Thursday 2 April 2015

Colour - Colour Saturation Analysis

HSL - Hue, Saturation and Luminance

When we talk about colour, we probably instinctively think of the bright, vibrant hues which make up a rainbow or the bright colour pallet of a child's paint set.  Colour vision allows us to appreciate these vibrant colours.  But there is far more to this subject than vibrant colours alone.  And, our vision is complex enough to appreciate the finer subtleties.

We are all familiar with the effect light has on the brightness of objects that we see.  Parts of an object which are lit are brighter and have brighter, more obvious colours.  Whereas, objects which are in shadow are darker, possessing darker colours.  This attribute is referred to as the luminance of colour.

There is a third element to the RGB colour triangle which we may be less conscious of and which is less clearly understood by most people.  This is referred to as colour saturation or the related term chroma.  Saturation is a measure of the colour purity or colourfulness of a colour.  It is directly linked to hue and is a measure of the purity of that hue, created by mixing hue with a certain quantity of neutral grey of equal luminance.  I must admit that I have found this concept a little hard to get my head around but I finally cracked it and I recommend reading THIS POST before continuing to read the detail below.

It is only when we start to really analyse colour theory that we begin to understand the relationship between the vibrant hues that we all easily recognise and their linkage to the bland, often ill-defined tones we commonly encounter in nature.  In the Birders Colour Pallet this linkage should be clear enough.  Olive for instance could be described as a desaturated yellow.  Similarly, maroon and crimson are both merely ruby with its colour saturation toned down by different levels.  If we accept this logic then it begs a simple question.  What factors influence colour saturation and hence our accurate analysis of colours in digital images?

Image Formation
In THIS posting I compared the structures if the human eye and camera sensor.  There is a remarkable similarity between the two.  Clearly the design of the camera sensor was based in part on the human visual system.  Both models require the filtering of the image through three types of colour receptor, green, red and blue in a ration of 2:1:1.  While the mechanism by which humans form an image beyond that point is not clear, we know how digital images are made.  RAW image data consists of two measurements - the light intensity reaching a cluster of photosites (a measure of luminance) and the colours of the filters associated with those sites (analogous with hue and saturation).

Saturation and Lighting
Lighting rears it's head in every discussion in this blog.  On a bright day colours appear bright and vibrant, whereas on a dull day colours are more subdued and less saturated.  Digital images typically are less saturated in appearance than conventional film or slide film. Many cameras offer three or more saturation defaults which an operator can choose from depending on individual taste.  Saturation can also be adjusted using post processing software.

Taking the bright, well exposed image below, it may appear that the part of the breast which is in shade is somewhat desaturated relative to the parts which are well lit.

In actual fact, both the well-lit and shaded parts of the yellow breast display near maximum colour saturation of 240.  That doesn't imply that the hue is the same right across the whole breast - and indeed it is not.  I suspect in this case reflection from nearby exposed wood or flowers is adding a pinkish flush to the shaded part of the breast, resulting in an orange hue overall.  The ventral area has a slightly colder-looking hue and this is possibly due to the naturally blue colour of skylight as illustrated HERE, or it may be reflection from nearby foliage, or both.  Once again, the colour saturation of the ventral area is still very close to fully saturated.  Perhaps there is another way of investigating this question?

Saturation & Exposure
Photographers working with film or slide would have detected changes in saturation with exposure due to the inherent properties of those media.  Do digital saturation algorithms attempt to emulate the variable properties of film?

I carried out a simple experiment involving progressively increasing exposure then sampling a number of discrete colour patches and comparing the saturation values of each.

The results suggest that, for the JPEG output of my Canon 70D at least, overexposed images have significantly greater colour saturation than normally or underexposed images.  This has implications for the use of the birders colour pallet, as one might be inclined to name a colour incorrectly based on the saturation value of the colour patch.  On the other hand, one is probably not likely to try and sample and name the colours using such an obviously poorly exposed swatch.  And, one is just as likely to have significantly blown or clipped colours as to have captured anything useful in such an overexposed image.

We know that while colour saturation is measured by the camera sensor there is also an adjustable saturation setting so photographers can choose to boost or reduce saturation to suit their tastes.  Are there other, hidden manipulations going on at the software level?

On the plus side, there are only three saturation increments provided for in the birders colour pallet.  Provided the user has a reasonably good camera, takes reasonably good exposures, uses colour calibration methods and is careful with the use of saturation controls, there is sufficient latitude built in to allow for reasonable saturation errors.

See also HERE and HERE.

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