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Thread: Posterisation

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    BPN Member William Dickson's Avatar
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    Default Posterisation

    Posterisation seems to be becoming more apparent now on the images. I don't know why. At first, I thought it was the 5K on my iMac, but now I see more people, are seeing it on non iMac monitors. Is it because of the new larger image size, I don't really know.. I would love to know how to prevent it. I have tried several ways, but to no avail, apart from cloning it out. It always appears on the BG for some unknown reason. Any Thoughts?

    Will

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    Hi Will... seems you are still waiting on a response on this. I'm no expert, but this is my understanding in case it helps. I hope others can chip and help.

    There is no reason that is obvious to me why posterisation would become more apparent recently unless you have changed something, for example, increased application of saturation. It's also not clear why a larger monitor would make it more visible either so I'm not sure that would be a fruitful area to investigate. Understanding what causes it is the best way to design a workflow that avoids it:

    In short, sRGB is not capable of displaying all colours on the colour spectrum. It is a relatively 'small' colour space. Have a look at the masp of colours in the charts on this webpage and you'll see what I mean:
    https://www.color-management-guide.c...-prophoto.html

    The colour space is smaller in sRGB than Adobe RGB for example which can be seen by the smaller area on the charts that is covered by sRGB. A simple interpretation of these charts is that sRGB is unable to reproduce the extreme reds and blues and especially the extreme greens as the colour space does not extend out into these areas of the charts. Posterisation will occur when these colours outside the sRGB space are 'mapped' into your file by your image software. In some cases different greens, for example, outside SRGB will map to different greens inside sRGB and so will look different when displayed as sRGB (which virtually all web browsers do by default and why sRGB is usually used for the web). However, sometimes the colours in your image, while quite distinct in the real world will map to very similar colours in sRGB. When this occurs in an area of smooth tone and colour, the small step in colour can be seen. This is posterisation. A smooth gradation in the original full spectrum image reduces to a stepped gradation of a few similar colours that sRGB is capable of displaying.

    This can be hard to conceptualise but I hope it helps a bit. Regardless, here are a few things the theoretical observations say will make it worse:

    1. Avoid really smooth, gradated backgrounds, especially in the green part of the spectrum. Posterisation is most likely in those. But it also applies to blues and reds to a lesser extent.
    2. Adding saturation pushes the colour further outside the sRGB colour space and is likely to worsen posterisation in smooth gradated tones.
    3. Try selecting and desaturating an area that is showing posterisation (especially if it is not on your main subject) as that *should* reduce the problem. Not tried myself but theory tells me it should help.
    4. Adding blur and denoising backgrounds reduces the variation between adjacent pixels and smooths the tonal gradients in that part of the image and is likely to make posterisation worse. A two-edged sword here as we all like those soft smooth background but they can come at a price. But do avoid adding blur if at all possible.
    5. Don't be tempted to save your image into a wider colour space like Adobe RGB. Pretty well all browsers are going to convert it and display it as sRGB - or worse, assume the colour space is sRGB and therefore render them in a way you didn't intend.

    Sorry but we are stuck with sRGB and just have to learn to 'live' with it.
    Last edited by Glenn Pure; 04-12-2018 at 06:19 AM.

  3. #3
    BPN Member William Dickson's Avatar
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    Thank you Glenn for a very informative comment, and also for the link, I'll read that in more detail later on. I haven't tried desaturating an area that is showing posterisation either. But, that sounds good, I will try that one next time I come across it. Looks like we will have to concentrate more on the BGs and if a choice is available, take one which has less chance of posterisation, again not easy. I suppose my workflow will adapt through time. I was wondering, would it make any difference, on what software was used to open the RAW file.

    Thanks again mate, much appreciated.

    Will

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    BPN Member Don Lacy's Avatar
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    HI William sorry for the late reply have been very busy at work and lost track of the last time I checked in. First make sure you are working on a 16bit file at all times so double check that your Raw processor is outputting a 16 bit file to your editing program second use a larger color space either Pro or adobe RGB. If you are only noticing it when downsizing for the web and it is a 16 bit file in a larger color space try downsizing in two steps the formula I use is to take my destination pixel size and times it by 1.66 downsize to that and then downsize to my final size. Also turn off any automatic sharpening that might be applied if using PS make sure you set the method to bicubic. For example if I want to downsize to 1500 pixels i will open the image resize box and choose 2444 for my pixel size select bicubic as the the method hit apply reopen the dialog choose 1500 pixels and bicubic again and apply I will then selectively sharpen the image. If I had to guess somewhere along the line your Raw converter settings got changed. let me know if this helps.
    Don Lacy
    You don't take a photograph, you make it - Ansel Adams
    There are no rules for good photographs, there are only good photographs - Ansel Adams
    http://www.witnessnature.net/
    https://500px.com/lacy

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