Masks and Channels in Photoshop CS3

Adobe Photoshop uses masks to isolate and manipulate specific parts of an image. A mask is like a stencil. The cutout portion of the mask can be altered, but the area surrounding the cutout is protected from change. You can create a temporary mask for one-time use, or you can save masks for repeated use.

Working with Masks and Channels

Photoshop masks isolate and protect parts of an image, just like masking tape prevents a house painter from getting paint on the window glass or trim. When you create a mask based on a selection, the area not selected is masked, or protected from editing. With masks, you can create and save time-consuming selections and then use them again. In addition, you can use masks for other complex editing tasks—for example, to apply color changes or filter effects to an image.

In Adobe Photoshop, you can make temporary masks, called quick masks , or you can create permanent masks and store them as special grayscale channels called alpha channels . Photoshop also uses channels to store an image's color information and information about spot color. Unlike layers, channels do not print. You use the Channels palette to view and work with alpha channels.

A key concept in masking is that black hides, white reveals. As in life, rarely is anything black and white. So: shades of gray partially hide, depending on the gray levels (255 equals black (hidden), 0 equals white (revealed)).

How to Creating a Quick Mask

You'll begin the tutorial by using Quick Mask mode to convert a selection border into a temporary mask. Later, you will convert this temporary quick mask back into a selection border. Unless you save a quick mask as a more permanent alpha-channel mask, the temporary mask will be discarded once it is converted to a selection.

1. Choose File > Save As, rename the file and click Save. Click OK if a compatibility warning appears.

Saving another version of the Start file lets you return to the original if you need it.

You'll mask the Buddha statue so that you can separate it from its background and paste it in front of a new background.

2. In the Layers palette, click the Buddha layer name to select the layer.

3. Click the Quick Mask Mode buttonin the toolbox. (By default, you have been working in Standard mode.)

A. Standard mode B. Change Screen mode

In Quick Mask mode, a red overlay appears as you make a selection, masking and protecting the area outside the selection the way that a rubylith, or red acetate, masked images in traditional print shops. You can apply changes only to the unprotected area that is visible and selected. Notice, too, that the selected layer in the Layers palette appears gray, indicating you are in Quick Mask mode.

4. In the toolbox, select the Brush tool. 5. In the tool options bar, make sure that the mode is Normal. Then, click the arrow to display the Brushes pop-up palette, and select a large soft brush with a diameter of 65 pixels. Click off the palette to close it.

You'll use this large brush to rough out a mask, and refine it in the next exercise.

6. In the image, drag the Brush tool to paint a mask around the halo; the brush size should match the width of the halo. A red overlay appears wherever you paint, indicating the mask you're creating.

In Quick Mask mode, Photoshop automatically defaults to Grayscale mode, with a foreground color of black, and a background color of white. When using a painting or editing tool in Quick Mask mode, keep these principles in mind:

• Painting with black adds to the mask (the red overlay) and decreases the selected area.
• Painting with white erases the mask (the red overlay) and increases the selected area.
• Painting with gray partially adds to the mask.

7. Continue painting with the Brush tool to add the Buddha statue to the mask. Don't include the background.

Don't worry if you paint outside the outline of the statue. You'll fine-tune the mask in the next exercise.

8. In the Layers palette group, click the Channels tab to bring that palette forward, or choose Window > Channels. If necessary, expand the palette by dragging its lower right corner so that you can see all of it.

In the Channels palette, the default color-information channels are listed—a full-color preview channel for the CMYK image and separate channels for cyan, magenta, yellow, and black.

9. In the Channels palette, notice that this quick mask appears as a new alpha channel, named QuickMask. Remember, this channel is temporary: unless you save it as a selection, the quick mask will disappear as soon as you deselect.