Tuesday 7 October 2014

Forensics - Shadow Anatomy

The multidimensional shadow

When we look at a bird in the field and we see the shadow which is cast by it, most of us would tend to consider the shadow as a simple, two-dimensional form.  In reality a shadow has a very complex, three-dimensional structure to it and what we see is merely a cross-section of this structure.  Normally in the cross-section there are three, overlapping shadow forms visible - a dark central one and two much paler outer shadows.  In overlap, these three shadows create a complex arrangement consisting of an umbra, two penumbrae and an antumbra, as illustrated below (I used a child's toy as a convenient prop).

The umbra (the dark central shadow) is effectively the sun fully eclipsed by the subject, so if a pinhole camera was in this position the sun would appear fully obscured.  Meanwhile, the two penumbrae are the shadows created by partial eclipses at either side of the subject.  A pinhole camera positioned within either of these shadows would see a sun in partial eclipse - sunlight would be visible from the outermost rim of the sun only (crescent-shaped).  Finally, within the antumbra, all of the sun's rim is visible (ring-shaped).  

Once again, remember we only see a 2D cross-section here.  The penumbrea actually occurs as a cylindrical shadow form around the darker umbra.  Like a slice through a cake, we only see the cylinder in cross-section on the ground.  Of course, as we know, birds perch and fly over complex terrain where the surface is rarely flat.  The cross-section through the shadow matches the complexity of the terrain upon which it falls.     

As the sun drops and the angle of the sunlight relative to the ground becomes narrower, the shadows lengthen and the penumbrae appear to diverge.  The opposite is true as the sun climbs in the sky.  Overhead, at it's zenith, the penumbrea appear to converge with the umbra, leaving one, short, tight, and fairly sharp, dark shadow.  Obviously, this principal only applies to flat, level ground.  The angle of terrain relative to the sun and observer can range from 0 to 360 degrees with all the complex shadow anatomy which that implies.

So where is the value in all of this from a bird identification perspective one might ask?  Shadows play a big role in identification.  I explored this a little HERE.  While HERE I made the important distinction between lighting and image artefacts.  Now that we know that shadows have a very complex anatomy we can anticipate the occasional surprise.  For, instance, the contrasting tones and prominent edges between the umbra and penumbra could be mistaken for a field mark on a bird's plumage.  For this reason it is always worth establishing where the sun is positioned in an image and how shadows are falling.  There might be nearby objects casting shadows on the subject or even a dappled lighting effect resulting in layers of overlapping shadows.  Normally shadow patterns stand out, but occasionally, where shadows align with plumage markings and patterns the situation can be very confusing.  I hope to explore some of these instances in future postings. 

For more see HERE.

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