Image enhancement
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March 10, 2008, 11:44 AM
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As Rolf recently mentioned in one of his videos, before the introduction of digital processing, the final image very much depended on the choice of film, printing paper and techniques and mixtures of chemicals at various temperatures and contact times. Also, a lot depended on the skill of the photographer in handling the equipment available. This is where I was some 30 years ago with my hobby, in the darkroom.
I have no difficulty in accepting the use of digital techniques in place of the film, paper and chemicals and other techniques such as burning and dodging and using various coloured filters on the camera. There is, however, one tool available to digital processing that I am having difficulty with, sharpening. In the pre-digital era an image was either in focus and sharp or it wasn't and I think the same should apply today. My understanding is that the process of digital sharpening is really an optical illusion and, as has recently been pointed out to me, may introduce unwanted, artificial artifacts.
I will stop here and I would love to read comments by others on this theme.
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March 10, 2008, 11:55 AM
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According to wikipedia sharpening techniques were used as early as the 30`s, alas nothing new to digital images.
You will also find that an out of focus photo is not possible to sharpen and still make it look good. A sharp digital image is however not as sharp as good old film, and therefore processing involving sharpening is for me nothing but good. There is however a very fine line between what looks good, and the cases of oversharpening that just looks terrible.
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March 10, 2008, 11:30 PM
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As Gunnar already said, there is a fine line between successful sharpening and overdoing it. I know two procedures which could help to prevent noise artifacts (to a certain degree of course).
1. Decompose the image to HSV and apply the unsharp mask only to the value channel. Then compose the hue, saturation and sharpened value channel again. This prevents colour noise which becomes quickly quite distracting. I have found this to work quite well. It permits to sharpen to a higher degree before noise becomes apparent.
2. Only sharpen the edges. Use a edge detector to find the edges in your image and use this as a mask for selective sharpening. This will not touch the areas without edges and thus avoids the building up of noise there. Although present noise at the edges is less apparent to the eye. This works also well but needs more fiddling, especially to find the "right" edges.
Another approach is deconvolution. Whereas the unsharp mask is rather a trick, deconvolution uses complicated mathematics to refocus an image. If I understood it correctly, it is close to the physical reality. Correct me if I am wrong here! I know of two plugins for GIMP, refocus and refocus-it, but I have never used them. Time to check them ;-)
I hope it helps.
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March 11, 2008, 12:02 AM
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Sharpening has been done in the darkroom too - it is built in into (most) films. If you look at a high contrast edge with a microscope you see a whiter and darker stripe running along the border. (I have never seen that myself but read it....) And the "unsharp mask" has its name from an old darkroom procedure.
It's necessary for most images from DSLRs and every RAW image. Most compacts do a lot of sharpening internally. But you can easily overdo it.
Up to now I have done these two videos about it:
http://meetthegimp.org/episode-6-selective-sharpening/
http://meetthegimp.org/episode-022-the-secrets-of-the-unsharp-mask/
Perhaps that needs some more covering.
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March 11, 2008, 03:36 PM
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I used to deliberately develop my film so as to produce high contrast edges. This was done by using a certain known contrasty developer, using it very weak and extending the development time. But then, I did refer to chemicals and processing in my opening conversation.
It is becoming clear to me from what has been written so far that I am tending to confuse image focus with sharpness. I realise they are not the same and sharpness certainly adds to impact.
I am very interested in trying out different sharpening techniques but I wonder if my sight is good enough to really differentiate between different amounts of sharpness and colour noise. I wonder if there is some sort of technique which enables the comparisons to be carried out that does not just rely on eyes, computer screen etc.
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March 11, 2008, 04:18 PM
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When it comes to sharpening I have found, mostly with portraits, that it is very important to have fine control over what areas to sharpen. I really like to sharpen hair, eyes, eyebrows, lips, and wrinkles that make up a person's characteristics. I have found that doing a high pass filter (desaturated) over the image (set to overlay or maybe hard light), with a black layer mask and painting in the detail I want with a white brush is very effective.
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March 11, 2008, 04:46 PM
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This seems to be very interesting. Is there any chance of illustrating the process in a video?
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March 11, 2008, 04:51 PM
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I have applied this technique to these two images:
The first one is a little overdone, but I kind of liked it with that particular portrait.
Steve Paxton has a nice videotutorial in PS, but it works fine in Gimp as well. Check it out at Paxton Prints under learning and "Learn Simple Techniques for Processing Urban Portraits".
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March 11, 2008, 05:24 PM
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Would it be possible to see the original images so that a comparison could be made? I like both of these especially the unshaven man. I remember trying to do similar things with film and paper combined with appropriate lighting. It was hard going.
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March 11, 2008, 06:20 PM
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Of course. here are the photos without my sharpening:
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March 11, 2008, 06:40 PM
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Thank you, I see what you mean by your comments regarding the man and the lovely lady becomes really beautiful. To be able to select where and how much to sharpen is a good thing.
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