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Thread: Gaps occur in between tonal peaks - explanation please

  1. #1
    Tim's Avatar
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    Gaps occur in between tonal peaks - explanation please

    In the http://www.cambridgeincolour.com/tut...hop-curves.htm there is an interesting section that deals with gaps between tonal peaks. It says:

    'If the gaps occur in between tonal peaks, then a unique ability with curves is that it can decrease contrast in these unused tones thereby freeing up contrast to be spent on tones which are actually present in the image. The next example uses a curve to close the tonal gap between the sky and darker foliage.'

    Excuse my obtuseness but I really don't understand quite what is happening, or the thinking involved to achieve this. Looking at the curve I guess the top 3 points lock the highlights in place, but then what happens? My own experiments don't seem to be able to replicate the effect.

    A step-by-step walkthrough would be appreciated.

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    arith's Avatar
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    Re: Gaps occur in between tonal peaks - explanation please

    Your not the only one

    I think of a luminance histogram as a pictorial representation of luminance, the horizontal scale 0-255 ranges from black to white, and the vertical scale is the number of pixels at each value.

    So if you have a U shaped graph then you have a lot of black and a lot of white but very little inbetween.

    The levels tool, I use a different version, seems to be a way of increasing or decreasing the population of certain tones, an n shape should depopulate high and low tones and increase the population of mid tones.

    But I admit I find this confusing, but just doing it isn't hard at all.

    Hope somebody can explain what is going on here

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    Re: Gaps occur in between tonal peaks - explanation please

    All this really means is that flat spots in the tonal curve don't use up any of the available output range. In other words, if there is a gap in the histogram, there are no tones in that area, so why waste the available output range on an area of no information? Flattening the tonal curve in such areas conserves the available output range for use elsewhere, so the available output tonal range is used to separate tonal values that are actually present in the image. I hope that helps.

    As for a walk-through, just look at the "before" and "after" histograms. In the "before" chart, there is a gap between two peaks. The idea is to use the tone curve tool to to create a flat response in the range of the gap; since there is no information there, the flat curve insures that all of the input tonal values in this range map to the same output value. You don't lose anything useful in the output, because there are no input values in that range, anyway. The flat spot doesn't use up any of the available vertical space on the tone curve, so the available output range is all used to separate input values that are actually present. Look at the "after" histogram, and you can see that the gap in the histogram has been filled in.
    Last edited by Snarkbyte; 4th May 2011 at 01:57 PM.

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    Re: Gaps occur in between tonal peaks - explanation please

    I think they're just talking about what you would normally do with tone mapping on a histogram. If you have a series of peaks, your initial curve probably is a step graph where (moving from left to right) the between-peaks part of the tone curve rises relatively slowly and the part over the peaks rises more sharply. Thus, you accentuate differences in the part of the tone curve where there are lots of pixels and downplay the differences in pixels that aren't very common on the image anyway. This approach doesn't always give good results, but is a pretty good place to start tone mapping adjustments. I used that kind of approach on this image. The large dynamic range coupled with the subtlety of tonal variation in parts of the image presented real problems when mapped into the 8-bit levels of a JPEG. So I squeezed some of the off-scale highlights and shadows into the 8-bit range in Capture NX2 and used a stepwise tone curve to allow the shadows on the near field and the range of irridescent shades in the near-field ice to still stand out while the brightness in the far field of the photo was mostly not blown out. This is not a great photo by any stretch of the imagination, but it does illustrate a tonally-demanding range that can be accommodated by selectively compressing and expanding the tone map. FWIW
    Gaps occur in between tonal peaks - explanation please

  5. #5
    Tim's Avatar
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    Re: Gaps occur in between tonal peaks - explanation please

    Sorry, I'm still missing something here.

    Snarkbyte - "All this really means is that flat spots in the tonal curve don't use up any of the available output range. In other words, if there is a gap in the histogram, there are no tones in that area, so why waste the available output range on an area of no information?"

    Yes, I understand that bit.

    Snarkbyte - "The idea is to use the tone curve tool to to create a flat response in the range of the gap; since there is no information there"

    Not sure if that's quite what the tutorial is suggesting...

    tclune - "...by selectively compressing and expanding the tone map."

    Well THAT's what I THOUGHT the tutorial meant. The 'After' image shows the range of darker tones have been 'moved up' the scale, and are now showing lighter values, compared to the sky, than they did before. A false image, yes, but a useful thing to do as it allows the dark areas more space to show their gradations of tone.

    This would be nice to do in Adobe Camera RAW, where the extra 'off the scale' data that normally gets discarded can be utilised. If not, then doing it in 16-bit PhotoShop would be the next best thing, I guess.

    I don't have Capture NX2, just do my processing in Adobe Camera RAW and PhotoShop CS3. Please can someone give a step-by-step guide to how you would expand the range of low values to replace a range of middle values, using 'Curves' or any other method.

    Am I right in thinking that the high ranges could be pulled down to replace a range of middle values as well? This would be useful to give more sky tones from available high-end data.

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    Re: Gaps occur in between tonal peaks - explanation please

    Quote Originally Posted by Tim View Post
    <...>

    I don't have Capture NX2, just do my processing in Adobe Camera RAW and PhotoShop CS3. Please can someone give a step-by-step guide to how you would expand the range of low values to replace a range of middle values, using 'Curves' or any other method.

    Am I right in thinking that the high ranges could be pulled down to replace a range of middle values as well? This would be useful to give more sky tones from available high-end data.
    Well, A 'step-by-step' guide would be difficult I think, as each image is different. Also, in my editor (GIMP) the only tool that gives me enough control to fill in gaps in the histogram would be the curves tool. The 'levels' tool would only remove empty regions at one or both ends of the histogram (and, as an aside, is a specialised example of the technique discussed here)

    I assume you know how to use the 'Curves' tool in your favourite image editor.
    Then it's just a case of creating a curve which is flat (near horizontal) in the 'empty' regions of the histogram. To decide what gets shifted into that empty region, you place the flat region either high (filling it up with dark tones) or low (filling the gap with light tones).

    Explanation: in order to have a curve that makes NO change in the image, you draw a straight line from bottom left to top right. Using that as a basis, every time your curve is above that line, you lighten the corresponding tones, every time it is below you darken the tones. The steeper the curve, the more contrast for those tones. (so an S curve will darken the darker tones, and lighten the light ones, increasing mid-tone contrast)

    Btw., every time you use a tool that influences your image globally (curves, levels, gamma, etc.), you're doing tone mapping...

  7. #7
    inkista's Avatar
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    Re: Gaps occur in between tonal peaks - explanation please

    Here. Try this tutorial on Levels and Curves. It's a bit more in-depth as well as a more gradual ramping up in describing the relationship between Levels and Curves.

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    Re: Gaps occur in between tonal peaks - explanation please

    Thanks Kathy That site is very informative for those of us who are not quite familiar on using the curves adjustment layer.
    Ron

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