Channel-Based Masks

When you're creating a selection, channels are often good candidates for the basis of a mask, and in some images, one might yield a mask without much work. For example, you might have an image of a red building against a blue sky. By finding the channel that contains the strongest contrast between the red and blue, you can create a copy of the channel, increase the contrast and "discover" a mask (Figure 9-3). To view individual channels, click each one in turn in the Channels palette. It helps to view them as grayscale when evaluating channel content. If you are seeing them in color, choose Preferences ^ Display & Cursors and deselect the option Color Channels in Color. When you find a likely candidate for a mask, create a copy by dragging it to the Create New Channel button, located next to the Trash in the Channels palette.

Figure 9-3: A—An image of the Tate Modern Gallery (London) that could benefit from having the burned out clouds replaced. B—A copy of the blue channel used as the basis for a mask. C—Levels used to crush shadows, expand highlights, and then stray levels painted with white and black to perfect the mask. D—Mask applied to a clouds layer. E—Final image minus burned-out clouds.

Figure 9-3: A—An image of the Tate Modern Gallery (London) that could benefit from having the burned out clouds replaced. B—A copy of the blue channel used as the basis for a mask. C—Levels used to crush shadows, expand highlights, and then stray levels painted with white and black to perfect the mask. D—Mask applied to a clouds layer. E—Final image minus burned-out clouds.

The copied channel might need a little bit of work to perfect the mask, which you can do easily with the Brush tool by painting with black or white (see "Using Painting Tools and Filters," later in the chapter). When you are happy with the mask, Ctrl-click (Windows), ^-click (Mac OS) to load it as a selection.

In addition to the Brush tool, you can use the Dodge and Burn tools on the channel to lighten or to darken detail around the mask's edges. To do so, select the corresponding range on the options bar (Shadows, Midtones, or Highlights) and then apply to remove unwanted detail. This way, you can preserve the mask edge while sending unwanted detail on either side of the edge to black or to white. This technique is particularly useful when you're creating masks with irregular edges, such as hair.

TIP You can use keyboard shortcuts to switch between the ranges for the Dodge and Burn tools. Shadows: Alt+Shift+S (Windows), Opt+Shift+S (Mac OS); Midtones: Alt+Shift+M (Windows), Opt+Shift+M (Mac OS); Highlights: Alt+Shift+H (Windows), Opt+Shift+H (Mac OS). You can also temporarily switch between the tools by holding down Alt (Windows), Opt (Mac OS).

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