Wednesday 19 August 2015

Human Bias - Brightness Illusions

Before reading this post please visit Lottolab Studio and have a look at a number of their brightness illusions.

I have already referred elsewhere to luminance and tonal illusions HERE, HERE and HERE among other postings.  I have discussed for instance the Checker shadow and Chubb Illusions.  These both depict lighting illusions (shadows and tonal gradients) in greyscale.  The work of Beau Lotto opened my eyes to related illusions concerning colour hue and tone.  It seems that very similar mechanisms are involved whether an illusion entails simple brightness or colour tones.

What is a colour?
Colour is an entirely human construct.  We are not born with an innate understanding of colour.  Colour is learnt, often quite slowly it seems.  From a basic vivid child's colour pallet we can learn to distinguish and possibly name many thousands of distinct colour hue, saturation and luminance tones.  For survival we have an innate attraction to certain colours in nature and are better able to see some colours more than others.  Meanwhile, physiologically our green colour detectors (cones) outnumber red and blue cones two to one so our colour vision is somewhat biased.

The Eye Versus The Camera
Unlike the camera, the human visual system is truly dynamic.  Perhaps the light sensitive cells of the eyes record light just as simply as the photosites of digital camera sensors but the way our brain then processes this information is far more complex and nuanced than that of the camera processor.  Most in-camera processing involved adjustment of all pixels (global adjustment), though there may be some finer tuning at the broad tonal level (highlights versus mid-tones versus shadows).  The Lottolab examples point to a far more targeted image processing by the brain, involving small (local) parts of an image.

Given this level of complexity human vision is perhaps less helpful for forensic analysis of images than we would like to think, particularly where there is significant individual variation between observers.  The amazing Dress Viral Phenomenon of early 2015 demonstrated just how polarised individual perception of colour and lighting can be.  We also know that a range of other Human Biases can come into play when an observer is requested to describe or recall what they see.

Optical Illusions - Fooled or Not Fooled
I have found myself a little confused by the relevance of some of Lotto's Illusions.  Among the various examples are three shown below.  Clearly these should evoke the same illusion as the Checker Shadow (particularly the top right example).  But in these cases I have failed to see the illusion.  To my eyes the tones involved appear the same whether the images are masked or unmasked.  That said I suspect if I come back to these again in a week or two perhaps I wont be as prepared and the illusion might 'kick in' then.

Where my eyes clearly do deceive me every time however are in Edward H. Adelson's Checker Shadow example and the very similar coloured cube tonal illusions from Beau Lotto.  Both are simply mind-blowing illusions.  Very often when I look at these I experience a transition over a second or two as my eyes, or rather my brain adjusts to the reality that these tiles are in fact identical in tone and therein lies a fundamental truth about human vision.  Our vision is entirely dynamic, designed to cope with an ever-changing world and to quickly adapt both globally and locally to almost any lighting conditions.

Computer Generated Images Versus Real World Images
It could be suggested that computer generated optical illusions are not so easily replicated in the more complex lighting environment of the real world.  It is worth watching Beau Lotto's lectures online to see how he masterfully brings illusions to life using simple colour cards, lights and filters.  So clearly these illusions don't just exist on the computer screen.  However, if these illusions do frequently occur in nature, why don't we experience the kind of 'eureka moment' in everyday nature that we do when the illusions shown above are unveiled to us.  Surely, every once in a while we should find that a certain colour tone on a bird has weirdly morphed before our eyes, revealing something of the illusory nature of our vision systems.  Well perhaps occasionally this does happen.  It is often reported that exceptionally pale birds never photograph properly.  Could it be that our perception of these birds as being exceptionally pale is merely an illusion and that the camera is provides a more accurate representation?  This is something which I have been trying to investigate for some time and hopefully I will be able to write about it in due course.

If these illusions are to be found all the time in nature they must be masterfully hidden for a lot of the time.  With this in mind I set about to find some of these illusions hidden within bird images.  For this experiment I have focused on images showing similar characteristics to the Checker Shadow Illusion.  I have selected birds in partial shade to try and uncover an illusion similar to the ones shown above.  Here is a flavor of what I have uncovered.

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