Mono vs Stereoscopic Depth Cues

A few months ago I wrote a post about Stereoscopic Depth Compression, the fact that our binocular vision and ability to judge depth reduces over distance. This phenomenon happens in real life too, and is not restricted to 3D Cinema, although in cinema it can often be exaggerated by using smaller interaxial settings. Due to this effect our three dimensional brain is forced to switch to the classic monoscopic depth cues to gather depth information from the frame. I have heard it said that 90% of a 3D films depth is actually based on the classic 2D depth cues. Cinematographers have had years to develop their methods to portray depth within a 2D cinema frame and as stereographers we should use that as our springboard to adding more depth.

So what are some of monoscopic depth cues? Well, many are based on the effects of perspective which I will not include in this list. So imagine all this was happening on a 2D orthographic level.

Relative Size

We as humans have developed a set of size expectations. A matchbox is small, a human is average and a building is big! So if we see a man standing next to a building, and his head is inline with the top of the building, our first assumption is that the man must be closer to the camera! Obviously our second option is to assume that he is a giant, but first we assume a physical reality exists.

Texture Gradient

If we have a repetitive texture created by objects, or scene elements (bushy landscape, etc) as those objects get further from the camera they will appear smaller, effectively following perspective. If the objects are similar in shape, your brain will assume they are further away.

Colour

Hue – Warmer colours advance & cooler colours recede.

Saturation – As you reduce the saturation of a colour it will appear to recede in your scene.

Occlusion

Its a big word, but its pretty simple. Basically if you see a bush in front of a house, it is occluding your vision of the house, then clearly it is in front of the house. If we were to move the bush so that it is occluded by the house, then your brain tells you it must be behind the house.

Motion

Two forms of motion allow our brains to see or assume depth. The first is based on our Point of View. Imagine you are sitting on a train looking out the window, near objects will be moving faster, or have a greater horizontal parallax, than those on the horizon. This illusion is used a lot in 2D side scrolling animations or games to this day. The second method is based on the objects speed. If we remove the movement from the viewer or camera, as in the first example, and use a static camera we are able to judge distance & scale based on the speed of an object. For example, an airplane in the distance appears to be moving very slowly, vs a airplane that fly’s directly over our head.

Depth of Field

Objects will often soften as they advance or recede with in the focus of a shot. This classic use of camera lensing has become a standard in the language of film, and viewers fully understand the depth cues created by depth of field.

Further Reading:

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