Although this article is primarily interested in the three-dimensional illusion created in lenticular printing, these same principles apply to other methods for creating 3D images.

As human beings we perceive objects in our world not just in relation to each other horizontally or vertically, but with depth as well. The main reason for this is that we have binocular vision, or in other words, we look at objects with two eyes.

Binocular vision occurs primarily in predatory animals, and is thought to help them judge the distance to their prey. Many herbivores, consequently, have eyes spaced far apart, or on opposite sides of their head. This allows them to have a field of view that is much wider, and therefore they are able to spot potential predators easier.

You can see the evidence of binocular vision for your self, simply by looking at an object and individually closing each eye. The open eye can still see the object. What's more important is that you will notice that each eye sees the object from a slightly different angle. This is because the amount of space between your eyes creates a different angle for each eye to see the object from. The difference in these points of view is called binocular disparity. The brain takes the image viewed from each eye and uses binocular disparity to give the perception of depth.

So in order to see the illusion of depth in a flat image, each eye needs to see a slightly different image. In a lenticular print, this is accomplished by vertical lenticules. Each eye sees a different image because each is looking at the image from a different angle.

This is not the only way we perceive depth. If we see a person and a house printed at roughly the same size, we will perceive the house to be farther because we know the size relationship between people and buildings.

Color can also play a factor in depth perception. Saturated, bright colors tend to appear to come forward while darker colors tend to recede. As objects move farther into the distance, they tend to have less saturated colors as well.

The most dramatic three-dimensional images utilize a mixture of many types of depth cues to convey the illusion of depth in flat images.