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Unsharp mask - Amateur Photography magazine article


zippie 17 1.3k England
1 Mar 2004 9:23AM
Pat (Timecatcher) did you try the action I emailed you, did it work ?

Andy
lobsterboy Plus
17 14.9k 13 United Kingdom
1 Mar 2004 9:55AM
Not sure what the ap artical recommended but I find LAB sharpening is really good:
Change the image mode to LAB
Create a new layer
Select the lightness channel only.
Sharpen it quite quite heavily.
show all the channels.

I have an action that does this for me (allowing me to change the sharpening levels) and it seems to work pretty good. Doing the sharpening on a new layer allows you to erase it to leave areas quite soft.

Chris
gibbsy 17 403 Wales
1 Mar 2004 2:56PM
Right, here goes with my sharpening advice, it's a lot longer than what Chris has described above. I'll give you the whole paper from night school.

We have all had an image/photo that we wished was a little bit sharper, or that little bit more rich in detail. Photoshop has a number of filters than can sharpen our image.

You may have already dabbled with sharpening filters already. They work by detecting changes in colour between pixels and then adjusting their contrast to make them stand out, basically finding edges/lines and making them more defined, thus giving the impression of more detail. However, the most commonly used method of sharpening is to sharpen the whole image at once. This can be done by selecting filter, sharpen then clicking on sharpen. This may be followed by sharpen more.

What happens however is that ALL of the pixels in the image are adjusted. The end result can be quite disturbing, a grainy image with garish colours. An all together better method is that of the unsharp mask. Contrary to its name, it is neither a mask or a means of unsharpening an image. It actually sharpens the image in much the same way as the standard sharpen filter. The main difference is that the unsharp mask affords you much more control. Not only can you control how much the image gets sharpened, you also have control over the radius of pixels it affects. The final control point is threshold.

Threshold supposedly lets you control how the computer decides what constitutes a line or edge, the higher the value the less accurate the computer is. For example, with a low threshold set, the slightest change in colour would mean that the computer would think that it is an edge or line. A high value would mean that the colour change would have to be slightly more drastic for the computer to consider it a line or edge.

My main 'beef' with this tool is that of threshold. There is no middle ground. Basically this means that if the computer deems an area to be an edge, it adjusts it. This applies to the whole image. Surely it would be better if we could tell the computer what areas were edges? This way the only areas that would get sharpened would be the areas that actually need sharpening. Well it just so happens there is such a technique. What is this magical technique I speak of? It is called the Edge Mask. It goes a little something like this...

Load up your image as you normally would. The first step is to make a duplicate of the background layer.

Now select channels. We need to select just one channel so we can work in black and white temporarily. This can be done by clicking on one of the channels, for example the red channel, right click and select duplicate channel, select ok.

Now, make sure that the channel copy is selected (highlighted in blue in the channels tool box). Now select: 1 Filter, 2 Stylise, 3 Find Edges. This now needs to be inverted by pressing Ctrl + I. Now select 1 Filter, 2 Noise, 3 Median, enter a radius, for most images a number between 2 and 7 is enough. This thickens our lines slightly to make them eventually stand out a bit more.

Next 1 Filter, 2 Other, 3 Maximum. This step thickens up the edges a bit more, again enter a value between 2 and 7 here. You may notice that the lines have gone slightly pixelated now. Not to worry, use Gaussian Blur to smooth them out again. 1 Filter, 2 Blur, 3 Gaussian blur. Enter the same value here as you entered in for maximum in the previous step.

Now press and hold the Ctrl key and click once on the red channel copy. This should turn all the white areas into a selection. Now click once on RGB from the top of the channel tool box. Your image should be restored into full colour and your selection will remain. Now click on the layers tab and select our backgound copy layer. Then choose 1 Filter, 2 Sharpen, 3 Unsharp Mask. Set the radius to a vlue of 2.0 LEAVE THE THRESHOLD AS IT IS...ZERO. Adjust the Amount to increase the strength of the effect. You can see the effect in the preview window. When satisfied, click ok. You can compare the difference in the image by toggling the visibility of the background layer copy. Finally flatten image.

As I said, it looks long winded, but the lecturer has convinced me that it is an excellent technique. Have a go and let me know what you think.

Now, as in the Oscar ceremony I have to thank:
Andrew, our whizz kid lecturer.
The 'Monotype' corporation for teaching me to touch type in the late 1960s.
You know, I really should get out more.
Now where the bloody tea?
keithh 17 25.7k 33 Wallis And Futuna
1 Mar 2004 3:09PM
Yep, it's long winded.

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