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Creating an Independent Mask Channel

The problem with masks generated using the quick mask mode and Color Range command is that they're here one day and gone the next. Photoshop is no more prepared to remember them than it is a lasso or wand selection.

Most of the time, that's okay because you'll use the selection only once. But what if the selection takes you a long time to create? What if, after a quarter hour of Shift-clicking here and Alt-dragging there, adding a few strokes in the quick mask mode, and getting the selection outline exactly right, your boss calls a sudden meeting or the dinner bell rings? You can't just drop everything; you're in the middle of a selection. Nor can you convey your predicament to non-Photoshop users because they'll have no idea what you're talking about and no sympathy for your plight.

The simplest solution is to back up your selection, save your file, and move on to the next phase of your life. In fact, anytime that you spend 15 minutes or more on a selection, save it. After all, you never know when all heck is going to break loose, and 15 minutes is just too big a chunk of your life to repeat. (The average person racks up a mere 2.5 million quarter hours, so use them wisely!) You wouldn't let 15 minutes of image editing go by without saving, and the rules don't change just because you're working on a selection.

Saving a selection outline to a mask channel

The following steps describe how to back up a selection to an independent mask channel, which is any channel above and beyond those required to represent a grayscale or color image. Mask channels are saved along with the image itself, making them a safe and sturdy solution.

STEPS: Transferring a Selection to an Independent Channel

Convert the selection to a mask channel. One way to do this is to choose Select Save Selection or right-click (Control-click on the Mac) in the image window and choose Save Selection from the pop-up menu, which saves the selection as a mask. The dialog box shown in Figure 9-32 appears, asking you where you want to put the mask. In most cases, you'll want to save the mask to a separate channel inside the current image. To do so, make sure that the name of the current image appears in the Document pop-up menu. Then select New from the Channel pop-up menu, enter any name for the channel that you like, and press Enter or Return.

Figure 9-32: The Save Selection dialog box enables you to convert your selection outline to a mask and save it to a new or existing channel.

If you have an old channel you want to replace, select the channel's name from the Channel pop-up menu. The radio buttons at the bottom of the dialog box become available, permitting you to add the mask to the channel, subtract it, or intersect it. These radio buttons work like the equivalent options that appear when you make a path into a selection outline (as discussed in the preceding chapter), but they blend the masks together, instead. The result is the same as if you were adding, subtracting, or intersecting selection outlines, except it's expressed as a mask.

Alternatively, you can save the mask to a new multichannel document all its own. To do this, choose New from the Document pop-up menu and press Enter or Return.

Regardless of which of these many methods you choose, your selection outline remains intact.

View the mask in the Channels palette. To do so, click the appropriate channel name in the Channels palette — automatically named Alpha 1 unless you assigned a name of your own. In Figure 9-32, I replaced the contents of a channel called Existing Mask, so this is where my mask now resides.

This step isn't the least bit mandatory. It just lets you see your mask and generally familiarize yourself with how masks look. Remember, white represents selection, black is the deselected area, and gray is a partial selection.

Return to the standard image-editing mode by clicking the first channel name in the Channels palette. Better yet, press Ctrl+1 (z -1 on the Mac) if you're editing a grayscale image or Ctrl+tilde (z -tilde on the Mac) if the image is in color.

Save the image to disk to store the selection permanently as part of the file. A handful of formats — PICT, Pixar, PNG, TIFF, Targa, PDF, and native Photoshop — accommodate RGB images with an extra mask channel. But only TIFF, PDF, and the native Photoshop format can handle more than four channels, all saving up to a total of 56 channels. I generally use the TIFF format with LZW compression when saving images with masks.

Both the native Photoshop format and TIFF can compress masks so that they take up substantially less room on disk. The Photoshop format does this automatically. When saving a TIFF image, be sure to turn on the LZW Compression check box. In both cases, this run-line compression is entirely safe. It does not change a single pixel in the image; it merely writes the code in a more efficient manner.

Converting a mask to a selection

To retrieve your selection later, choose Select Load Selection. A dialog box nearly identical to the one shown in Figure 9-32 appears except for the addition of an Invert check box. Select the document and channel that contain the mask you want to use. You can add the mask to a current selection, subtract it, or intersect it. Select the Invert option if you want to reverse the selected and deselected portions of the mask.

Want to avoid the Load Selection command? Ctrl-click (Win) or z -click (Mac) the channel name in the Channels palette that contains the mask you want to use. For example, if I Ctrl-clicked the Existing Mask item in Figure 9-32, Photoshop would load the equivalent selection outline into the image window.

You can convert color channels to selections, too. For example, if you want to select the black pixels in a piece of scanned line art in grayscale mode, Ctrl-click (Win) or z -click (Mac) the first item in the Channels palette. This selects the white pixels; press Ctrl+Shift+I (z -Shift-I on the Mac) or choose Select Inverse to reverse the selection to the black pixels.

Viewing mask and image

Photoshop lets you view any mask channel along with an image, just as you can view the mask and image together in the quick mask mode. To do this, click in the first column of the Channels palette to toggle the display of the eyeball icon. An eyeball in front of a channel name indicates that you can see that channel. If you are currently viewing the full-color image, for example, click in front of the mask channel name to view the mask as a translucent color coating, again as in the quick mask mode. Or if the contents of the mask channel appear by themselves on screen, click in front of the composite name (RGB, CMYK, or LAB) to display the image as well.

Using a mask channel is different than using the quick mask mode in that you can edit either the image or the mask channel when viewing the two together. You can even edit two or more masks at once. To specify which channel you want to edit, click the channel name in the palette. To edit two channels at once, click one and Shift-click another. All active channel names appear highlighted.

You can change the color and opacity of each mask independently of other mask channels and the quick mask mode. Double-click the mask channel thumbnail or choose the Channel Options command from the Channels palette menu. (This command is dimmed when editing a standard color channel, such as Red, Green, Blue, Cyan, Magenta, Yellow, or Black.) A dialog box similar to the one shown back in Figure 9-11 appears, but this one contains a Name option box so you can change the name of the mask channel. You can then edit the color overlay as described in the "Changing the red coating" section earlier in this chapter.