Monday 31 March 2014

Colour - Colour Sampling

I remember getting into an argument some years ago about the apparent leg colour of a Shearwater.
The question was were the legs blue or pink?  In reality they were probably best described as pale bluish-pink but the question came down in the end to a measure of the number of pixels on the pink side versus the number on the blue side.  And, of course, given the difficulty in accurately capturing colour in nature using a camera, the image may not even have reflected the true colour of the bird's legs in life.  Nonetheless, for me it was an interesting exercise and I have recently been thinking a bit more about the question of how we should sample colours from an image of a bird.

Colour in nature is rarely uniform.  A patch of colour will, generally consist of smaller patches of slightly different colours, further complicated by the texture and structure of the surfaces of the subject.  The angle of light falling on the subject will create subtle colour gradients.  A feather for instance as we all know is neither flat nor uniform.  It has a complex structure at a microscopic level.  One has to be relatively close to see the shaft and vanes on a feather and these are just the larger, more obvious features that make up this marvellous design.  

However, as the observer withdraws, details tend to merge and flatten with distance.  If you open any field guide and closely observe the plates, generally you will note that the artist has not resolved detail down to the level of individual feather veins.  Not only would this be extremely laborious but it should not even be necessary.   After all we generally don't tend to get close enough to birds to see these fine structures clearly in life.  

So, in summary we are used to observing and identifying birds without having to resolve extremely fine detail.  We are also more used to describing birds based on large, apparently uniform patches of colour.  The question is, how do we define the colours we capture in photographs.  In COLOUR BY NUMBERS I described an accurate, standard method for classifying colours in digital photographs.  This method however only applies to individual pixels.

As the example above shows, though an area may look quite uniform in colour, there tends to be a lot of variation at the pixel level so simply placing a "dropper" over a patch of colour gives no guarantee that the actual pixel sampled will be representative.

Selective use of the dropper
If at first you don't succeed you can keep sampling pixels until you find what might be considered a good match.  This approach however is obviously subjective.

Make the image much smaller  
When we make images smaller the pixel count drops but the overall colour representation remains fairly true.  Unfortunately, we still end up with far too many choices.  Below is a good example of why this method simply doesn't work.   

Postarize the image
Obviously using an artistic postarizing tool will dramatically alter the image.  This is a means to an end.  Our primary motive here is to take subjectivity out of the process.  By subjecting the image to a standard algorithm we can replicate the process for different images and expect the same results time and again.  So we now have a qualitative method for sampling colour in bird images.

A Result!

See also HERE.

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