Tuesday, 17 November 2015

Colour - Saturation Finally Explained

Not for the first time I stand corrected!  I have found it hard to get hold of a clear explanation for colour saturation.  Sure it's easy to visualize colour saturation when we see it illustrated graphically, as in this simple depiction below.  Saturated colour is rich and pure in appearance while desaturated colour looks washed out, fading eventually to greyscale.  But what actually is colour saturation and how is it measured by the camera?

In earlier postings I fell into the trap of assuming that colour saturation is not actually measured at all by the camera but is one of the camera pre-sets.  This statement is partially true.  Saturation, like contrast is one of the pre-sets that is laid down as the raw image data is converted to JPEG.  It is also one of the settings that needs to be adjusted in a RAW workflow using Camera Raw or whatever you have.  

It is also evident, in the design of the camera sensor that there are only two elements used to measure colour.  Each photosite directly measures luminance, one of the three parameters that defines a colour.  Then we have the Bayer filter array.  In the majority of cameras the Bayer filter array is the critical element that defines colours accurately in digital images.  Without it the image would be black and white.  Over each photosite sits either a green, a blue or a red filter.  The filter blocks all of the visible spectrum, apart from the region of the spectrum that corresponds to that filter.  So in effect each photosite measures light intensity over a limited region of the visible spectrum.  So, how does the camera actually decide based on this limited information what the hue and saturation for a given pixel should be?

The answer is interpolation, or more specifically de-mosaicing.  Each pixel on the final image does not correspond to a single photosite on the digital sensor but rather to a cluster of neighbouring photosites, generally two green filtered and one each of adjacent red and blue filtered photosites.  This starts with the creation of a Raw Bayer Image as illustrated below.

So, what is Colour Saturation Exactly?
Having been researching colour for some time I kept finding the trail going a little cold at this point in the journey.  Then, finally I found a proper explanation for what colour saturation actually is, and it all starting to fall into place.  When we look at a depiction of a saturated colour compared with a desaturated equivalent it is easy to assume that saturated simply means 'more of the same colour'.  That is kind of true, but more specifically what saturation is is 'purer colour'.  When we analyse the spectral distribution of a colour in nature what we find is that the colour is often made up of a range of different colour wavelengths, not just the wavelength that corresponds most to the colour we see.  We are all familiar with the concept of mixing paints to create different colours.  Colour is itself a mixture of different quantities of other coloured wavelengths.  Saturation is a measure of the purity of the most dominant wavelength.  If a colour is almost entirely made up of one wavelength of light (eg. a laser) it will appear richly saturated.  If however the colour consists of one dominant wavelength plus a lesser amount of a range of other wavelengths then that colour may still have the same dominant hue but it will appear less saturated, i.e. less pure.  What an extraordinary and somewhat counter-intuitive revelation!  And yet I kept missing this vital point while researching colour and saturation.  I finally found all of this neatly explained by the experts at Cambridge in Colour (once again) under their tutorial on Colour Perception HERE.

How do cameras measure colour Luminance, Hue and Saturation?
So its all finally fallen into place.  Cameras measure luminance directly at each photosite based on the number of photons collected.  Then, taking the colour information gathered from the green, red and blue filters the camera can measure both hue and saturation to a high degree of accuracy.  If for example the object being photographed is a pure, saturated green colour then only the green filtered photosites will likely register an image of it.  If however the object is a dull desaturated green then chances are its spectral signature will register to a greater or lesser extent across all three colour filtered photosites.  During interpolation this data can be combined to identify both the correct hue and saturation level for that colour.

Saturation pre-sets and other post-processing 
The story doesn't end there.  Most photographers would agree that digital images are not as well saturated as equivalent film or slide images.  Manufacturers leave the choice of saturation preferences up to the photographer.  Those who like more saturation in their images can select a stronger saturation pre-set if they prefer.  Different camera manufacturers are likely to put their own non-adjustable pre-sets in place depending on exposure and other factors to boost the overall quality of the images offered.  So there is probably a limit to how well we can really analyse the true saturation of colours based on digital images.  Hopefully though the overall accuracy is good enough for our purposes.

Colour management is a process that aims to maintain accurate colour from the scene to the camera or scanner, to the screen and ultimately to the printer.  Doing this properly takes a great deal of effort and starts with the proper calibration of the camera sensor.  No two cameras generate colours exactly the same way.  Sensors vary slightly and are not calibrated by the manufacturer to any globally recognised standard.  For some reason manufacturers don't think this is important.  The most well recognized tool needed to calibrate a colour sensor to a standard is the X-rite (formerly Gretag) Colorchecker Passport.  Meanwhile, in the field the human eye is constantly adjusting to changing light temperature (white balance).  The camera also tries to compensate accordingly but often fails in that regard.  Once again there are ways to correct camera white balance properly and to a recognized standard.  I have discussed these various processes in detail HERE.

In summary
So, once again I have finally jumped another stubborn hurdle on my journey of discovery.  I have found colour saturation to be one of those conundrums that didn't quite fit in to my understanding of light and of digital photography.  Having established that the key word in the definition of colour saturation is 'purity', finally it all now makes sense.  The camera can indeed measure very effectively all three parameters that go to make up what we call a colour.

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