Human Vision & Our Perception of Visual Art

by

or – Is What We Think We See Really There?

I am fascinated by how the eyes and brain work together to create what we call vision. The eyes collect input and send it to the brain for processing. It’s the brain that decides what it is we see (or think we see). The brain makes its decisions based largely on perceived color and contrast data sent to it by the eye’s sensory elements such as cones and rods. Sometimes these decisions don’t match reality (at least as defined objectively if one measured the visible light’s actual physical characteristics at any given moment) – which occasionally give rise to what we know as optical illusions among other things.

A simple photographic example – The brain adjusts automatically for ambient light effects on the colors we think we see. With our camera, we have to compensate manually with White Balance adjustments. Shoot something white in a tungsten light environment with the WB set to daylight and what color does your camera see for white – orange! Ditto if you’re a film shooter and don’t shoot this scene with tungsten film. But – how about your own vision? What color do you see for your white shirt as you move from outdoors to a room lit by tungsten bulbs? It doesn’t change colors; it’s still white! Your brain did the required adjustments without any intervention on your part. You took it for granted even though (unbeknown to you) colors were changing, i.e. the brain’s decision was to alter reality for you. Amazing!

I had written previously about aspects of vision -

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Using what we know about how vision works is useful for painters – they have control over what appears on their canvas via their use of contrast & colors. Photographers don’t start with a blank canvas as does the painter and, further, what  can be done  with colors and contrasts in-camera is limited. The best we can do at the time of image capture is to be aware of possible pitfalls related to color interactions and the use of negative space and, thus, not fall into avoidable traps.

Here is a famous painting by Monet – Impression: Sunrise. It is the work that led to the name Impressionism for this style of painting.

What I plan to do in this series, Human Vision & Our Perception of Visual Art, is explore the possibility of applying techniques used by impressionist painters to “fool” the viewer’s eye. A very specific technique is used in this painting where the actual painting is shown together with one showing only its luminance (lightness). If you’re not familiar with the HSL – hue, saturation, lightness – color model now might be the time to take a look).

In Monet’s painting, note especially the luminance of the sun. It is virtually identical to that of the sky behind it – surprising considering the appearance of the color version. This is one of the impressionist’s techniques that can result in fooling the eye. It’s called equiluminance for obvious reasons. And so – why might equiluminance fool your eye? The human vision system has two functional areas -

  1. The WHAT part which determines
    1. Color and
    2. Form
    3. It uses the H & S parts of the HSL color model; not luminance
  2. The WHERE part which determines
    1. Spatial properties (location, depth)
    2. Motion
    3. It uses the L part of the HSL color model; not H & S

Our vision determines location & motion based on the output of the Where system. Here’s the kicker – the Where system operates entirely on luminance values (color does not play a role; it’s a completely B&W process). Do you see how that might cause some issues in terms of what we think we see? In the extreme, as our eyes scan the painting (the eyes scan continuously), the Where system may make a slightly different decision regarding where the sun is located each time the eye returns to the sun. Repeat this over and over (as we do) and some people (it varies) may see a slight jumping or shimmering of the sun for this reason. This may explain why movement and shimmering light are often words used to describe impressionist paintings. If you’d like to see an interactive display which allows you to vary both the luminance and color of this Monet painting to see the effect click here.

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In subsequent parts of this series, I intend to explore possible ways to use equiluminance in photographs. Obviously this will be done in image processing. The basic idea is simple.

  1. Create a B&W l(lightness) version of a color photograph.
  2. Adjust the luminance to create equiluminant areas where desired
  3. Save this new luminance version as a layer above the original color image
  4. Blend the equiluminant version with the original color version using the Luminosity Blend mode.

The theory of Photoshop’s luminosity blending mode is that it uses lightness values of the “blended layer” (equiluminant layer in our case) and the hue & saturation values of the lower layer (original color image). This is exactly what we want. We’re, in effect, creating an impressionist equiluminant film. The resulting image will be identical to the original except in those places where the luminance was adjusted – or so the theory goes. Will it make any difference in how we perceive our photograph?

Tune in in a day or two and be surprised (or not). I’m not done with the experiment so I can be surprised along with you. In theory this should work, but in practice- who knows?? We’ll find out (once an engineer, techie – always an….). Here is Part 2.

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