Friday, 22 April 2016

Gestalt - Gestalt Keys (Primary Projection)

For those new to gestalt and gestalt keys please read the introductory post (HERE) and the essential principles of gestalt keys (HERE) before proceeding.

Primary Projection (PP), or Primary Extension is one of the tougher measurement challenges.  All the structures that influence PP are fluid.  I have discussed the limitations of primary projection as a measurement tool in some detail HERE.  But despite those limitations it is still possible to measure PP accurately and consistently within certain, tight constraints.

A Question of Axes
If we accept that primary projection only makes sense as a measurement on an orthographic projection we are already part of the way to identifying a proper method for measuring PP from digital photos.  As two-dimensional projections go digital photographs may not be perfectly orthographic but they often approach it closely.  For the first do we make our bird sit in perfect side profile along our measurement axis - the X axis?

X Axis Control
We actually have a very useful and reasonably reliable way of judging if a bird is sitting in perfect side profile.  The answer is contained in a bird's wings and tail.

This little exercise reveals how primary projection should be approached from a digital image.  By viewing a bird from a slight elevation (or with it's body tilted towards the camera) to just about reveal the top of the far wing, we have a ready made formula not just for controlling the X axis, but also for gaining a perfect view of all the salient features.  Okay so this is not always 100% reliable.  A bird can hold it's wings slightly askew relative to one another.  Or the tail may have a jagged end to it.  We do however have a number of points of reference here to help make a sound judgement, including the tips of each of the tertials, primaries and tail feathers.

It is also true to say that by shifting elevation, effectively raising the camera above zero mark on the Y axis, we are introducing an angle of perspective.  This will introduce some foreshortening on the Y and Z axes, throwing our measurements out slightly in both.  However, in this case we are only interested in the preservation of orthographic (or parallel) measurements in the X axis.  PP measurements should be unaffected.

As a rough stab at PP measurement we have probably done enough already, simply by confirming X axis alignment.  Provided the structures of the wing are aligned reasonably naturally it should be possible to take a pretty good PP measurement.  But if we want a more solid measurement, plus perhaps some additional confirmatory clues, I recommend going a step further. 

Y and Z Axis Control
This is where things get tricky and we start to look towards a Gestalt Key solution.  While we may have cracked the X axis, locking down the bird's body and overall wing orientation.  We haven't yet dealt with the position of individual feather tracts of the wing.  An individual bird may hold its tertials slightly drooped or in varied alignment.  Secondaries and primaries may be clumped or fanned in a variety of positions.  To the eye, looking at a bird in side profile (even slightly elevated) it may be difficult to tell if the primaries are resting on the side of the rump or crossed over the top.  All these points have a significant bearing on PP.

I started approaching this piece of the puzzle by assessing the relative position of the joints of the wing in the hopes of finding a few suitable surface points to lock onto.  Unfortunately this seems to have led to a dead end.  There doesn't seem to be any way of accurately linking the position of the bones and joints of the wing to surface features.  Time for a simpler approach.  Having established that the far wing represents the locus for the X Axis, we only really need to be certain of the position of the base and tip of the tertials and assert some control over the primary tips and we should have our solution.

Accurate PP measurement is undoubtedly tricky but not impossible.  The wing itself plays a useful role in confirming that a bird has been photographed in side profile.  After that its a matter of constraining the wing structures.  That's where the concept of a Gestalt Key comes into play.  Having an overlay map as it were we can introduce useful additional morphological landmarks such as the approximate position of emarginations on the various primaries.  No tool of course is ever 100% fool-proof.  Feather wear, misalignment, moult, including growing or dropped feathers and individual variation all have a say.  But I hope that tools like this give a new perspective on an old subject.  And who knows there might even be a few new tricks and discoveries to be made with the help of Gestalt Keys.

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