SkyDome's 3 Dimensional Illusion

The SkyDome projection screen is visually engaging & IMMERSIVE

because it's BIG & it's circular.

That's unusual in itself, and enough to catch the eye.

But there's more.

Even though the SkyDome is concave, it creates a convincing 3 dimensional ILLUSION.

A planet, slowly revolving, looks SPHERICAL. You can even see this in the video below.

The illusion doesn't require any special equipment. It works for most people under a variety of viewing angles and distances - in fact, viewing from farther away seems to help set the illusion going. Like all other dimensional illusions, people whose eyesight depends primarily on only one eye are usually left out. :(

We've been thinking about why the illusion appears.

Here's our working explanations, not mutally exclusive, and none probably entirely the sole explanation...

a) Humans are conditioned to expect to see "roundness". Look around right now: are there more things in your surroundings that are convex vs things that are concave?

b) Certain classes of very common objects ALWAYS present as convex: faces and planets among them. In fact, we find that projecting rotating planets on the SkyDome is one of its best and most reliable illusions. Faces are another interesting class, and there's an entire class of theatrical "haunted house" illusions based on the "hollow face" illusion.

c) Projections in slow movement or rotation seem to work better than still.

d) Compared to an ordinary flat projection screen, all parts (center vs edges) of the SkyDome screen are more closely in simultaneous focus because they're at closer to the same distance from the projector.

e) Compared to an ordinary flat projection screen, all parts of the SkyDome screen are more equally illuminated because they're at closer to the same distance from the projector.

f) Compared to an ordinary flat projection screen, ray tracing reveals more light is returned from the SkyDome screen's center region to the viewer than the edges. This is a common perceived appearence when light reflects off convex surfaces like balls, fruit, or bubbles.