Emphasizing Edges

Sharpening works by increasing the contrast around edges. Edges in images always involve darker tonal values adjacent to lighter ones. We can emphasize the edges by making the dark tonal values darker and the light tonal values lighter.

In the analog darkroom, this was accomplished using an unsharp masksee the sidebar, "Why Is It Called Unsharp Mask When It's Used to Sharpen?" for details on just how this process worked. In the digital domain, we sharpen by identifying the dark and light pixels that represent edges, and lowering the value of the dark pixels to make them darker while raising the value of the light pixels to make them lighter.

When we do this, we create a "halo" that makes the edges, and hence the entire image, seem sharper. The concept is simple, but as with many things in digital imaging, the devil is in the details that we discuss throughout this book!

Figure 1-1 shows the same image before and after sharpening. (The image also illustrates the pitfalls of driving in rural Scotlanda good metaphor for the myriad things that can go wrong when we use sharpening inappropriately!)

Figure 1-1. Before and after sharpening

The only difference between the two versions is the sharpening. Figure 1-2 shows a zoomed-in comparison with an accompanying graph of the values of a single row of pixels, before and after sharpening.

Figure 1-2. Sharpening up close

Notice that the tonal range of the sharpened versionthe distance between the lightest and darkest tonesis wider than that of the unsharpened version. Notice too that the biggest differences occur at the edge transitions of the capital "O" while smaller differences emphasize the texture of the sign's background.

Sharpening is closely related to contrast, but simply increasing the contrast over an entire image just produces an over-contrasty image. Successful sharpening demands that we localize the contrast boost to those parts of the image that actually represent edges.

Why Is It Called Unsharp Mask When It's Used to Sharpen?

Sharpening predates digital imaging by decades. If you've often wondered why one of Photoshop's main sharpening tools is named the "Unsharp Mask" filter when it's supposed to make the image sharper, rest assured that you're not alone.

The name originates in an analog wet darkroom technique. It increases the apparent sharpness of a photographic print using a duplicate of the negative to create a mask that increases contrast along the edges.

The original and the duplicate negatives are placed on either side of a piece of glassoften just plain old window glassand the entire sandwich is placed in the enlarger's negative carrier.

When the enlarger is focused on the bottom negative, the top, out-of-focus copy creates a contrast mask that boosts the contrast along the edges in the image as the out-of-focus dark contour burns the dark side of the edges and the out-of-focus light contour dodges the light side of the edges.

The technique is called "un-sharp masking" because the maskthe top, out-of-focus negativeis out of focus and hence isn't sharp. In short, it's an unsharp mask that has the effect of increasing the apparent sharpness in the print!

So the "unsharp" in "unsharp mask" refers to the mask, not the result, and it's this analog technique that Photoshop's Unsharp Mask filter replicates (only with a great deal more control than its analog counterpart!)

Real World(c) Image Sharpening with Adobe Photoshop CS2(c) Industrial-Strength Production Techniques
Real World Image Sharpening with Adobe Photoshop CS2
ISBN: 0321449916
EAN: 2147483647
Year: 2007
Pages: 71
Authors: Bruce Fraser

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