HUD symbology and reticle adjusts to head position?

I noticed the other day that heads up displays in combat aircraft seem to have an accurate reticle regardless of the pilots head position and angle. It always seems to shoot the right way no matter what I do with my head. (Where as of course if I hold a rifle in real life, and move my head, the rifle's line of fire would no longer match up with where I am looking, which is why you need to line up the two sight markers to aim)

Can someone please explain to me the theory/method of how this works? Does the HUD somehow know which way the pilot is looking, and adjusts the symbology to offset the angle of the pilots head in real time?

The HUD is focused to infinity. And optically it is "far away". Meaning that for the pilot eye focusing on the distant horizon or the HUD is the same.

To compare to your gun analogy, it works the same way a red dot or holographic sight works.

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That makes perfect sense. Thank you :-)

That's what reflector sights with collimating lens were invented for very late in the WWI - to relieve pilots of iron sights alignment problem. What we have today is just a further, fancy development of that idea.