Friday 20 November 2015

Birding Image Quality Tool - Rev. 3.0 Colour

Having recently 'bolted on' a Lighting Quality Tool to the Image Capture Quality Tool I figured it was as good a time as any to drive on with a third tool, this time devoted to Colour Quality.

As with the Lighting Tool the Colour Tool is in effect a summary of the findings of the Spotlight - On Colour thread and a way of drawing a line under that chapter.  And, as with the other tools the Colour Tool attempts to provide birders with a representative, quantitative tool for analysis for colour quality and accuracy in your bird images.  

Before I start of course I have to point out once again that digital colour is only a representation of natural colour.  Of the potentially infinite array of colours produced by light humans can only perceive a certain colour gamut.  Within this range digital cameras can only capture a much smaller gamut of colours.  Finally, within that cluster of colours we have a smaller gamut again called sRGB colour space.  Most digital imaging devices including cameras, display screens, scanners and printers tend to operate in sRGB for the most part.  sRGB is also the colour gamut used by the internet.  So that is the colour space which I have restricted myself to in this blog.  

The colour parameters which I have selected for the tool all narrow down the accuracy with which colours are captured and selected within this sRGB colour space so as to approximate, as closely as possible, the colours captured in nature.  After all, we can expect no more than this from our camera equipment.

Sensor Calibration
No two cameras are identical.  Due to slight variations in the way individual camera lenses, sensors, filter arrays and processors capture colours every camera is unique.  This is our first stumbling block on the road to 'accurate colour'.  Professionals use a tool called the X-rite (formerly Gretag-Macbeth) Colorchecker Passport to get over this first hurdle.  The Colorchecker is a standard calibration tool.  Having photographed the Colorchecker in RAW, at 100 ISO the photographer uses software to assess the performance of the camera setup.  From this a special colour profile is created (called a DNG Profile) which can then be used to correct for any slight variations in the camera setup when compared with a recognised professional colour calibration standard.  This profile only needs to be created once for a given camera, lens and lighting setup.  Afterwards, any time a RAW file is opened in a RAW work flow the DNG profile can be selected and this will automatically calibrate the colours in the image to that recognised standard.   For anyone interested in bench-marking and analysing colours from their images this tool is a 'must-have'.  For more on DNG profiles see HERE.

White Balance Calibration
In theory a DNG profile should be the only calibration needed to capture colours as accurately as any camera can.  The problem is, as humans we don't see the world quite as it actually is in nature.  Sunlight is ever-changing owing to the position of the sun in the sky.  Humans can correct for this changing light using a white balance adaptation.  We also use this to correct for the unnatural colour of artificial lighting indoors.  Camera manufacturers needless to say aim to produce images which match the world as the human eye sees it so cameras are equipped with white balance correction.  Unfortunately cameras are not as adept at this skill and frequently get this calibration wrong.  The only way to be absolutely certain the camera has corrected white balance approriately is to use another calibration tool called a Grey Card.  White balance correction then becomes the second prerequisite for accurate colour capture.  

Of course white balance correction can be closely approximated, particularly if there is some reasonably neutral grey in an image.  But this approach can be a bit 'hit and miss', particularly if it is being done by eye and particularly if the display screen used is not itself perfectly neutrally calibrated.  I have made an allowance for manual white balance correction with Colour Quality Tool, the caveat being that one would hope the observer is exercising caution and that the correction is reasonably accurate.

The X-write colourchecker comes with a colour grid (shown above) also containing two neutral grey patches.  This panel flips over to reveal a large neutral grey card beneath, so the passport caters for this dual purpose of DNG profiling and white balance correction.  The white balance of this image from the Collins Bird Guide second edition was created very simply by placing the white balance eye-dropper cursor in Adobe Elements over one of the neutral grey squares and clicking the mouse.  Having also corrected the colours using the camera's DNG profile the colours of this image are now matched to a professional standard.  Note how pure and saturated the colours are and how neutrally white the pages appear.  For more on white balance see HERE.

Image Manipulation
The next challenge is that, having corrected colours as accurately as possible the temptation might be to start manipulating an image further to correct for slight lighting, shadow, or exposure issues.  If this is done carefully and with a great deal of attention it is certainly possible to improve an image and draw closer to accurate colour.  But it can just as easily go wrong and lead the observer away from the target objective.  Where the only manipulation of a RAW image is it's DNG profile and white balance correction that is considered a perfectly acceptable manipulation, as this is the minimum correction needed to calibrate colours.

If an image on the other hand requires some additional lighting, hue or saturation manipulation to try and draw out representative colours there is a risk of deviating on quality so the image scores lower in the Quality Tool.  If the image being measured is not a RAW file at all but a Lossless image file format like a PNG file then manipulations are going to result in some clipping of image colour data so once again the quality score is affected even more.  Lastly if the image being manipulated is a lossy file such as a JPEG image then the impact on colour quality is greatest so all forms of manipulation will damage colour accuracy and drive the colour quality score down to its lowest setting.  So to achieve the maximum quality score the goal should be to capture a good quality exposure that requires little or no manipulation other than colour calibration in the RAW workflow.

Lighting Quality
Lighting has a huge impact on colour capture.  Simply take the score obtained using the Lighting Quality tool and apply it here.  The lighting tool captures everything from the quality of scene lighting to lighting direction and shadows on the subject to dynamic range and multiple lighting issues.  The best lighting conditions overall provide the best colour capture.  For more on the Lighting Quality Tool see HERE.

Sample Point Quality
Last but not least we have to consider the quality of the sampling method.  Since coming up with an effective way to sample colour (see HERE) I have completed an analysis including coming up with an effective quality control method for choosing appropriate sample points (HERE).  It simply involves using artificial defocus to test sample homogeneity.

I have also tested the effect of varying image resolution on the effectiveness of the sampling method (HERE).  So far the analysis points to a sampling method that is very robust.

Summary and Conclusions
By gathering together various elements that define accurate colour capture and presenting them here as an Image Quality Tool I can now draw a line under this chapter.  Along the way I have learnt an awful lot about colour in birds and the processes involved in human and digital imaging.  No doubt I will continue to add further insights to this thread and may update the quality tool along the way.

Important to stress that the calibrations referred to here manipulate the image content to reveal accurate colours.  These individual colour pixel contains an RGB value and these values can be identified using any standard photo editing software (eg. MS Paint).   However there is one more calibration required to view your images properly.   Correct visual presentation of colour on a screen or printer depends on the quality of that device and its accurate calibration.  Obviously if you decide to bring your images to a lab to have them printed you are relying on the lab to have properly calibrated its printer.  If you decide to print them at home using a high quality inkjet printer at least you have ultimate control over its calibration.  

Having calibrated and sampled the colours of an image the obvious next step is to put a name to each colour.  You could simply decide to name the colours subjectively.  However, after having gone to so much trouble to calibrate your images at every step why leave the final stage to chance.  Through the blog I have developed the Birder's Colour Pallet as a standard reference tool for colour nomenclature in the sRGB colour space.  Using RGB values it is possible to assign a name using a scientifically repeatable methodology to any colour in your image.  The real beauty of this approach is that your display device doesn't even have to be calibrated.  The RGB values remain the same regardless of how they are displayed on your screen.  For more on the Birder's Colour Pallet see HERE

All that remains now is to provide you with the tools so you can play around with them and have a go at scoring the quality of your own bird images.  Feedback is as always much welcome and appreciated.

(Note you will have to download the file and open it in MS Excel for the tool to work properly).

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