try to find its Vision Attribute
  5. Create a ring around the Entity using its radius
  6. Then we try to draw a line from the center position to all of the ring positions
  7. And only take the positions from the line while the vision is not blocked at that point. Note that we use do/while here since we need to take one more position after we find a vision-blocking Tile. Otherwise we would not be able to see the walls!
  8. We return an empty list if the Entity has no Vision Attribute

Wait, what is this flatMap thing? What happens here anyway? What you see here is a bit of functional programming. What flatMap does is that it flattens all the collections into one big collection from what we return from the takeWhile. So for example listOf(1, 2).flatMap { listOf(it, it) } will return [1, 1, 2, 3] in contrast to listOf(1, 2).map { listOf(it, it) } which would return [[1, 1], [2, 2]]

Now that we have the means to determine what the player can see, let’s add a Fog of War effect which the player can reveal as they explore the cave!

Adding the Fog of War

Let’s think about how Fog of War works. When we start the game everything is shrouded in darkness and only a small part is visible around the player. As we traverse the cave each step reveals some of the shadows and it will stay visible.

We could have a Field of View as well, but personally I’ve always found it annoying so we’ll stick with Fog of War only for this tutorial.

We’re going to implement it in a very simple way. By default we’ll cover everything with a shroud. Then the world will have a behavior that’ll reveal the shroud using the player’s vision attribute! Let’s start by adding the necessary tile and color:

// add this to GameColors:
val UNREVEALED_COLOR = TileColor.fromString("#090909")

// and this to GameTileRepository:
val UNREVEALED = Tile.newBuilder()
    .withCharacter(' ')

For the FogOfWar itself we’re going to use the TOP tile of our Blocks, so let’s add this to the GameBlock:

init {
    top = GameTileRepository.UNREVEALED

Technically we could make an entity called Shorud and add it to all Blocks but it would create hundreds of thousands of new objects. This is not ideal, so we’ll optimize a bit.

Now if we start the game, we’ll see the map is covered with Fog of War, so our next job is to create a new Behavior that will autonomously reveal the shroud around the player on each update:


import com.example.cavesofzircon.extensions.position
import org.hexworks.amethyst.api.base.BaseBehavior
import org.hexworks.amethyst.api.entity.Entity
import org.hexworks.amethyst.api.entity.EntityType

object FogOfWar : BaseBehavior<GameContext>() {
    override suspend fun update(entity: Entity<EntityType, GameContext>, context: GameContext): Boolean {
        val (world, _, _, player) = context
        world.findVisiblePositionsFor(player).forEach { pos ->
                    x = pos.x,
                    y = pos.y,
                    z = player.position.z
            ).map { block ->
       = GameTileRepository.EMPTY
        return true

How this works is that it simply finds the visible positions around the player and sets the top tile of the block to EMPTY effectively revealing the tile underneath.

Now to make this work we’re going to add this to our EntityTypes:

object FOW : BaseEntityType(
    name = "Fog of War"

and this to EntityFactory:

import com.example.cavesofzircon.attributes.types.FOW

fun newFogOfWar() = newGameEntityOfType(FOW) {

Now this fog of war entity won’t belong to a Block, and it will have no position. In order to see this in our game we have to have a way of adding a world entity to the World:

// add this to World
fun addWorldEntity(entity: Entity<EntityType, GameContext>) {

and initialize it in the GameBuilder:

fun buildGame(): Game {


    val player = addPlayer()


    return Game.create(
        player = player,
        world = world

If you look at this now you’ll still see the blackness. The reason for this is that the Player haven’t moved yet and no update happened to our game world, so the shroud still encompasses the whole map.

An easy solution for this is to simply make an initializing update to our world. For this we can add the update in the PlayView at the end of the init block, we just have to make sure that we use a key that we can’t type:
    screen, KeyboardEvent(
        type = KeyboardEventType.KEY_TYPED,
        key = "",
        code = KeyCode.DEAD_GRAVE
    ), game

Now let’s see what we created:

Revealing the Shroud


In this tutorial we’ve added a very simple vision system to our game which we put to good use by implementing a Fog of War effect. In the next article we’re going to add wandering monsters which will definitely lead to some jump scares!

Until then go forth and kode on!

The code of this article can be found in commit #10.