Saturday 26 March 2016

Gestalt - Limitations of CPA and 3D Modelling

In my two most recent postings I have showcased two novel and potentially powerful techniques for exploring the thorny subject of gestalt from digital images.  Both techniques attempt to tease apart questions about the subtle shape or structure of our subjects, but in slightly different ways.   In those postings I merely presented these concepts.  Now it's time to critique them. 

Comparative Photographic Analysis (CPA) techniques use real digital images, presented together perhaps in the form of a collage or photo montage to try and convey some of the overall gestalt characteristics of different candidate species, hopefully making it easier to choose the correct identification for our subject.  But there are potential pitfalls with these methods, some of which I will explore here.

3D modelling takes a rather different approach.  By immersing us in a three dimensional space we might perhaps explore the proportions and dimensions of our subjects from different perspectives.  Once again there are pitfalls but the analysis provides different clues to those obtainable using the CPA technique.  Hence the two techniques are somewhat complementary.

Once again I continue to work on a really challenging subject, that of the Dublin Bay Swift, found by Sture Persson on Christmas Day, 2002.

3D Modelling - The Pros and Cons
While no doubt heavily time-consuming I think the potential for 3D modelling to aid in bird identification is massive.  I have learnt an awful lot about the techniques and the potential short-comings of 3D modelling in just a few short weeks.  And no doubt I have a hell of a lot to learn.

The big advantage with 3D is of course the ability to immerse oneself inside an image and explore detail from any angle.  Working in 3D I guess one begins to appreciate what it's like to pilot a helicopter!  The task of simply trying to locate a point in 3D space requires constant rotation of the model for reference because there are three axes to consider.  Point positioning in 2D is straightforward.  In  3D it seems next to impossible!  One quickly starts to realise a digital image in reality offers a very narrow window onto our subject.   Any assumptions we think we can make about the true shape, size and relative proportions of our subject based on single images are patently false!

But of course the major flaw with the 3D models which I have generated is that they are purely artists impressions.   In order to make truly accurate 3D representations of different birds one would have to either scan subjects in 3D or painstakingly measure and reconstruct them.  For my experiments I didn't set out to prove that the Dublin Bay Swift was one species or another.  This was not a process of elimination but a process of inclusion.  I wanted to explore the possibility that different species could be ruled in based on the evidence and I think I have proved that.

Taking three of the five available images I hope I have shown in the video below that it is just as possible the Dublin Bay Swift could be a White-rumped as a Pacific, based solely on relative proportions, posture and viewing angles.  While one might imagine the rump patch to be overly large and extensive for a White-rumped Swift, the pattern could theoretically be explained entirely by posture and angle relative to the camera.  We don't even have to resort to theories about complex moult or wear to explain the pattern.


CPA, The Pro's and Cons
Having considered the extraordinary power of 3D modelling as illustrated above, surely 2D photo montage is a poor substitute.  There are some similarities between these two techniques.  For instance it was possible to find a number of images of different species taken from very similar angles which hinted at the possibility these birds could be indistinguishable when viewed from certain angles.  Note for example the left hand images of Pacific and White-rumped Swifts below.

3D modelling has allowed me to explore that problem far more closely.  It has 'rescued me' from one of the traps I had fallen into during my analysis of the Dublin Bay Swift.  In the bottom row second and third from left the Dublin Bay Swift is flying towards the camera with what appears to be forward-swept wings.  From such an angle might one risk making the assumption that foreshortening is not such a problem?  The wings appear quite short.  In fact, compared with many similar images of Pacific Swift, the wings might appear too short for Pacific.  Had I found the clue that would clinch the identification of the Dublin Bay Swift?  Sadly it was not to be.  The 3D analysis above confirms that a Pacific Swift could carry it's wings in a manner that would match the Dublin Bay images.  

The best lesson for me from this whole exercise is that foreshortening plays a role in EVERY bird image.

Notably, when one studies the biometrics of these Swifts an interesting pattern emerges.  The body length of many Apus Swifts more or less matches the wing length (measured on the closed wing).  What differs between them is mainly their body and wing proportions, tail length and shape etc, but not their relative body to wing lengths.  Another potential problem with this photo montage method is that there is a high dependency on maintaining a common scale in all these images.  The benchmark which I used was body proportionality.  One of the most puzzling things about the Dublin Bay Swift is it's rather barrel-chested, bull-headed appearance.  While few would probably doubt this bird is either A. caffer or A. pacificus, its body proportions don't seem to match either species perfectly.  This got me thinking about the circumstances of this record.  A powerful aerial insectivorous bird with a high energy intake requirement, in Northern Europe, on Christmas Day!  Surely this bird was barely surviving.  Such a bird might fluff up its body feathers and withdraw its head and wings towards it's centre to try and conserve heat.  On that basis the images chosen below for comparison may not really match the gestalt of a bird lost and struggling to survive!

Perhaps as the arguments above suggest I didn't pick the best subject to showcase two novel techniques for exploring the challenges of gestalt from bird images.  

Or then again, perhaps I did!  I have persevered with the development of tools having resisted the notion that this mystery Swift could not be identified.  Along the way I have been forced to admit defeat.  I now believe it cannot be safely identified with any certainty.  While the circumstances including prolonged southerly winds, the presence of a Common Swift (A.apus) on Guernsey at the same time and the fact the observer described 'a small swift' flying just a couple of meters above head height all tend to support the conclusion the Dublin Bay Swift was a White-rumped Swift.  On the other hand, that niggling rump pattern, the fact pacificus remains the most likely white-rumped species in NW Europe and the potential a cold, lethargic and puffed up pacificus might easily masquerade as a smaller species all creates more than sufficient doubt for acceptance as a 1st national record.  It would be a terrible shame were such a star record to remain shrouded under a permanent 'pend' categorisation.  Provided there is sufficient evidence maybe the best compromise would be to accept it onto the Irish list as a White-rumped/Pacific Swift.

As for these techniques.  The challenge to put a name to this bird has forced me to look far more deeply into the subject of gestalt than I had anticipated when I set out.  And I have developed new skills and insights along the road.  I wouldn't rule out to possibility that these techniques could be used very effectively to aid a tricky identification but at the moment I am coming away from these experiments with the niggling doubt that for every clue these tools might reveal there are probably many more false hopes.  That before one finds the path that leads to a firm identification one must travel down many blind alleys or cul-de-sacs.

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