The Unsharp mask is a good example of how bitmap images can be enhanced or improved by functions that alter or add pixel information intelligently.

 

 The only real information we have for a bitmap image is the information captured when the image was created (scanned or captured with a digital camera). Any improvements or enhancements we make to the image come from manipulating the original image information in some way. The Unsharp Mask is a Photoshop feature that can seemingly improve an image by sharpening the detail.

 

 As we know, if the sharpness was not captured when the image was created, it must be simulated in some way by manipulating the image pixels. The Unsharp mask does this by enhancing contrast (or creating more intense colour changes) along the edges of an image. The increased edge contrast makes the image detail stand out more, creating the impression of a sharper image.

 

To use the unsharp mask, choose Filter ->Sharpen ->Unsharp Mask from the Photoshop menu bar. This will display the Unsharp Mask dialog box.

 

 

If you put a check in the Preview check box, you will be able to see the current results of your settings in the main image area on the Photoshop screen. If it is unchecked, you will see the results of your settings in the dialog box’s preview window only.

 

The Unsharp mask function is controlled with three slider switches: the amount slider, the Radius slider, and the Threshold slider.

 

The amount slider controls how much the contrast will be intensified at the edges. If you do not set the amount slider high enough, you will see little change in the image. If you set the amount slider too high, you can introduce unwanted information in your image, degrading its appearance. Often, the amount slider will be set quite high (200 to 300) relative to the radius (often set to .5 to 2.5) and threshold (often set to 1 to 5). Remember, the numbers given here are only a guide, and for best results you should rely on your own judgment to find the most appropriate setting for the given image.

 

When you have finished making your settings and are happy with the preview, click the OK button to sharpen the image.