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Selections and Masks

Last month, I discussed masking and compositing in Adobe Photoshop. This month I will continue with some additional techniques for masking images. All of the selection tools in Photoshop are essentially masking tools. They allow you to select an area to manipulate while protecting another area from changes. These selection tools include the Rectangular and Elliptical Marquees, the Lassos and the Magic Wand (Fig 1).

Fig. 1

Using the Marquee tools is very straightforward. Draw a box or oval by clicking and holding the mouse button once for the origin point and then drag and let go of the mouse button to finish the shape. You will see the familiar dotted outline (or ‘marching ants’) indicating a selection is active. You can use any of the selection tools to click in the selected area and move the selection frame. This will not change the image; it will only move the dotted line border of the area to be selected.

You can use modifier keys to add to or subtract from an existing selection. These too work with any of the selection tools. Hold down the Shift key to add to a selection, hold down the Alt key (Option Key of a Mac) to subtract from a selection. This allows the user to create selection areas that are not simple rectangles or ellipses.

Any selection can be saved for future use. Choosing Save Selection from the Select menu will save the selection as a new channel. You can reload the selection at any time by choosing Load Selection from the Select menu or by hitting the Load Channel as Selection button (it looks like a dotted outline of a circle) on the Channels pallet. Saving selections in this manner will increase the file size of your image by as much as 50% on a grayscale image or 33% on an RGB image. Some file formats do not allow extra channels, so to preserve these channels be sure to save your image first in the Photoshop PSD format.

The Magic Wand tool is popular with many beginning users and is pretty simple to grasp. It will select an area of contiguous color within your image. The Tolerance setting ranges from 0 to 255 and controls how close in value the colors need to be to be selected. At zero tolerance, only the few pixels around your cursor will be selected when you click the Magic Wand; at 255 your entire image will be selected.
Sometimes, the Magic Wand’s selection area will yield a coarse edge. You can create your own custom mask using the image’s own channel information. My example, a bird against a blue sky, is a good choice for this technique. Since the sky is blue, I know there will be good contrast between the bird and sky in the Blue channel. You can look at each of the channels in the Channels pallet.

Fig. 2

To make this into a mask, duplicate the channel by dragging it to the New Channel button (it looks like a document) in the Channels pallet (Fig 2). Using the Levels (Image: Adjust: Levels) increase the contrast in the image so that the sky is white and the bird is mostly black. Move the Black Point triangle and the White Point triangle under the Histogram closer together (Fig 3). There will still be some white areas in the bird so paint them out black with the Brush tool (Fig 4). Once completed, you can load this selection. In the channel, anything white is selected, anything black is not. Since the sky is white, it is selected when you load the selection (Fig 5). To then select the bird, choose Select Inverse from the Select menu.

Fig. 3

Fig. 4

Fig. 5

Paul Vaughn is a freelance graphic artist, writer and web designer. If you would like to see the Graphics Guy address a specific topic email Paul Vaughn at Color examples can be seen at

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