Saturday, 11 July 2015

Human Bias - Tonal Gradient Illusions

In recent postings on this blog I have been interested in the causes and control of tonal variation.  What really started this thread was a look at Grey Scales and Gulls.  Aside entirely from the complexity of trying to accurately capture and reproduce exact tones, we also have various different elements impacting on the uniformity with which tones are distributed across an image.  

The properties of light
We know that the angle of a surface relative to the light source impacts on surface illuminance as explained by Lambert's Cosine Law.  But the lighting in a scene comes from multiple directions.  On a bright sunny day the lighting in an image is often overwhelmed by uni-directional sunlight.  In the shadows we may detect a bluish tone and this is due to blue light from the sky dome.  On an overcast day the sun's influence is diminished.  Sunlight is scattered by cloud and arrives at the ground from all directions as diffuse light.

The properties of surfaces and perspective
Surfaces are rarely uniform and may consist of a combination of specular highlights and more diffusely-lit areas.  Surfaces may also vary in terms of reflectance and absorption of light.  But even if objects are considered virtually identical and are carefully aligned in front of the camera, they rarely if ever look identical in the final image. Unless a surface is perfectly Lambertian light will not be reflected evenly in all directions, and so the angle of view of the camera relative to each subject comes into play. Tones vary slightly even between identical subjects, simply because of perspective.

The properties of the lens
To further compound the problem we have artefacts introduced by the camera including the 'Cosine Fourth' Law of Illumination Falloff and other forms of vignetting.

The properties of the human visual system
Last but not least we turn to the human visual system.  Consider the following tonal optical illusion.  

If we are interested in studying and comparing tones across an image this type of illusion is worrying.  While many optical illusions might not appear to have practical consequences I think this illusion is particularly relevant to birders.  The three elements that make up this illusion are the presence of a tonal gradient, a regular pattern of objects and a contrasting white or bright background.  As birders we encounter these three elements together in almost every one of our field experiences.  Lets replace trees with feathers and repeat the experiment.

It is sometimes stated that the Checker Shadow Illusion and similar computer generated graphics eg. used by the Lottolab Studio could not be replicated in nature because lighting and surfaces are more variable in nature than in the optically perfect, Lambertian, theoretical computer-generated world.  I am not so sure personally.  I think these illusions probably happen all the time but are simply harder to detect in nature than in these computer-generated examples.

For more see HERE.

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