ALF, it's all about angles and 2 different pictures to compare (different angles give different pictures). Object size doesent really matter - imagine standing 10km away from Mt. Killimandscharo (spelling? LOL) - although it will fill a good proportion of your view you won't see it 3-dimensional - the 2 pictures from your eyes will be (almost) identical (your eyes fixed to "infinity" thus paralell view lines). OTH, imagine a pin 50cm away from your eyes. The stereoscopic effect does not come from seeing different sides of the object - but acutally seeing it in different positions (close one eye after the other). We don't see our world (above 10 metres or so) 3-dimensional in terms of stereoscopic but using clues we learned: e.g. 2 cars --> the smaller one is farther (if we ID them as aprox. same model), tree blocking LOS to a house --> tree is in front of house. That can cause confusion sometimes when things don't look like we learned em - extensively used in art as so called optical illusions (term?): 2 identically tall people drawn near 2 oblique lines --> our brains "thinks" perspective: we see the 2 people differ in size (try to find the picture).
A good hint are stereoscopic glasses for computer games. Let's take "MIG alley" for expample (one of the few sims the glasses really work worth playing). In 3D mode you get the impression looking into a box (monitor) with toy planes flying around because you get a (neat) 3D effect but it's not how we see the outside world thus the "box-impression".
Or real life example: ReVi gunsight in (WW2) fighters. The sight is reflected in a way that your eyes fix to infinity --> that trick makes it possible to aim at planes correctly in the first place. Iron sights do not work because your eyes near fixes on it - or infinity fixes on the target --> either one not seen sharply.
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Oedipus is right - in IL2 limited (but historical) setup up no icons work - in AH it wouldn't, unless we'd get a historical setup. In IL2 standard missions (especially VEF, to an extend IOW) you're right that the formation you run in is highly probable to be enemy but there is a online arena called DiD (Dead is Dead) which features a huge battlefield with lost of AI - there you cannot be sure if the formation you run in is enemy or not. And still icons are OFF and it works. And even when dogfights are hot, with communication and custom skins maybe, you can coordinate quite well. I've never experienced friendly kill 'till now (collision yes - when one gets tgt fixated) - that might be connected to the fact that I fly a mission format that sponsors fly to LIVE but still much less friendly fire than killshooter in AH!