Posterization

Colour images can potentially hold up to millions of tones, though in most cases the number is far smaller.  Where colours gradually change from darker to lighter areas enough tones are required to make the transition appear smooth.

FIGURE 1  Here we see how blue skies, for example, need many tones to give the impression of a gradual change from dark to light and form a blue gradient.

Sometimes, however, there are not enough tones to keep the gradients smooth, resulting in steps or blocks of colour - a phenomenon commonly known as Posterization. This can be a result of not capturing enough colours in the scan or, more likely, when heavy colour correction is used. The deliberately posterised image on the right has a number problems often seen in images: skies separating into bands of colour, midtones dropping into shadow or black, and excessive reds in the warm tones.

FIGURE 2  Here we see how blue sky is a common problem, especially when colours are exaggerated. Sky tones can be forced to separate from one another, often into a completely different range of hues altogether, resulting in the sky becoming quite unrealistic.  

The two examples above show cases where the posterization is very obvious. The image on the left has lost all reality in the sky ranging from an over saturated blue at the top, through cyan, down to almost grey at the bottom. A result like this would render the image useless, unless you are willing to re-create the blue sky manually.

The image on the right has localised posterization so could be saved if the affected area is small enough or can be cropped. The same problem is occurring where the sky goes from blue, through cyan, grey and then to pink, although the blue in this case has not been exaggerated. The pink may give the impression that there is a sunset effect, but this distortion appeared in all the highlight areas of this image where sunset effects could never have occured.

FIGURE 3 In the example on the right, the red at the top has gone altogether and the gradient has broken into distinct bands further down.

Extreme blue skies do not necessarily end up as different colours but posterization can still occur. If you are not sure that there is a problem then try looking through the separate channels for a clearer view. The red channel is usually the common problem here as it is the colour furthest from blue and will be damaged first.  

FIGURE 4 In the example image here it looks like the red channel is a problem, but in reality it is the blue, and even the green channel (right), that have crashed leaving just the red.

Posterization can force shadow detail into solid blocks of black. One of the common areas this occurs in is with skin tones where the three colour channels drop into black at different points. As the red is the dominant colour, it can be left isolated, giving the effect of a bad skin rash.