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Thread: Unsharp mask on value channel

  1. #1
    GiacomoD's Avatar
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    Unsharp mask on value channel

    While postprocessing my photos in GIMP, I'm rarely satisfied of the results of the sharpening. Often I find it excessive, giving to the image the look of an ordinary post card. So, I'm still looking for the optimal settings
    This afternoon, trying to learn more on sharpening techniques, I've found on the GIMP User Manual the following tip:

    Sometimes using Unsharp Mask can cause color distortion where there are strong contrasts in an image. When this happens, you can often get better results by decomposing the image into separate Hue-Saturation-Value (HSV) layers, and running Unsharp Mask on the Value layer only, then recomposing.

    Does anybody knows more about this approach?

    I made a test on an image applying the traditional Unsharp Mask and then Unsharp Mask on Value channel only. I intentionally exagerated the sharpening settings in order to more easily evaluate the differences.
    Here the results: do you notice any significant difference or improvement?

    #1 No sharpening
    Unsharp mask on value channel

    #2 Unsharp mask (Settings: Radius=3 pixels, Amount=0.40, Threshold=20)
    Unsharp mask on value channel

    #3 Unsharp mask on value channel (same settings as #2)
    Unsharp mask on value channel

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    Re: Unsharp mask on value channel

    HI Giacomo-- I do see a difference with #3 being the best (IMO). Sharpening is tricky I've found. Not much forgiveness- it can be going good, good, good, then wham it's over done. I think #3 has the right amount of contrast in the shadows and crispness in the edges- looks good. Sorry I can't help with GIMP- never worked with it--mary

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    Moderator Donald's Avatar
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    Re: Unsharp mask on value channel

    Quote Originally Posted by GiacomoD View Post
    While postprocessing my photos in GIMP, I'm rarely satisfied of the results of the sharpening. Often I find it excessive, giving to the image the look of an ordinary post card. So, I'm still looking for the optimal settings
    This afternoon, trying to learn more on sharpening techniques, I've found on the GIMP User Manual the following tip:
    Sometimes using Unsharp Mask can cause color distortion where there are strong contrasts in an image. When this happens, you can often get better results by decomposing the image into separate Hue-Saturation-Value (HSV) layers, and running Unsharp Mask on the Value layer only, then recomposing.

    Does anybody knows more about this approach?
    Giacomo

    I have seen the text you quote, but do not know about the approach, as I have never used it.

    All my sharpening using the GIMP is done with the Wavelet Sharpen plug-in. It was recommended to me by another GIMP user some years ago and I have never used the Unsharp Mask Tool since then. I think it is excellent.

    You can access the plug-in here. At least you might want to install it to then assess how useful you find it.

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    Re: Unsharp mask on value channel

    Hi Giacomo,

    I am not a GIMP user, but I think, in Photoshop or Elements, your settings translate to Radius 3, Amount 40%, Threshold 20.

    I have to say that for output/final sharpening I would never use a radius that wide, rarely use an amount that low and never use a threshold that high.

    For 'creative sharpening' (which I rarely do given what I shoot), I would still never use a threshold that high because (assuming GIMP numbers work like Photoshop numbers for the threshold levels), it means you're leaving a lot of fine, low contrast detail completely unsharpened.

    However, I shoot RAW, if you are starting from a jpg capture, then really, all bets are off, because that's already been sharpened by the camera - and looking at the first 'no sharpening' shot, I think this might be the case.

    To me #2 looks slightly 'more sharpened' than #3, although the effect is subtle and (for my eyes), only apparent in the high contrast edges.

    Hope that helps,

  5. #5
    GiacomoD's Avatar
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    Re: Unsharp mask on value channel

    Thank you all for you replies.
    Mary, the procedure mentioned for GIMP can be surely applied also in Photoshop. I don't use it, but i suppose it should not be difficult. If you want to try...

    Donald, this plug-in sounds interesting. I used in the pst the wavelet-decompose for softening skin and I was very happy with that plug-in. If this one uses the same approach, it will be perfect! I see that the compilation for Windows is also available, for sure I will try it. Thank you for the suggestion!

    Dave, unfortunately I don't know what are the equivalent settings in Photoshop. What I can tell you is that Radius is expressed in pixels (so it depends on the image size), the Amount bar range is from 0 to 10 (so, it could mean that 0.40 equals to 4% in Photoshop, but I'm not sure at all) while the Threshold range is 0-255 and it's the minimum difference between pixels values that indicates an edge where sharpen must be applied.
    I shoot always in raw and I apply a slight sharpening in raw conversion. Are you suggessting to set to zero the sharpening in this phase?
    Anyway, in this test I purposely used the "heavy hand" in settings with the goal of making more evident the differences between the two techniques. Actually, my trend on images of similar size is to use settings such as Radius=3, amount=0.3 and Threshold between 10 and 20. I think I will have to make more and more tests to find out the optimal combination of parameters.

    Giacomo
    Last edited by GiacomoD; 30th May 2013 at 11:32 AM.

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    Re: Unsharp mask on value channel

    For me, examining files of this small size leads only to inconclusive results when considering the question that you posed about seeing differences between them. If you post crops of each full-size image viewed at 100%, you have a better chance of deciding for yourself which method you prefer and I think you'll also get more meaningful responses about the two treatments.

    By the way, that was Donald that offered the idea of using the plug-in, not Mike (not me or any other Mike).

  7. #7
    GiacomoD's Avatar
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    Re: Unsharp mask on value channel

    Quote Originally Posted by Mike Buckley View Post
    For me, examining files of this small size leads only to inconclusive results when considering the question that you posed about seeing differences between them. If you post crops of each full-size image viewed at 100%, you have a better chance of deciding for yourself which method you prefer and I think you'll also get more meaningful responses about the two treatments.

    By the way, that was Donald that offered the idea of using the plug-in, not Mike (not me or any other Mike).
    I apologize both with Donald and with every Mike on this forum for my mistake, I don't know how it could happen.
    Mike (Mike? .... yes!), I will follow your suggestions and I will post a small crop of the full size-image with the two treatments.

  8. #8
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    Re: Unsharp mask on value channel

    Hi Giacomo,
    I've never used Gimp, but reading your post I've linked the "Hue\Saturation\Value sharpening" to the "Lab sharpening" in photoshop.
    Don't know how these two procedures are similar or not, but maybe, reading something outhere on the web about Lab sharpening in photoshop can help you understand better.
    I suppose you already know what is Lab in potoshop... don't you?

    hope this helps, or some guys here will deny me...

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    Re: Unsharp mask on value channel

    Nicola seems to have mentioned it first but I equate what you are doing in GIMP with LAB in Photoshop where one sharpens the L layer as A &B are colour layers, and with Paint Shop Pro I have CMYK where sharpening is done on the K layer. But I have not bothered to do this for some years now and I think the undo button is the most important tool for when you go overboard. I also have HighPass in PSP.

  10. #10

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    Re: Unsharp mask on value channel

    You can do something similar on Photoshop, and probably other editors too.

    Apply USM, with a larger amount than looks right, then Edit>Fade>mode Luminosity>Opacity 50-80%.

    Or go to CMYK, and sharpen the black(K) channel, plus the unwanted colour.

    I got both of these from Dan Margulis' book, Professional Photoshop.

    HTH

    Peter

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