Wednesday 11 March 2015

Field Marks - False Field Marks

Before I get into this in detail, a quick recap.  Bird identification may be based on a range of factors and field marks are certainly an important consideration.  We can often neatly characterise field marks as being either BOLD or BLAND based on a few characteristics.  These characteristics may include luminance, contrast relative to other surrounding features, the range of tones that go to make up the field mark (i.e. tonal range), and lastly how sharply defined the edges of a field mark happen to be (i.e. it's edge definition or sharpness).

As the analysis so far has shown, bold field marks are robust and remain detectable even in many low quality images.  The characteristics that define bland features however also tend to make them susceptible to being masked or lost as image quality deteriorates, be it through exposure, focus or white balance errors for example.

In addition to our image quality parameters, we also have to consider the overall lighting context and how light interacts with the anatomy of the bird, including translucent as well as opaque features.  In this posting I am going to broadly explore the factors that give rise to false field marks in digital images, hopefully setting the stage for the next few postings in this series.

A Trick of the Light
I have a whole page devoted to birds and light.  I intentionally started this series of postings all about field marks by looking at the interactions between light and avian anatomy.  I chose this as a starting point because lighting is such a fundamental part of this challenge.  Awkward lighting is often seen as a menace for identification but it needn't be that way.  If we have more than one image we normally have sufficient basis to rule out lighting tricks.  

The supra-loral shadow is such a consistent structural feature that is is often listed as a field mark for the identification of Booted Warbler (Iduna caligata), yet it is just in essence a trick of the light which can vanish with a slight movement of the bird's head.

The image above illustrates a few other important points.  The definition of the feathers on the crown and breast of this bird are made possible by shading.  An inexperienced observer might confuse these shadows as being fine streaking but this is one of the very first pitfalls we start to overcome as bird observers.  Shadows can also be mistaken for larger field marks.  The dark spot at the cleft where the wing meets the breast (centre image) or the cleft at the base of the throat (top left image) again might both confuse inexperienced observers.  Again, the more we familiarise ourselves with avian anatomy the less likely we are to be thrown by these features.  The two dark shadows running from the nape onto the breast are again something which an inexperienced observer might mistake for an actual plumage pattern, especially as these shadows are present in all these images.  Again, with experience comes the realisation that this pattern is not found in any species and even a simple analysis of the markings compared across two or more images will reveal that they are not located in the exact same place on the bird in any two images.  Thus, this could only be explained as shadow cast by nearby twigs.  And yet, as experienced observers we all still regularly fall foul of such tricks of the light, both in the field and in photographs.

While shadows tend to be the biggest cause of false field marks, highlights and glare can also introduce an element of confusion.  The subtle reflection of light off the base of the feathers of this Little Stint (Calidris minuta) mirror the pattern of juvenile Semipalmated Sandpiper (Calidris pusilla) to an extent.  Again, an experienced observer might pick up on this false impression based on the lighting of the primary feathers, or at the very least might seek confirmation from another photograph to determine if this is a real effect or merely lighting.

Incidentally some birders might be inclined to refer to this example as a lighting artefact.  I think this use of the word artefact is incorrect for reasons explained HERE.

Lighting and exposure are closely linked.  If lighting confuses the novice then image exposure errors give rise to even greater consternation.  Good exposure control can be key to the accurate capture of field marks in bird images, especially bland field marks.  Incorrect exposure can certainly make the challenge of identification more difficult.  But on the other hand exposure adjustment allows an observer to see beyond the dynamic range of the eye and peer more deeply into shadows and highlights.  So there may be positives to be gained even from under or overexposed images.  The key is to be able to carefully and properly interpret the evidence.

Image Lighting Tools can be used to pull more detail from JPEGs but can rapidly introduce weird and false impressions including the appearance of false field marks.  Take the heavily modified image below for example.  By using lighting adjustments it is possible to work out some additional detail from the shadows but the colours are now wildly incorrect and misleading.  Without due care the ID of this bird can take a rapid turn down the wrong path.  The ideal for image lighting adjustment is to work on maximising image content from RAW files.

Relatively low resolution may not itself introduce false field marks to an image.  There may be a temptation to try and interpret detail that isn't really there.  One of the initial motivations behind the Image Quality Tool was to try and define a point at which pixel resolution really starts to undermine image quality for identification purposes.  Extreme low resolution does introduce artefacts including pixelation, aliasing, moiré and others and these could certainly be mistaken for real field marks.  But, at that point one would have to be pretty foolhardy to try and make definitive identification judgements where fine field marks are concerned.

While it may be highly tempting to say that this Diver (Gavia sp.) has a dark chinstrap can we really be certain or should we err on the side of caution?  At this pixel resolution I think it is just too close to call, especially were we to have just this one image from which to base our conclusions.

There are a variety of different mechanism that affect the sharpness of an image including environmental, lens and in-camera artefacts.  Each of these introduce their own particular suite of anomalies, some of which might give rise the false field marks.  I hope to delve into this in some detail to try and establish some analytical boundaries.

This image of two Magellanic Snipe (Gallinago paraguaiae) has been modified.  The bird in the background is naturally defocused by the camera lens.  The bird in the foreground on the other hand has been artificially defocused using a Gaussian blur tool.  The rear bird appears to show a double eye-ring which is not replicated by simply defocusing the forehand bird.  The double eye-ring effect could be due to motion blur or simply due to imperfect bokeh created by the aperture as light passes through the lens (for more see HERE and HERE).  Clearly, when it comes to focus we have to concede at some point that an image is simply too blurry to be analysed.  The question is, where do we draw the line?

Colour (White Balance)
Colour errors during image processing are a standard problem.  Incorrect white balance will inevitably lead to an incorrect representation of colours.  This is particularly relevant if the colour field mark is bland and if we are trying to sample and read the colour as accurately as possible.  The problem is explored in detail HERE.

Accurate colour management is essential for accurate colour rendition.  In order to be able to meaningfully compare these two Chiffchaff images for example, perhaps with other images taken with different cameras in different locations and settings, all images must be calibrated to a standard.  The standard requires the creation of a DNG profile using an X-rite colorchecker passport and then also using a grey card for white balance correction in the field.  There are also some additional considerations that might require the creation of what I call a Colour Profile (CP) for trickier subjects such as this.  For more see HERE.

Artefacts come hand in hand with reduced image quality and are most likely to be confused with real field marks where the subject is depicted in very low resolution.  The nature of most artefacts is that they are unlikely to be mistaken for a recognised field mark.  But some have characteristics that make them easily confusable with the real thing.  Such as the similarity between sharpening halos and feather fringes or moiré and scalloping.

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