Saturday 24 September 2016

Field Marks - Grey Scales and Gulls (Part 5)

In this series of postings I have been concerned with trying to replicate the famous (among gull enthusiasts) Kodak Grey Scale in sRGB colour space.  The intention has been to try and directly measure gull upper-wing and mantle tones from digital images, in a manner consistent with studies using an actual Kodak Grey Scale card alongside a gull in the hand.  Why?  Because, upperparts tone can be instructive in gull identification, and if reliable measurements can be taken from digital images it will help in some gull identifications. To date I have written four other blog postings on the subject, parts OneTwoThree and Four.  In the most recent posting I took a conceptual look under the hood as it were, focusing on the various parameters that together explain the non-linearity of tonality in digital images.  

For starters, human perception of brightness is non-linear (covered by the luminosity function).  Next we have gamma - a non-linear function applied to images to cater for the non-linear properties of older display monitors.  Lastly we have the characteristic curve, used in photography to make subtle tonal corrections and get the best out of our photographs.  In this posting it's time to get 'down and dirty'.  Would the real Kodak Grey Scale card please stand up?

From left to right, the X-rite (formerly Gretag-Macbeth) Colour Checker Passport is the modern professional photographer's quality control tool for exposure and colour calibration.  I have used it in discussions about colour in this blog.  Centre, the Kodak Grey Scale together with the Colour Separation Guide (not shown) represent the original quality control tool for many photographers.  Though the Kodak tool has tended to be surpassed in more recent times by more robust, all-in-one tools like the X-rite Colourchecker Passport, the Kodak grey scale tool has long been favoured by gull researchers as a tool to aid in the separation of taxa based on mantle shade.  And so it remains.  Lastly to the right I have included a cheap and cheerful, Mudder white balance card, consisting of a white card, an 18% grey card and black card.  Time for a more practical, direct look at the Koday Grey Scale.

Online Resources - sRGB Guideline Values
Having spent a long time trying to obtain appropriate sRGB values for the Kodak Grey Scale, I finally stumbled upon an excellent resource from Berkeley, University of California as outlined in my last instalment on this subject (HERE).  

 The sRGB values certainly appear to replicate Berkeley's high quality copy of the Kodak Grey Scale.  However, in attempting to apply those values in my analysis of gulls, something didn't quite fit.  It proved necessary to darken my gull images before applying the tool.  Considering that I had been able to obtain surprisingly consistent results using just a linear grey scale model, and without having to darken the images drastically to read off the mantle tones, something seemed to be amiss.  Hence the research has continued, and hence I find myself writing yet another chapter on Grey Scales and Gulls.

A Comparison of Multiple Grey Card Captures
For my first experiment with the Kodak Grey Scale card I have simply taken a series of bracketed exposures with my Canon 70D and 300mm lens, then selected the most representative one.  I then took another image of the card with an Iphone 6.  I found it was necessary to adjust the brightness of the Iphone 6 image slightly to obtain a matching exposure (note I used the 18% greycard in both images as a standard exposure reference).  Next I converted both images to greyscale in Adobe Elements before samplling each swatch from each image (using the sampling procedure HERE).  Lastly I compared each image both visually and graphically.

The results showed a clear difference between the camera's in terms of tonality.  What more, both differed markedly from the Berkeley image.  Of the various parameters I had explored in my earlier post only one could account for this vast difference - the camera's Characteristic Curve.

Though it's difficult to make a meaningful comparison between my eye's perception of the tonality of the actual Kodak Grey Scale card and various on-screen depictions of it, I nonetheless gave it a go.  To my eyes the Canon 70D gave the closest match to the actual Kodak Grey Scale card in terms of mid-tones, say from level 2 or 3 to level B.  Whereas the Iphone did a much better job in depicting the highlights and shadows, i.e. levels A - 2 and B - 19.  So I decided to average the Canon 70D results and Iphone results and graph the averages alongside each of the different captured versions.  The resulting compromise certainly has the classic sigmoid or S-shape of a characteristic curve and it looks elegant.  But are we any closer to that elusive ideal sRGB Grey Scale after all of this?

What Next?
 It may be tempting at this point to throw in the towel and say that, as all camera's have differing characteristic curves surely it's impossible to accurately reproduce and measure tones along any comparable scale?  And yet, all the results to date have been surprisingly effective using just a purely linear model (the blue scale in the graph above).  So it's not all doom and gloom.

Once again...more to come.

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