How to avoid posterization

[trink408]

What are the best ways to avoid posterization when working with your images in photoshop?

I'm working in 16 bit, using adjustment layers, and I still see my historgrams get choppy making adjustments. I don't notice anything visually in the images, but the histograms definitely show posterization.

How critical is it to avoid this?

[Tyler Boley]

Are you refreshing the histo? I believe it just works off the monitor image, or something like that.
The only histo view I really trust, if I'm extremely concerned, is after flattening and refreshing.
Try that once and see what happens, you may be fine...
The way the histo works changed at some point in the last few versions, sorry I'm not up on it's precise behavior.
Tyler

[Ken Lee]

All adjustments will result in choppiness, because we are dealing with a computer. That's why they call it "digital". Even before any adjustments, the histogram is choppy: but the steps are rather small, so it appears rather smooth to our perception.

With 16-bit, you start out with 65,536 shades of grey. Even if you lose 90% of them, you still end up with more shades, than you ever start with in 8-bit, which is only 256 shades.

[trink408]

Sounds like I shouldn't spend much time worrying about the histogram as long as the image looks fine visually...

Thanks guys

[paulr]

Are you making radical adjustments? If not, I'm guessing it's a histogram refresh issue, as Tyler suggested. With 16 bit images and adjustment layers, I never see anything remotely resembling posterization or comb filtering. Even without adjustment layers, the only times I've gotten choppy histograms was on purpose, to demonstrate the effect.

[jetcode]

Are you making radical adjustments? If not, I'm guessing it's a histogram refresh issue, as Tyler suggested. With 16 bit images and adjustment layers, I never see anything remotely resembling posterization or comb filtering. Even without adjustment layers, the only times I've gotten choppy histograms was on purpose, to demonstrate the effect.

The reason you can't see the changes in the histogram is because there are 144-196 dots in the histogram window or so vertically to represent 65536 values. My guess is PS distributes the points in the histogram according to the display size.

In my eye a B/W image with 32-64 levels is sufficient in certain images with buildings. For smooth tones in flowers and skin more resolution is required.

[Bruce Watson]

I'm working in 16 bit, using adjustment layers, and I still see my historgrams get choppy making adjustments. I don't notice anything visually in the images, but the histograms definitely show posterization.

No actually, they don't. Visible posterization (http://en.wikipedia.org/wiki/Posterization) occurs when there are enough adjacent pixels with the same hue, saturation, and value (HSV) that you can see them as a "flat" region in the print.

The histogram only shows numbers of pixels with certain values (per channel if you are in RGB). What it does not do is indicate where these pixels are, in particular where they are in relation to each other. And it's the pixels' proximity to other pixels of the same HSV that defines posterization.

You may or may not have visible posterization after your adjustments. The only way to tell is to make a print and examine it.

[trink408]

thanks again guys.

Sounds like I was mislead to believe that because the histogram shows the comb effect that it means there is posterization in the image. Posterization is really a visual attribute that can only be determined after printing?

[paulr]

No actually, they don't. Visible posterization (http://en.wikipedia.org/wiki/Posterization) occurs when there are enough adjacent pixels with the same hue, saturation, and value (HSV) that you can see them as a "flat" region in the print.

The histogram only shows numbers of pixels with certain values (per channel if you are in RGB). What it does not do is indicate where these pixels are, in particular where they are in relation to each other. And it's the pixels' proximity to other pixels of the same HSV that defines posterization.

You may or may not have visible posterization after your adjustments. The only way to tell is to make a print and examine it.

In my experience the histogram will show comb filtering long before there's any visible banding or posterization in the image. If you actually posterize a 16 bit image, even with a large number of grays (say, 125) you'll see radical comb filtering in the histogram.

[keith english]

Anytime the hosto gets that "comb-tooth" look it means data is being corrupted, or new interpolated pixels are being created to fill in stretched data. That is why it is important to have the best image possible to start with and make minor adjustments. As said, if you work in 16 bit and don't notice the degradation you are probably ok.

[Bruce Watson]

Anytime the hosto gets that "comb-tooth" look it means data is being corrupted, or new interpolated pixels are being created to fill in stretched data. That is why it is important to have the best image possible to start with and make minor adjustments. As said, if you work in 16 bit and don't notice the degradation you are probably ok.

Corrupted? Hardly. Changed? Yes. What it really means is that you are working the image, making adjustments to various parts of it. It shouldn't be surprising that the HSV of the pixels changes in interesting and odd ways according to the histogram. The test of corruption IMHO is the print -- are your adjustments to the image leaving visible artifacts? If they are then a smooth histogram won't help you. If they aren't then a choppy histogram isn't something to worry about.

That said I fully agree that it is very important that one start with the best possible image capture and best possible scan (if film). Image editors can do interesting and sometimes amazing things, but they can not create new information -- just new data.

It's perfectly possible to work in 8 bit spaces but it really puts a premium on the quality of the image capture and scan. It's much easier to get good results when working in 16 bit -- more headroom makes it easier to maneuver.