Nearly every single image you see selling you the magic of 3D displays is a lie.

Now, it’s not the same kind of marketing lie that tells you to buy an expensive car to attract women. Conceivably there could be a sexy woman who digs you for your car. Really. It’s happened. Once or twice, maybe.

But 3D Displays literally can’t do what they’re often depicted as doing: namely making 3D images appear in space. They just can’t, outside of science fiction.

Case in point (from the esteemed MIT Technology Review no less — shame shame…)

Here’s a simple rule of thumb. There needs to be a pixel on the display behind every part of the 3D image you see. By "behind," I mean you can draw a straight line from your eye through the virtual object and hit a pixel of the display. If there’s no pixel at the far end of that line or the line hits something else instead of the display, there’s no virtual 3D object in that point in space. Except through gravity, lenses, and mirrors, light does not bend.

So here’s what the phone above would actually look like in reality. The nice glow-y characters are suddenly cut off by the sad, cruel laws of physics. (The white robot’s left arm would also be cut off by the bezel, but you get the idea)

Not so beautiful now, is it? Here are a few more examples from a quick image search:

This example is slightly more passable. Here they’re trying to show you what the guy and his son would see. Including the audience (esp. with realistically goofy glasses) makes a difference in terms of the message conveyed. [The front bumper of the blue car would still be cut off behind the bottom bezel, IMO, but that’s nitpicking.]

Astute readers will tell me that there are volumetric 3D displays that can make stuff appear in air. Yes, in an enclosed volume with spinning mirrors, moving LEDs, or whatnot. But actually what’s happening here is that there’s a display pixel behind the object as required — but the pixel itself may be moving or obscured.

So for the spinning mirror, for a microsecond light reflects from a hidden display into your eye at that precise 3D spot in space. Next microsecond, the light comes from somewhere else as the mirror moves.

Even the old swinging LED trick has a pixel light up in space — it just moves to fast you don’t think of there being a display there, at least not a solid one.

Fog screen? Similar issue, different optics. A traditional projector hits fog or water particles and they refract light from that point in space into your eye. The particles are the pixels in this case, but the light source is remote, often hidden. Also, fog screens suck at 3D because, even though they can do field-sequential active stereo if needed, they can’t restrict the glow to just one 3D point, but rather work like god rays on a cloudy day. That appears a lot like cross-talk, which is the fatal flaw of a lot of TVs on the market.

Parabolic mirrors can make an object seem to float above an opening. That’s pretty cool. But you’re effectively seeing through the hole, light bouncing off mirrors, to a pixel of a display (or a real object hidden inside). Even holograms have this property, that there needs to be a holographic pixel behind (or in front of) the virtual object you want to see, relative to your eye.

The only true exception would be a laser projector that somehow shines into your eye from across the room, and even then this would require some neat tricks to get the right spot on your retina lit up. No one has done this yet, and I recon it would still require a lens near or on your eye.

Even my favorite "VR Contact Lens" idea from way back is essentially a set of pixels, the only difference here is that the pixel is in front and you’re seeing through it.

I predict a big 3D backlash, not just for this, but for the undue crosstalk I’m seeing on the market today. Long-term, though, I’m still bullish on 3D. Just look very carefully before you buy anything.