Thursday, June 28, 2007

Contour Masks

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

For the most challenging selections, consider creating a contrast mask. Based on the luminance values of individual channels (R, G or B), a contrast mask can be used to define a contour by accentuating its contrast so dramatically that all shades of gray are eliminated, leaving only black and white values. Duplicate the channel with the best contrast in the area you wish to isolate. Or load it as a selection by dragging it to the Selection icon in the Channels palette and create a layer mask. Then accentuate the contrast of the alpha channel or layer mask using Curves. When accentuating contrast, pay particular attention to using increased contrast to define the contour you're concerned with. Avoid the temptation to use contrast to drive broad areas to white or black if doing so will adversely affect defining the contour precisely. On occasion, you may want to select a specific area of an alpha channel or a layer mask to accentuate contrast locally. Where unwanted values remain, select and fill or use a brush to paint an area with the appropriate value (black or white).

All selection tools can be used as a starting point from which contours can be manually refined. While the goal is to keep hand work to a minimum, increasing both efficiency and precision, some hand work is often required. Either save the selection and refine the resulting alpha channel or create a layer mask and refine it using a black or white brush to eliminate or add an area to be included.

The way you view an alpha channel or a layer mask can help you refine it. To see an alpha channel, click on it in the Channels palette. To see a layer mask, Option/Alt click on it in the Layers palette. You can see both the image and the alpha channel or layer mask while you're refining a contour. To view a layer mask and an image together, Option/Alt click the mask in the Layers palette. To view an alpha channel and an image simultaneously, click on the alpha channel in the Channels palette and then activate the eye icon to the left of the RGB composite channel. By default, the mask or alpha channel will be seen as an overlay of 50% red; however, both percentage and hue can be modified by double-clicking the mask or alpha channel  and entering alternate values.

A word of caution: Make sure only the eye icon is activated; don't highlight the RGB channel or you'll paint on the image rather than the alpha channel. You can even view both the image and the mask or alpha channel separately and simultaneously. To do this, create a new window for the image (Window > Arrange > New Window) and Option/

Alt click on the mask. If you're zoomed into a smaller portion of a larger image, you get both windows to mirror both zoom ratio and position (Window > Arrange > Match Zoom and Location).

Once defined, selected areas can be combined in a variety of ways. First, load one alpha channel as a selection (Select > Load Selection). To load a layer mask as a selection, target it in the Layers palette before loading the selection (or Command click on it). Second, load another selection, checking any of the following: Invert, Add to Selection, Subtract from Selection and Intersect with Selection.

You can reposition a contour using the filters Maximum and Minimum (Filter > Other > Maximum or Minimum). Think of them as affecting white, where Maximum makes the white area bigger and Minimum makes the white area smaller. Often, one or two pixels is all it takes to reposition a contour more precisely.

You can also reposition a contour by first blurring it (Filter > Gaussian Blur) and then lightening or darkening it using Curves. Increasing contrast with Curves will reduce the blurred effect, making transitions sharper. With this technique, you can precisely refine both the placement and edge quality of any contour. Rather than feathering selections (guessing at numbers), I prefer to blur masks (viewing the effect). When deciding how soft to make a contour, look to other nearby contours within the image as your guide.


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