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Tutorial - Posterization



What is posterization ?


Posterization or "banding" is an undesirable series of stepped colour gradients in what should be a smooth transition of colour. It will most frequently be encountered in backgrounds to images that should have nice smooth out-of focus colours. Also it will especially occur when images are being heavily compressed from a large file to a small size for web viewing.

This tutorial offers advice on how to prevent posterization from ruining your images and also gives some ideas to reduce it once it has crept in.

Posterization may be summarised as "Insufficient colour data spread too far apart". Think in terms of your Photoshop levels histogram representing the tones in an image. If there are not many tones present and you adjust the levels to open up the histogram you will see gaps appearing giving a comb effect. The gaps are missing data. Other technical articles on the web will explain this thoroughly, but if like me, you are more interested in knowing how to prevent posterization or fix it once you have it ....read on.

As I shoot in 16 bit RAW rather than 8bit jpeg , I rarely encounter posterization but a recent photoshoot of a very cute little owl with a smooth blurred background gave me all sorts of problems to edit and get my pictures onto the web without looking awful. Here is the little chap proudly holding a mouse before flying back to feed it to its young.



Take a careful look at the background and you will see some nasty colour transitions. Below is a 100% crop of part of the image and you can clearly see the rainbow coloured steps.



I first noticed the posterization in the image in the Adobe RAW converter while I was viewing at normal screen size and I knew it might be troublesome. However, when I viewed at 100% or full size the problem went away. So, first lesson is that the image from the camera was not really posterized but I was seeing a Photoshop artifact through not viewing at actual size where one pixel in the image is represented by one pixel on the screen.
However, had I have shot this image in jpeg or under-exposed it and had to open up the levels I might not have been so lucky.

If you look at the owl in the image you will notice that as the late afternoon sun was coming from the left, the opposite side of its body is in shade. I therefore boosted the brightness of the dark side by moving the shadows slider in the RAW converter to the right. Also I added some saturation to bring out the colour intensity a bit. Finally I added some noise reduction before opening it in Photoshop. Once in Photoshop I further improved the contrast issues with the shadow/highlights tool and sharpened the image. Unfortunately these are all steps that can aggravate posterization in an image ! When I came to reduce the size of the full sized image to a tiny web-sized jpeg the posterization was very prominent and totally unacceptable.

What can be done to minimise posterization?

The single most important thing that can be done to prevent posterization is to shoot in RAW. This is a 16 bit format whereas jpeg is 8 bit. This means that there is twice the data to work with. 8 Bit is not always sufficient to allow the smooth transitions. Once processed in 16bit, save in a lossless format such as Tiff.

When editing in Photoshop, use adjustment layers where possible and generally keep editing to a minimum. The most problematical editing tools which will aggravate posterization are any that effectively open up the levels, thus exposing gaps in the data - so significant brightening through levels, curves or shadows/highlights should all be used with care.

Do not under-expose your images as you will be forced to brighten them later in post processing. This will open the levels and will invite posterization. You will also minimise noise into the bargain by "shooting to the right."

Noise reduction also enhances posterization so only use the minimum required and go easy on those sliders in post processing ! I set the colour and luminance detail sliders to zero and used just a whiff of Luminance and colour noise reduction.

Go easy with Hue/saturation tools and do not add sharpening to the problem areas. This will usually mean painting on sharpening using layer masks.

You can make a selection of the problem area of the image and leave it alone, this usually means the background. Then select inverse (which will usually select your subject) and process as required. This is what I did with my little owl image. I brightened the dark side of the owl with shadows/highlights and increased the hue/saturation without affecting the background. I then flattened the image layers again and saved as a Tiff file. You can also use a layer mask if you are familiar with this process.

I also had to resize the image and save to web using as little compression as I could get away with, without making the filesize too large for web viewing.

Here is my second attempt at processing the image - much better I think you will agree.



How can Posterization be removed ?

There is an old saying that "prevention is better than cure." This is very appropriate in the case of posterization. It is certainly easier to prevent it in the first place than to try to remove it later as all you can really do is disguise it.

My first tip would be to go back to the original image and reprocess it again using the suggestions in the previous paragraph - as I did with my little owl image above.

So what can be done to remove posterization ? Surprisingly, the best way of dealing with it in Photoshop is to disguise it by adding noise to the parts of the image showing the effect. This means first making a selection of the area or using a layer mask and then applying noise as described below. It is important that the noise should be added after all editing has been completed and the image has been resized its final size but before the final sharpening has been applied. To add the noise, select Filter/Noise/Add Noise and the Add Noise dialog box appears:

Amount determines how much noise is added. Only a small amount needs to be added but this will depend on the size of your image. I usually add more than is necessary, then go to Layers and back off the percentage opacity from 100% to what looks best.Too little and the posterization is still visible, too much and noise becomes intrusive.

The Distribution control should be set to Gaussian. Finally, the Monochromatic box should be checked. Checking this box makes sure that the noise that is added is tonal rather than colour noise. Click OK.

Finally, as an alternative, you can download a Photoshop plug-in to repair posterized histograms (Google it). You will probably have to pay for these plugins and I do not know how affective they are as I have never had to resort to using them.


 

 

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