Gameplay Proposals/Visibility

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Author: BTAxis (talk, contrib)

This is a proposal for the implementation of visibility (and lack thereof) in the game.

At the time of writing, a target is visible when:

  • There is a LOS (=Line Of Sight) from an allied actor to the target.
  • The allied actor is facing the target.

I think most people will agree that this is not desirable. Stealth and detection must play a role in tactical combat, especially at night.

Whether an actor spots a target should be a function of two factors: the actor's ability to detect and the targets visibility.


The ability to detect should be fixed for each species (human, Ortnok, Taman, etc), but can be modified by equipment such as IR goggles (better detection) or gas masks (worse detection) or implants. In addition, the (unaided) values should differ depending on whether it is day or night. Here is an example of a detection ability table:

Species Day Night
Human 50 25
Ortnok 60 60
Taman 25 75
Shevaar 50 50
Hovernet 75 75

These values are the base values that apply to the actor's own square. The value should gradually decline with distance from the actor. Say, by 1 per metre.

Note that this might unbalance multiplayer. Perhaps all actors should be given the same values in multiplayer to prevent this.


A target's visibility (though, given the counter-intuitive values in this section, a better term would be "stealth factor") should be influenced by:

  • The lighting of the space the target is on. A square in full daylight will have stealth factor 0, and a square right underneath a street light will have a stealth factor of -10 (since these squares are especially noticeable in the darkness), while a square in a deep shadow at night could have stealth factor 20.
  • The stance of the target. See partial obscurity.
  • Any stealth bonuses the target receives from camouflage or other cloaking methods, such as smoke screens.
  • Aliens could get small stealth bonuses or maluses as a result of the difficulty setting.

If the unit's visibility is equal to or less than the actor's detection value at that range, and the conditions mentioned at the start of this article are met, the target is spotted. Otherwise, the target remains hidden.

Partial obscurity

An issue is partial obscurity. For example, a prone soldier lying on a roof would be hard to spot while a prone soldier in a pit would be really easy to spot.

If either full obscurity or the simpler stealth factors mentioned above result in non-visibility of the target, we're already done. When the target is deemed visible, however, we must do more work.

Option 1: Ray-casting

In total, ten rays must be traced:
- One for either foot of the target
- One for either hand of the target
- One for either hip of the target
- One for either shoulder of the target
- One for the target's torso
- One for the target's head

A ray must be traced from each of these ten points to the eyes of the spotting actor. For every ray that can't be traced, the target receives a stealth bonus, which may or may not bring his stealth factor above the spotting actor's detection ability. If so, the target remains hidden. If not, the target is visible. Note that if the target could not possibly become invisible even with the ray tracing modifiers, the ray tracing step may be skipped as the actor will be visible anyway.

I believe that with a limited amount of rays as described here, we can implement a fast partial obscurity model that is nevertheless accurate enough to be convincing.

Option 2: Image rendering

I believe this is something that Arisian spoke about before, and I may not be exact on the details. But it seems pretty straightforward. It involves rendering an image from the unit's point of view. All non-enemy items are rendered black, all enemy units are rendered white. Then it's just a matter of comparing how many pixels are rendered white. If enough are rendered white the enemy unit is considered visible.

In addition, a second image could be rendered with proper colors to test camoflauge. These images could be compared to see which areas of the second image represent the enemy unit. Then some calculations could be done to see if the enemy unit's colors match the colors around him. If so, then his visibility decreases.


This is an attempt to organize the proposal here and several of the suggestions from the talk page into a staged timeline for development. Since visibility is such a large and complex system, this roadmap attempts to lay out a series of intermediate systems on the way to a more complete system. These steps would be more easily implemented and would allow us to move forward without committing to such a major project in one release.

Mark I

Implement reduced visibility based on distance, time of day and race attributes (listed above). A unit's probability to see another unit will reduce depending on how far away from the unit he is, whether it is day or night and his race's visibility attribute. Player should receive some visual feedback on which tiles are visibile and which tiles are not. Very easy to implement and will function as expected in most situations. But it does not take into account lights on night-time maps or the benefits of hiding part of an alien's body behind an object.

Changes to team_*.ufo files to add race sight attributes
Adjustments to visibility code to take into account these factors
Basic UI display of tiles visible. This could be as simple as a green circle displayed on each visible tile when the player clicks a button. It could be a cloud-like particle that is put over tiles the player can not see.

Mark II

Implement support for adjustments to visibility based on movement and adjustments to visibility during reaction fire. When a unit is moving, his visibility should increase dramatically. Moving while crouched should still increase visibility, but less than moving while standing. A unit should only be able to perform reaction fire if he has direct visibility of the unit. Visibility should not be shared across the team in this case (not sure if it is at the moment, but this is responding to a comment on the talk page from BTAxis).

Adjustments to visibility code based on movement
Adjustments to reaction fire code

Mark III

Implement support for smoke grenades, visible weapons and enhanced UI visibility display. A soldier's probability to see an alien will reduce if smoke from a smoke grenade exists between the soldier and the alien. If a unit fires a highly visible weapon (flamethrower), he should beome more visible. Any areas not visible at the beginning of the map should be covered in black. When the player has seen an area it becomes visible, but areas the player has revealed but can not yet see should display with some kind of cloudy or grey overlay indicating the player can not currently see them.

Adjustments to LOS code to take into account whether or not the ray cast passes through smoke.
Particle effect for smoke.
A way for smoke to grow/fade over time.
Adjustments to weapons.ufo to indicate how much a weapon show "expose" a unit when fired.
Ability to calculate whether an area should be unexplored/explored/visible.
Some way to make the transition between the unexplored/explored/visible look nice.

Mark IV

Implement support for adjusting visibility based on the unit's lit state. If a unit is standing in the light, he should be more visible than if he is standing in the dark. This would supercede the visibility calculations based on time of day of the map.

Option 1: Rendering engine must be able to apply lights from all sources (light entities, lights created from surface textures during rendering time, sunlight) and a unit's visibility should be based on that.
Option 2: The game compiles information about each grid tile's visibility during compilation (like the pathfinding, this is embedded in the .bsp). The game can then determine a soldier's visibility based on the grid tile they are in.

Mark V

Implement partial visibility based on ray-casting or image rendering. Units with parts of their bodies behind cover should be less visibile. For full details, see Partial Visibility above.

Depends on whether ray-casting or image rendering is implemented.


There is already CM_GetVisibility. This function is not fully implemented yet.