The basic functionality of “display relevant into on windshield” is nothing novel — cars have had heads-up displays for decades now, all working on the same core concept. What shifts Apple’s implementation from HUD to AR is the intention of placing virtual, projected images over real-world elements. Basically, imagine if every speed limit sign were to suddenly become one of those radar scanners that shows your actual speed compared to the limit.

The company has filed a patent, spotted by The Drive, to use car windshields as augmented reality displays. But the way Apple has decided to integrate AR tech is a decidedly tricky one — less obtrusive if done correctly, but difficult (and likely expensive) to get right.

This is relatively easy to do in an augmented-reality headset, but far harder with a full windshield. The issue is parallax — drivers of unequal heights will see the same objects through different parts of the same windshield, making it difficult to determine where on the windshield to project display elements. Let me explain with a picture: