Saturday 3 September 2016

Colour - Pros and Cons of Boosting Saturation

As a young boy I can remember being told that white light can be scattered by a prism into all the colours of the rainbow.  Like most kids, I found that an incomprehensible concept.  For a child, used to subtractive mixing of coloured paints, the additive mixing of coloured light to produce white light is totally alien.  For more on additive and subtractive colour mixing see HERE.

In the typical model of colour that most of us work with in image processing we have three axes which together describe all the colours that we see.  The classic rainbow is defined by the property of colour referred to as hue.  This represents colours at their purest and most vibrant (fully saturated).  Luminance is merely a measure of the brightness of a colour.  If we take away hue what we are left with essentially is a B&W image made up of levels of brightness of each pixel along a grey scale.

The third axis, saturation is a little harder to grasp, but, actually I have just described it in the previous paragraph.  Desaturation of colour is the gradual removal of colour to reveal a grey scale.  Scientifically, saturation is a measure of the purity of the most dominant wavelength of light.  The presence of other wavelengths of light desaturates the dominant wavelength making it less vibrant.  It's totally counter-intuitive.  By adding more colours we end up with grey scale.  If this sounds a bit like the process involved in creating white light that's because it is the very same process.  A prism splits apart different wavelengths of light so they become individual, vibrant, saturated colours.  Take away the prism and all these wavelengths intermingle again, reducing their individual vibrancy or saturation levels until what remains is pure luminance, without colour.  I have written a bit more about saturation HERE.

Boosting Colour Saturation
As birders we put a lot of demands on our digital cameras.  We bolt on a long lens and ramp up aperture and shutter speed in the hopes of capturing an elusive, often small and fast-moving subject, using minimal levels of light.  Thankfully, modern digital cameras use advanced processing to boost the sensitivity of the camera sensor to increase it's versatility in low light situations.  Part of that process may include a boosting of colour saturation.

 In the illustration above I have taken a typical exposure and boosted saturation beyond normally acceptable levels.  It reveals a number of pros and cons about the tool.  On the plus side, colourful objects like the bareparts of the gulls are boosted in a positive way.  We also see a boosting of other natural colours including the mantle shades of the gulls (these are not neutral greys as it turns out), plus the colour of the sand and sky reflection on the water.  These are 'over-cooked' here for illustrative purposes.  Taking saturation back a few notches will render them more acceptably.  

On the negative side we can see how boosting saturation makes colour noise more apparent and makes shadows appear unnatural in colour.  In reality even shadows have underlying colour in them which only becomes apparent when saturation is boosted.  Provided we have an understanding of each of these inherent pros and cons saturation can be used as a forensic tool.

To illustrate that true neutral greys are not altered by the saturation tool note I have added six grey boxes, three of which are neutral grey.  The other three have a minimal, almost imperceptible colour cast applied, which is revealed when the saturation is boosted.

So, what can boosting saturation tell us about the image above?  
  • It tells us that the mantle shades of these gulls are not neutral grey.
  • We can better visualise leg colour, not always clear from low saturation images
  • We can see there are a number of things impacting the shadows including the blue sky and reflected sand.  We  often think of shadows as grey but in fact they generally have underlying colour in them.
  • We may be better able to detect a white balance error
  • If there are any true neutral greys in an image these will be revealed

In Camera Saturation Processing
Processing from RAW, saturation is one of the parameters requiring setting by the operator.  RAW data files are naturally low in contrast and saturation.  When the camera outputs a JPEG from RAW the processor uses proprietary settings for saturation.  These may not always be easy to anticipate.  For instance in an earlier posting HERE, I carried out an analysis of the relationship between exposure and saturation, with some unexpected results.

 In another posting HERE I explored the intrinsic interrelationship between brightness, contrast, saturation and sharpness.  Adjusting any one results in a knock-on effect for all the others.

 In summary, saturation is an intrinsic part of colour.  It is also yet another tool which we can use in the forensic analysis of images.  There are of course limitations which we need to understand in order to use this tool effectively. 

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