Page 108 - Enhancing CAD Drawings with Photoshop

4386.book Page 91 Monday, November 15, 2004 3:27 PM

SHARPENING AND BLURRING 91

Sharpening and Blurring
Sharpen and blur at the end of your digital darkroom work flow because they can significantly alter
image data. Correct the tone, balance the color, and retouch before sharpening and blurring to min-
imize banding and data loss due to overmanipulation.
Sharpening and blurring are opposites—one clarifies, and the other obscures. You use both tools
to your advantage in Photoshop. You sharpen images to make the edges more visually defined. We
prefer images with distinct edges because they resonate with the perception of form in our minds.
Photoshop sharpens images by comparing pixels that differ in value from their neighbors and
increases the contrast between them. Within the radius of this comparison, the bright areas get brighter
and the darks get darker. Even though no true hard edge is really defined in the matrix of pixels, our
visual systems ultimately perceive areas of higher contrast as an edge.
The amount of sharpening required varies in each image and depends on subject matter, resolu-
tion, and quality of the digital camera or scanner used to import the image into Photoshop. Every dig-
ital photograph benefits from at least a little sharpening.

WARNING You cannot effectively sharpen severely blurred images because there is not enough
detail to enhance.
As the contrast between pixels is increased by sharpening, sometimes undesirable color halos
appear, especially in images that have extreme levels of sharpening. Halos tend to spread around
areas of overly saturated color in sharpened images. To reduce this unwanted effect, you can blur the
color halos to remove noise.
TIP When the woven grid of a garment’s fibers is close to the scale of the pixel grid, interference pat-
terns, called moirés, result. You can remove a moiré pattern by blurring the color of the affected
regions.

Sharpening and Removing Color Noise
You will now learn techniques to both sharpen and blur images to enhance edges and remove
unwanted color noise.
1. Open the file Basilique St Nazaire de Carcassonne.jpg from the CD. A version of this
image is included in the color section.
2. Choose Filter Sharpen Unsharp Mask to open the Unsharp Mask dialog box, as shown in
Figure 3.22.

3. Change Amount to 100%, Radius to 2.0 pixels, and Threshold to 3 levels, and click OK.

NOTE For best results when using Unsharp Mask, the Amount slider should range only from 50%
to 100%, Radius between 1 and 3 pixels, and Threshold between 3 and 10 levels. Although greater
amount and radius values result in increased sharpening, threshold is counterintuitive with
greater sharpening occurring with fewer levels.
4. Immediately choose Edit Fade Unsharp Mask, or press Shift+Ctrl+F to open the Fade dialog
box (see Figure 3.23). Fade only works immediately after you perform an action. If you select
another tool, Fade will not work.