What is the Unsharp Mask?

[Ansel]

Dear Togu and other Gurus of PS:

I am always puzzled by the Unsharp Mask function, known affectionately as USM, not related the other USM of Canon fame.

What exactly is the "Unsharp" mask and why do people use it to "Sharpen" their photos? Strange for people like me. Why would I choose the "Unsharp" function to perform a sharpening fuction? What does it actually do, algorithmically or optically?

I notice that their are 3 sliders when I pull up the USM function. The first one, Amount, is quite obvious. When I slide back and forth I can see the preview effect immediately.

It is the other two sliders that really made me scratch my head until I botak. What is the meaning of Radius and Threshold? I move the sliders for these two back and forth, I can see some changes to the picture, subtle changes, which seem a bit like contrast settings sometimes and brightness settings at other times. What exactly is happening when I adjust the radius and threshold?

What do I look for to determine the correct settings of all these sliders?

Thanks to all for any comments, instructions, and teaching.

Sincerely,
Your humble PS Goondu.

 

[Kei]

Might be helpful to you

http://www.bythom.com/sharpening.htm

 

[togu]

The Unsharp Mask filter helps to corrects blurring introduced when you took your photographs, scannings , resamplings. It basically locates pixels that are different from surrounding pixels by the threshold you specify and increases the pixel's contrast based on the amount you specify. The radius refers to the region to which each pixel is compared.

For the settings,

Amount - Enter a value to determine how much to increase the contrast of pixels. High res printed images will need an amount of 150% to 200%.

Radius - Enter a value to determine the number of pixels surrounding the edge pixels that affect the sharpening. A small value will most likely sharpens only the edge pixels, where a bigger number will sharpen a wider area of pixels. For High res printed images, 1 or 2 will be just nice.

Threshold - Enter a value to determine how different the sharpened pixels must be from the surrounding area before they are considered edge pixels and sharpened by the filter. Note that to avoid noise, you can try a value between 0 and 20. The default 0 value will sharpens all the pixels in the image.

Sometime, USM might caused bright colours to appear overly saturated. If that happens, you can try to convert the image to Lab mode and apply the filter to the Lightness channel only. The image will be sharpen without affecting the colour components. I had translated the method from Tom Shen's recommended site in this thread,

http://forums.clubsnap.org/showthread.php?t=45100&page=2


Hope this helps.