Generally, colors in an image blend gradually from one color to the next. Is impossible for the human eye to tell where one color stops and the next one picks up. However, in some cases, the transition from one color to the next can actually be seen. This usually occurs in areas of little detail (e.g., featureless skies). The result is an image where bands appear to run across the image. This problem is known as posterization (also known as banding). An example of posterization is exhibited in Figure 1.
Posterization is highly undesirable. However, posterization can usually be prevented. For those cases where posterization has already occurred, it can usually be corrected if you know the proper technique. The purpose of this article is to cover the cause, prevention, and cure of posterization.
The cause of posterization can be explained in one little sentence: too little information spread too far apart. The "information" refers to the tones contained in an image, and the "too far apart" refers to how far apart the tones are from each other. Figures 2 -- 5 illustrate this concept. Figure 2 (JPEG image) shows a color gradient from green to red. The gradient has a smooth transition of color across the gradient. Figure 3 shows the histogram of the image. The histogram shows no tonal gaps. In Figure 4 (JPEG image), the color gradient was subjected to some editing that caused posterization. The histogram in Figure 5 shows why the posterization occurred. This histogram shows that the tones are now spread far apart.
It is easier to put on a coat than to cure pneumonia. Similarly, it is easier to prevent posterization that to fix it after it has occurred. There are three ways to prevent posterization.
Now, this may not seem like the most brilliant or technical solution, but posterization can, sometimes, be significantly reduced or even eliminated by reducing the amount of image editing. For example, reducing the amount of Curves or Hue/Saturation used might help reduce or eliminate a posterization problem. Another possibility is to use a tool that doesn't spread the tones apart (e.g., use Brightness/Contrast instead of Curves). By reducing the amount of editing, or using a tool that does not spread the tones apart, the distance between the tones will be less and the posterization problem may disappear.
While a bit simplistic, and not possible with all images, this method may save the day for some images.
One of the easiest ways to help reduce or eliminate posterization is to shoot images in raw. Raw has sixteen times more tones than JPEG. Therefore, the tones are much closer together than when the JPEG format is used. Thus, images that start as raw and are converted to high bit TIFF (or some other high bit format) for editing are much less likely to exhibit posterization.
The effect of having more tones can be seen by comparing Figures 2 -- 5 to Figures 7 -- 10. Looking back at Figure 2, we see an eight bit, JPEG image of a smooth color gradient that shows no tonal gaps in the associated histogram. However, after editing, the image exhibits heavy posterization (as shown in Figure 4) with large tonal gaps (as shown in Figure 5).
Figure 7 shows the exact same color gradient as that shown in Figure 2, except that this image is a sixteen bit image. The image has a smooth transition of color across the gradient. Figure 8 shows the histogram for the image. The histogram shows no tonal gaps. In Figure 9, the color gradient was subjected to the same editing done with Figure 4. This time, there is no posterization due to the larger number of closely spaced tones in this image, and the histogram in Figure 10 shows a lack of tonal gaps.
Please keep in mind that these color gradient images were created in Photoshop for illustration purpose. They were not created in a camera (e.g., Figures 7 and 9 were not created from raw images; however, they behave similarly to raw images that were converted to sixteen bit TIFF files). Thus, these images illustrate well the impact of having more tones.
Have you ever noticed that posterization tends to occur mostly in the darker parts of an image? There is a reason for that. The different tonal regions contain different numbers of tones. This is shown in Table 1. For a five stop dynamic range, JPEG image, there are 128 tones in the highlight tonal region (before application of tonal curves) but only sixteen tones in the shadow region (before application of tonal curves). Since the darker tonal regions have so few tones, posterization is a bigger problem in these regions.
|Three quarter tones|
The posterization issue is resolved by adding something that, under normal conditions, everyone tries to get rid of: noise. The noise is added in very small amounts to only the areas that show posterization (the sky in this image). The noise breaks up the edges of the bands so that the eye can no longer detect the edge (kind of like how camouflage breaks up the outline of a human figure so that it is not discernable). On the other hand, the noise is added in such small amounts that it is not readily noticeable at the normal viewing distances for prints. Furthermore, since posterization is most noticeable in dark areas that have little detail, the noise is being added to areas where there is not much detail with which the noise can interfere. This is seen in Figure 11. The noise will be added to the sky where it will not hinder any significant detail.
Prevention is always better than a cure. By shooting in raw and maximizing the exposure or shooting multiple exposures, a photographer can greatly reduce problems with posterization. For those cases where posterization can not be prevented, this noise technique can save an otherwise great image.