Wednesday 6 April 2016

Gestalt - The Limitations of Structural Angles

As all the postings in this series have shown, gestalt or 'jizz' (also spelt J.I.S.S.) is a particularly difficult thing to measure.  In many ways, the strong proponents of a 'jizz-based' approach to identification have been dealt the short straw as it were.  Gestalt is exceptionally difficult to define, to describe and to nail down in terms of scientific measurement.  

Many of the postings under the subject banner Gestalt have contained the words "the limitations of...".  I honestly set out studying this branch of the blog with the belief that I could tie down gestalt using some of the commonly applied tools such as primary projection, bill to eye ratio, leg proportion analysis, beak structure and shape, comparative morphology, etc.  So far it seems that any real attempt to define the morphology of a bird based on digital images is fraught with difficulties.

There is one popular identification tool that I haven't really given much consideration to before now.  Occasionally one comes across the use of angles evident in bird morphology for identification purposes.  I am  not sure what the generic term is for these angles so I refer to them collectively here as Structural Angles.

Gular Pouch Angle (GPA)
Many experienced birders would probably have heard of GPA or 'Gular Pouch Angle' used in the sub-specific identification of Great Cormorants (Phalacrocorax carbo)  here in Europe including the Atlantic (P. c. carbo) and Continental Cormorant (P. c. sinensis).  

Following my recent successes with 3D modelling for assessing the impact of perspective and angle of view on our subjects, I decided to use this technique once again to test the usefulness of structural angles in bird images.  For this I needed to generate a rudimentary 3D Cormorant.

Based on preliminary assessment the GPA is a useful and seemingly fairly robust feature to assess from photographs.  That doesn't guarantee of course that a bird is assignable to one race or another.  The value of GPA as an ID feature remains under constant scrutiny thanks to the thorny subject of racial hybrids.  For the purpose of this blog however it is interesting to finally find a gestalt-based tool that might actually work quite well.

Can the same be said for other structural angles?  As it happens I am not aware of many other structural angles in regular use and I'd like some feedback if anyone knows of others that might be worthy of analysis.

Loral Angle
Another structural angle that recently came to my attention is the Loral Angle used to assist in separating Dowitcher (Limnodromus) species.  Once again I have taken to generating 3D models to put this one to the test.

Note that in the analysis of Cin-Ty Lee and Andrew Birch (HERE) they observed that the eye in Short-billed Dowitcher is positioned in the head just slightly above the position where it is found typically in Long-billed Dowitcher.  Based on my analysis I am inclined to conclude that the eye in Short-billed Dowitcher should actually be a little more forward in the head in addition to being a little higher, but that's only of minor consequence.  This looks like it might be another really effective tool, but how well does it stand up to some 3D testing?  Specifically how does camera perspective alter the loral angle, with the potential to render a false identification.  In the video below I present some initial findings.

The results of the 3D analysis indicates that the loral angle is heavily influenced by camera perspective.  As the eye in Short-billed Dowitcher appears to be positioned marginally closer to the bill one might expect a discrepancy to occur more frequently while measuring the loral angle of that species.  Worth mentioning of course that, as the authors point out, this feature should only be used in combination with others to form a firm identification.

Structural Angles are an interesting approach to the complex problem of judging gestalt or morphology from photographs.  Foreshortening occurs when three-dimensional objects are projected onto a two-dimensional plane such as a photograph or screen.  This means that only objects which are flat and perfectly parallel with the camera can be measured accurately.  This principal applies equally to the measurement of angles from photographs.  As reliable identification features go GPA appears a bit more robust than Loral Angle but both tools clearly have their limitations.  I believe there may be a slightly more accurate way to approach Structural Angles from photographs.  I will elaborate more in a forthcoming posting.

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