Pictorial depth cues constantly surround us, yet the human brain is so adept at making these instantaneous calculations and assumptions that we hardly realize we deciphered potentially random visual data at all. It’s only when these depth cues are misaligned or incomplete (inadvertently or not) does our perception take a step back and need to be reprocessed.
Animators, graphic/motion designers and filmmakers need a solid command of identifying the most common depth cues to allow their work to ‘read’ and be understood as quickly as possible– and with the least amount of viewer confusion. Most of us are unconsciously aware of these cues, but to quantize and analyze these visual phenomena will help use utilize them more effectively, and even break them for effect when desired. Let’s take a look!
This shouldn’t be confused with volumetric lights that exhibit participating media in a small area – such as God Rays through a window or a flashlight beam on a foggy night. The underlying concepts are similar, but volumetric lighting does not give an indication of distance.
Overlap and Occlusion
However, this information only allows the observer to create a ranking of relative nearness – it can’t be used to determine actual size discrepancy.
This concept eluded artists through the Middle Ages until 1435 when Leon Battista Alberti provided the first theory of accurately reproducing linear perspective in his book On Painting.
A good example is how the rails on railroad tracks appear to get closer together in the distance.
The general rule is the closer to the level of the horizon line, usually near the center of the frame or field of view, the father away the object appears. Of course this is dependent on a traditional composition – scenes with no horizon line (such as airplanes in flight) or objects that do not adhere to the ground plane would render this cue potentially invalid.
We assume that in the two similar objects, the smaller objects (as it appears on the retina or camera recording device) appears to be more distant.
Shadowing & Shading
The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects, provide an effective cue for the brain to determine the shape of objects and their position in space.
Nearby things pass quickly, while far off objects appear stationary as they move at a much slower rate. When driving, consider the moon never seems to move at all or actually seems to be following you.
Textures and Details
There are normally three types of texture gradient cues that can be analyzed: Perspective (the width of texture elements decrease with distance), Compression (texture elements will shrink in height as the distance increases), and Density (the number of elements per unit area will increase with density).
For example, on a long gravel road, the gravel near the observer can be clearly seen of shape, size and color. In the distance, the road’s texture cannot be clearly differentiated.
Depth of Field / Defocus
The depth of field does not abruptly change from sharp to unsharp, but instead occurs as a gradual transition. In fact, everything immediately in front of or in back of the DOF convergence point begin to lose sharpness — even if not perceived by our eyes or the camera. Contrast decreases as defocus increases, contributing to the effect.
Because this article focuses on the pictorial aspects of depth cues, we’ve left out a few of the stereoscopic clues that would only exist in the real world; cues that would not exist on a two-dimensional image plane. These additional cues would include Accommodation (the brain processing the tension in the muscles that adjust the lens in the eye) and Convergence (the eyes moving slightly inward), and others.
Having a broad understanding of the types of depth cues will greatly increase the readability and instant viewer comprehension of your pictorial images – whether they are posters for print or animated footage.