Thursday, June 28, 2007

Contour Masks

Use this powerful tool to affect precise areas of your image smoothly

Contour MasksDigital imaging offers the ability to define complex contours efficiently and precisely, enabling users to affect an image in very specific areas. What was once tedious and challenging is now quick and easy. Once you learn a few essential selection and masking techniques, few contours will elude your grasp. Before I continue, let me caution you against defining contours too precisely. Remember, contours in continuous-tone images are often quite soft. What's more, many times photographers simply need to define broad areas to work smooth transitions into surrounding areas. Just because you can define contours precisely doesn't mean you should, but it's advantageous to have the option when you need to.

Defining Contours

While it's sometimes unavoidable, besides defining an area by hand, either with a Selection or Brush tool, there are a number of more efficient and precise ways to isolate contours: using the Magic Wand tool, the Magnetic Lasso tool or the Color Range feature, and deriving a contour mask from a contrast mask. Try these first.

All of these techniques use contrast within an image to isolate a contour. Contrast in any one component of color can be used—luminosity (light and dark), hue (warm and cool) or saturation (intense or desaturated). If contrast helps define a contour, then you can accentuate the contrast of any one component of color within an image to make it easier to define a contour by using an adjustment layer. Once the contour has been defined, you can throw away the adjustment layer that was used to facilitate selection rather than enhance the image.

Many images contain broad areas of contrasting color that make selection easy—a blue sky, for instance. Found in the toolbar, the Magic Wand tool is ideal for such simple selections. Two check boxes give you control over how the Magic Wand tool behaves: Tolerance (which defines the range of related values away from the sampled color that will be included in the selection, based on 256 levels); and Contiguous (which limits a selection to areas of similar color that abut one another).

The Magnetic Lasso tool can be used for slightly more difficult contours. Using it, you can add an extra degree of discrimination manually. The tool will define the majority of the contour for you if there's adequate contrast between it and surrounding areas. While the default settings often provide excellent results, Width (the distance from the path drawn where contours will be detected), Edge Contrast (the amount of contrast required for a contour to be detected) and Frequency (the number of points placed while defining the contour) can be used to modify the tool's sensitivity.

Drawing a contour too quickly will reduce the accuracy of the tool. While defining the contour, if points are placed that are undesirable, you can hit the Delete key to eliminate previously placed points, one at a time. When using the Lasso and Marquee tools, practice drawing selections in closed loops. If you define only part of a contour and then let go, a straight line will snap between your start and finish points. All selection tools can be used multiple times to define a selection and can be used in combination with one another; hold the Shift key to add to and the Option/Alt key to subtract from an existing selection.

Found under the Select menu, the Color Range feature is very useful for complex contours involving multiple areas. Color Range has predefined settings to automatically detect reds, greens, blues, cyans, magentas, yellows, highlights, midtones and shadows. In addition, it has a Sampled Colors feature that can be used to define a custom range of colors that can be increased or decreased by using the Add to Sample (+) and Subtract from Sample (-) droppers or expanded and contracted using the Fuzziness slider. The selections Color Range makes are often more sophisticated than the previous tools mentioned, defining areas not just with black and white but also with shades of gray. Sometimes it's difficult not to have shades of gray where you don't want them. In this case, once the selection has been converted into an alpha channel or a layer mask, the contrast of either can be accentuated using Curves to eliminate unwanted values.


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