Sharpening Using Luminance & Density Masks

There are countless ways to sharpen an image.  Unsharp Mask, Smart Sharpen, High Pass.  A while back I wrote an article on sharpening using Smart Objects to make your sharpening more versatile.  Today we’re going to look at a different way of doing creative sharpening using luminance and density masks.

Luminance masks are masks that are created using the pixel information in the image itself.  A density mask is the inverse of a luminance mask.  Luminance masks work on the lighter areas of an image.  Density masks work on darker areas of an image.  Simple, right?

Let’s look at a couple examples.  In the two images below, I’ve created a Luminance Mask on the first and a Density Mask on the second.  After creating the mask I hit the Q key on the keyboard to enter into Quick Mask mode so you can see the difference.

Luminance Mask (click for larger version)

Density Mask (click for larger version)

You can see that the masks on the two images are different.  To create the luminance mask, press CTRL+ALT+2 in PS CS5 on your keyboard.  In earlier versions the shortcut is CTRL+ALT+~.  Alternatively you can hold CTRL and click on the RGB thumbnail in the Channels palette.  Creating the density mask is simply a matter of inverting the luminance mask.  Go to Select>Inverse or use CTRL+SHIFT+I on your keyboard.

What these masks do is select parts of the image based on the pixel values.  For the luminance mask pixels with higher brightness are more selected.  With the density mask, pixels with lower brightness are more selected.  The cool thing about these masks is that they’re what are referred to as ‘self-feathering’.  For example, the luminance masks selects pixels with values higher than 128 more than others but pixels with values below 128 are still selected to a lesser extent.  This allows the effect of any adjustment made using these masks to softly roll off.  If you were to select an area using a painted mask and apply an adjustment, you affect only those pixels under the painted mask.  You can feather the selection which will help but it’s not the same.  Luminance masks are useful when you want to affect the entire image.  Painted masks are more useful when you want to localise an adjustment to a specific area.  The histograms for these two selections show the selection and how soft the rolloff is to other areas.

With these masks you can sharpen different parts of the image differently.  This can be useful, for example, if you have an area of open sky, that you don’t want sharpened.  The sky will typically be a lighter area of the image.  Using the density mask you can sharpen other areas of the image and leave the sky basically unaffected.

You can also refine the masks.  In Quick Mask Mode you’ll see the Quick Mask as a channel on your Channels palette.  Holding CTRL+ALT+SHIFT on the keyboard and clicking on the Quick Mask thumbnail you’ll cut the active selection in half.  You can do this to either the density or luminance masks and you can do it multiple times.  This way, if the sky in our theoretical image is a bit darker and gets selected with our density mask, we can cut the density mask in half deselect the sky.

In the image below, I don’t want the sky or the white water of the waterfall sharpened.  But I do want to sharpen the trees and rocks.

Selective Sharpening (click for larger version)

Using a density mask I can achieve this selective sharpening.

Waterfall Sharpened (click for larger version)

Try out this neat creative sharpening trick for yourself and let me know how you like it.

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