Why icons are needed in combat flight simulators

By cos / VLeLv Icebreakers

UPDATE 22.2.2017: Found 2 more sources for eye angular resolution, updated calculations based on it. 0.3 -> 0.6 arc-minutes.

In recent years it’s become a trend to force icons off in many multiplayer flight simulator arenas. The icons are said to be unrealistic and not needed for realistic simulation. As a real life pilot I have noticed that in real life you can see an aircraft several kilometers away and see it’s color, attitude and larger details. In flight simulators the same is not possible - you just see a pixel or few.

Seeing your opponent as soon as possible is paramount in air to air combat. I decided to study the issue and found out that icons must be used if you want to fly realistic air combat. Icons are a compromise caused by limitations in current display technology. You can fly without icons, but then the visual detection and identification ranges are much shorter than in real life.

Resolution comparison

Human eye is not a digital camera. It is analog device and has various smart features. The eyes move constantly and paint the image in our brain. Regardless of this, we can calculate a resolution for eyes for comparison.

According to [1], human eyes have FOV of about 200 degrees horizontal and 130-135 vertical.

There are several different values for the visual angle or one “pixel” in human eye.
According to [2] it's 0.3 arc-minutes. According to  [7] it's 0.6 arc-minutes.
According to [8] (page 16 table) it's 0.63.

Let's assume the correct value is 0.6 arc-minutes which is 0.01 degrees. If we limit our study to a more moderate usable FOV of 120 degrees in both directions, the human eye resolution is 144 megapixels [3].

A average modern monitor has resolution of 2560*1200 pixels. This is about 3 megapixels. The FOV is usually about 75 degrees in a flight simulator.

We can calculate how many pixels we would need in a monitor to match the eye angular resolution for 75 degree image: 7500*7500. That would be 56 megapixels.

One more geometry exercise: How many “pixels” wide is an aircraft with wingspan of 15m at 10km distance?

On a 2560*1200 monitor with 75 degrees horizontal FOV one pixel represents 0.0293 degrees [4].

The aircraft is 0.0859 degrees wide. This gives 8.6 “pixels” with human eye [5]. You should be able to see its orientation and larger details quite well.

With a monitor, this would give about 2.9 pixels [6]. You might be able to see aircraft color, but not orientation or other details. Even noticing the aircraft against terrain would be difficult.

In summary:

	Human eye	Typical monitor
Horizontal FOV (degrees)	200 (120 used in calculations)	75
Resolution (pixels)	7500*7500	2560*1200
Megapixels	144	3
Angle of one pixel (degrees)	0.01	0.0293
Size of 15m object at 10km (pixels)	8.6	2.9

With first generation VR headsets (OR CV1, HTC Vive, etc) the resolution issue is even worse - they typically have resolutions of about 1200*1200 which is less than half of the typical monitor used here. FOV is luckily better (110 degrees on both).

Peripheral motion detection

In addition to resolution one important factor is the peripheral motion detection. Eyes and brain are able to detect targets moving against background outside the high-resolution central vision area. When flying, this is very important as it allows noticing other aircraft  when looking elsewhere.

It's difficult to measure any significant numbers on the motion detection so it's excluded in this study. I also didn't find any data on if or how well this works on monitor compared to real life. At least the FOV is significantly smaller on monitor which naturally has an effect on target detection.

Suggestions for simulator developers

As the chapters above explains, icons are mandatory for realistic air combat in visual range. Many simulators implement icons in sub-standard way, and they often either unrealistic (too long range / visible through cockpit) and ugly (line of text).

Here’s my suggestion:

Use these ranges as baseline. Make sure that in multiplayer situation all players have the same settings forced. Players should have option to turn off icons, if they want.

Range	Icon
5km	Detect small aircraft
3km	Show aircraft type and friend or foe
1km	Show any other details as needed (weapon loadout, etc)

• Make the icons look good. Use alpha blending to gradually display them.
• Darkness, clouds, fog and sun glare should decrease the distances.
• Icon must not be shown if the target is behind obstacle (mountain) or in other way not visible from cockpit.
• Use visual means to display distance (a bar or arc instead of text). Distance is difficult to judge visually so this doesn’t have to be very accurate.
• Keeping the target in view for longer period can increase the distances. For example you might be able to detect a target or identify it's type if you look at it for several seconds.
• It is more difficult to notice targets against ground than against sky. Reduce the distances, if the target background is ground.
• Large aircraft should be detected and identified from longer distance.

For an example of well working icons, take a look at World War 2 Online. See videos of the simulator to see how the icons work.

Thanks for reading!

[1] https://en.wikipedia.org/wiki/Human_eye#Field_of_view

[2] http://www.clarkvision.com/articles/eye-resolution.html

[3] (120 / 0.01)^2

[4] 75 / 2560

[5] 0.0859 / 0.01

[6] 0.0859 / 0.0293

[7] https://en.wikipedia.org/wiki/Visual_acuity#Physiology

[8] http://www.icoph.org/dynamic/attachments/resources/icovisualacuity1984.pdf