Practical Application for a Knowledge of Perceptual Mechanisms

I am often asked how an awareness of the our perceptual mechanisms might contribute to a observational realist effort. Let’s look at an issue that some are having in my studio this week regarding our challenging ellipse chart exercise.

Several of my students are currently invested in one of our exercises known as the ellipse chart. This exercise involves the construction of a series of ellipses from a provided schematic (an evolution of ellipse orientations) which is then populated with value from “live” models under a single light source.

Two common problems that are quick to arise during the value phase of the exercise are a problematic exaggeration of the light drop-off on the top plane of the cylinder when that plane is nearing a somewhat perpendicular orientation to the light source as well as the recording of the perceived value “ramping” at certain edges.

I know that some of this sounds can sound a bit dense so I will make an effort to simplify as much as possible. First, in the upright cylinders shown, the elliptical plane on the top of the cylinder is lit evenly. Without going into specifics of light source/surface reflectance/etc., we can say that in this context/orientation we may expect to see some light drop-off (light diminishing on a surface as distance increases from the light source). However I kept the entire top plane a single value. Even so—most can indeed perceive a difference in value from one side to the other (so as you can imagine, even a slight drop-off will amplify this perception.) If an artist adds this perceived disparity to the drawing, the top of the cylinder beings to look “bowed” or bent (turning away from the light.) By understanding how our perception of “lightness” is governed by context (surrounding values) we may better realize why differences like this appear—and thus may make more informed decisions during an observational drawing or painting effort. (Shown above the cylinder on the right is a standard simultaneous brightness/lightness contrast demonstration connected to the areas of the elliptical plane that may be perceived as different due to their context.)

A second common issue in the value stage is the addition of the perceived value “ramping” that may be perceived when two contrasting values meet. When examining the cylinders during the ellipse chart exercise, artists report seeing this ramping along the contour of the ellipse. It is often most salient in the region where a bright light inside the cylinder meets the area of the elliptical plane that is furthest from the light source (region of the right circle in the right image). This value ramping is often described as a Mach band. Named after the nineteenth century physicist, Ernst Mach, who discovered the phenomena, this perceptual effect is a perceived exaggeration of contrast at the edge of regions that differ in lightness or brightness levels. Artists may not realize that they are already quite familiar with this effect as it is the same effect that causes value scales or charts to appear “fluted”.

But if we see things like this should we not draw/paint/record them like this?

It depends on what your goal is of course—but if you would like to have your representation yield a percept that is as similar to a relatively standard percept of the original subject as possible—then no. Here’s why: If I observe an object (A), my brain will generate a percept (A1) (this is the image (percept) in your mind that INCLUDES all of these psychophysical phenomena). Therefore if I paint true to that percept (A1), the viewer of my representation will not “see” A1, but a new percept (A2) (or (A1(artist)+1(viewer)). While this is not “bad” thing in and of itself—the more distant (A1) is from (A), the less “realistic” the resulting effort may appear.

There are many more such effects that come into play for the practicing observational realist, but hopefully the pondering of these two for now (in a practical context) may convince you how helpful these considerations can be.

Happy Painting/Drawing!

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