Saturday 16 April 2016

Gestalt - Gestalt Keys (The Principles)

For those new to gestalt and gestalt keys please read the introductory post HERE before proceeding.

A Secure Solution
By now hopefully you will have had time to digest this concept which I have been developing and you can see it's purpose and potential.  The principal by which gestalt keys work is much the same as the pin tumbler lock and key.  In order for a door key to work the little serrated hollows or divots on the edge of key lift tiny key pins in the lock mechanism as the key is inserted.  Provided these pins all then align along the shear line the key can be turned to open the latch and unlock the door.  The security of any lock depends on the quality and sophistication of it's design.  I am hoping to apply these same principals to the design of effective gestalt keys.

At the moment birders already use a range of gestalt measurement tools to aid an identification.  While these tools no doubt are rooted in sound logic they all tend to lack a secure means for ensuring they are applied correctly at all times.  If we take any one of these tools, e.g. Primary Projection (PP), and analyse it's limitations as a measurement tool (as I have done HERE) the lack of rigor applied surely equates in security terms to having no lock on a front door.  The correct application of PP becomes a matter for trial and error and debate, and rightly so.

Orientation And Perspective
In the posting on lighting and perspective (HERE) I outlined one of the important distinctions between human vision and the camera.  The world as we perceive it is actually a distorted reality.  Our brain assimilates the slightly different perspectives of both our eyes to create one image and one perspective.  Not only does the brain have to balance two slightly different viewing angles but it also plays around a bit with normal perspective lines in the process.  The result is an image which never quite matches reality.  Of course the camera image isn't totally accurate either.  As light passes through a camera lens it is distorted in order to focus detail onto the small, flat surface of the sensor. This is especially true of smaller focal length and fish-eye lenses, which attempt to gather a much wider angle of view than telephoto lenses.
So how should we best present our subject for gestalt measurement?  In order to preserve scale and therefore accurate measurement we must minimize image distortion.  We therefore must design Gestalt Keys so that they approach perfect axial orthographic (or parallel) projections of our subjects.  Generally, a perfect axial side profile is what we are after.

Camera images will always contain some element of both perspective and lens distortion.  Images of birds captured at long range with a long telephoto lens help minimize perspective distortion.  Furthermore, modern DSLR cameras typically carry out image processing to correct for known lens distortion.  So, many images do approach an orthographic projection in their preservation of scale.

Digiscoping continues to be very popular, particularly as phone cameras start to produce images on a par with compact digiscoping cameras.  While the image produced by the scope or binoculars may be reasonably orthographic, thanks to very long focal lengths and high quality lens components.  It is worth bearing in mind that the short focal length of the compact camera or phone camera may give rise to lens distortion of the image beyond that point.  For more on perspective and image distortion see for instance the postings HERE and HERE.

One would hope that all gestalt tools currently in common usage are originally based on sound principles requiring preserving scale, using a proper axial profile and taking account of a bird's natural posture.  On the other hand, it can be extremely difficult to gauge a perfect side profile on a bird be it in the field or even in the hand.  In some cases, a tool might have been developed based on a small sample size.  Or, perhaps some flawed logic was applied.  For instance, if measurements were taken by manipulating feathers to align perfectly along a straight ruler, in a manner that doesn't match how these features align naturally in the field, then such measurements clearly would not be readily applicable to ID of a bird observed (or from images captured) in the field.

'Alternative' Gestalt Keys
Having already established that gestalt measurements tend to have been made based on perfect side orthographic projections this does not exclude the possibility of generating 'alternative' gestalt keys from different perspectives.  It is always tempting to try and gauge a gestalt measurement such as primary projection from other viewing angles.  We may not always be fortunate enough to capture an image in side profile.  For instance, in the images below the angle of view provides for a reasonably good view of all the relevant structures but clearly foreshortening is at play so the standard primary projection measurement will not apply.

The advantage of gestalt keys is that they apply rigid principles so that a measurement can be replicated consistently across many images.  One could develop multiple gestalt keys for the same identification problem but using different perspectives such as side, front, rear, plan and any angle in between.  Accuracy and reliability comes down to an appropriate selection of loci.  The loci used for locking down a rear profile view will differ from those used for a side view.  Needless to say the actual measurements from alternative gestalt keys will differ from the original, standard side profile key.

Hard Versus Soft Loci
Bareparts, including structures of the bill and legs have the potential to make for the best loci.  Obviously given that these are reasonably solid structures, with minimal moving parts there is a good chance of using them to lock down image orientation.  This is especially true of the bill as it is typically aligned with perfect symmetry along the Y axis of the head, i.e. it's side profile.  So when it comes to the features of the head the bill is certainly a 'go to' locus.  Having said that, we may require the resolution of some fairly minute structures of the bill to ensure that the bill is actually axially aligned.  A combination of the bill width at the base, nares (nostril) and cutting edge of the bill should normally suffice to align a bill correctly.  In the cormorant example above we don't have the benefit of a nares (as Cormorant's don't have one).  Instead the culminicorn, the bony plate encasing the culmen on top of the bill, provides an additional locus to lock down any lateral rotation of the bill and head (around the X axis).  While the overall proportions of the various structures should hopefully take care of longitudinal rotation (around the Y axis).  Of course we have intra-specific variation to consider.  So, once again development of an effective key will require some trial and error.

Soft loci are far more challenging.  In future posts I will be tackling primary projection among other challenges.  For many gestalt tools we compare the relative position of feather tips.  Clearly feathers and the tracts they lie in are highly dynamic.  Determining appropriate loci for these soft structures will be a challenge but I am confident solutions can be found.  I expect it will take a good deal of experimentation, trial and error to find the right matches.

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