Striking perspective shift illusion

I’ve just stumbled across a remarkably simple yet fiendishly effective visual illusion that seems to give flat images the illusions of 3D depth. It works by quickly shifting between two images of the same scene taken from slightly different perspectives.

As we’ve noted when discussing other illusions, our brain generates the experience of seeing a 3D world by making the best it can out of the two fairly poor quality flat images that fall on the back of the retina.

Visual illusions usually have their effect by taking advantage of the brain’s processes for inferring visual features.

These processes are really nothing more than educated guesses, and illusions essentially give the brain red herrings, misleading it to guess in the wrong direction, so we experience one visual feature in a context where it wouldn’t normally appear.

In this case, the illusions give the misleading impression of depth in the context of an entirely flat image.

One clue the brain uses to infer depth is occlusion – things that are near the front of a visual scene will block those at the back. They just get in the way.

In fact, the lines in cartoons or diagrams are often just a representation of the occlusion contours – the edges of where one surface hide another.

However, although we can see that a cartoon drawing of a head is a 3D representation, we don’t experience it as actually having depth. It doesn’t seem to stand out from the page.

The perspective shift illusion flips between two images taken from slightly different perspectives and this seems to add a dynamic aspect – the illusion of movement.

Movement is often crucial for determing something’s depth. Have you ever moved your head side-to-side when you see a puzzling sculpture to try and better understand its shape?

This allows us to see the relative depth of each of the occlusion contours by experiencing how much the foreground moves in relation to the background. Things nearer the front seem to ‘move quicker’ – something known as motion parallax.

I’m guessing by adding an impression of movement – some motion parallax – to a detailed photograph that already has other depth cues from the natural environment, the perspective shift illusion produces an impression of real depth.

If you want to know more about the cognitive science of 3D shape perception, there’s a great review article by available Dr James Todd available online as a pdf file.

Otherwise, just spend a few minutes checking out the impressive visual effect.

Link to perspective shift visual illusion.