As we have two eyes, two different images reach the brain and we see everything doubled except the object we have the focus on, because the eyeballs converge until the two images of the object overlap.
Despite of this, the image of a real HUD must be readable as you are looking at the world outside the cockpit. To achieve that the image is projected as it were placed several meters away from the pilot. This is called light collimation. What happens this way is that the convergence of the eyes while looking at the outside world and while looking at the HUD are the same (well, to be more exact, the eyes are parallel because the outside world objects are far away from the viewer while flying).
In a computer simulator everything is placed over the surface of the display, you don't have to change the convergence of your eyes to look at objects at different distances. It's all in flat 2D.
But, with a 3D display, you are facing the same problem with the HUD than in real life, and when you look at the HUD the world is doubled or, in reverse, when you look at the world the HUD is doubled and this is because the HUD image is very near of your face, just at the HUD glass. The result is that, apart of the difficulty to read flight data, symbols like the velocity vector or target box are not over the corresponding object even if the developer took care of making the HUD simbology conformal with the outside world.
For example, when you look at the runway threshold in a landing you see two velocity vectors at each side of the runway.
To avoid this the HUD in a simulator must be placed several meters away. In the AS F-16 it's placed a bit farther away of the tip of the nose. That creates the fake collimation effect that, when I saw it in 3D for the first time, shocked me because of the realism feel. I couldn't stop thinking "so that's how it would look like to be in front of a real HUD".
They made it this way to more easily keep conformality with the outside world when the user leans his head left/right etc thanks to TrakIR, but unconsciously they create a HUD that looks gorgeous in 3D.
In the middle of my 3D ecstasy, I rushed to the aerosoft forum to make a post to thanks them for that, but passed away unnoticed.
I feel like a spammer when I recreate with so much detail on topics like this, but I love to talk about stereoscopic graphics and force feedback because they add so much to my simulation experience that I want everybody to know about them.
