Using Channels and Masks

Viewing channels

You can use the palette to view any combination of individual channels. For example, you can view an alpha channel and the composite channel together to see how changes made in the alpha channel relate to the entire image. By default, individual channels are displayed in grayscale.

Changing the display of the palette

You can show the individual color channels in color (rather than grayscale) in the Channels palette and specify the size of the thumbnails. Using thumbnails is the most convenient way of tracking channel contents; however, turning off the display of thumbnails can improve performance.

Adding spot colors.

Spot colors are special premixed inks used instead of, or in addition to, the process color (CMYK) inks. Each spot color requires its own plate on the press. (Because a varnish requires a separate plate, it is considered a spot color, too.) For information on printing spot color plates, see “Printing color separations”

To change a spot channel’s options:
1 - Do one of the following:

• Double-click the spot channel name in the Channels palette.
•  Select the spot channel in the Channels palette, and choose Channel Options from the palette menu.

2 -  Click the color box, and choose a color. For more information, see “Using the Adobe Color Picker” in online Help. By selecting a custom color, your print service can more easily provide the proper ink to reproduce the image. For more information, see “Choosing custom colors (Photoshop)” in online Help.

3 -  For Solidity, enter a value between 0% and 100%. This option lets you simulate on-screen the solidity of the printed spot color. A value of 100% simulates an ink that completely covers the inks beneath (such as a metallic ink).

Using the Calculations command

The Calculations command lets you blend two individual channels from one or more source images. You can then apply the results to a new image or to a new channel or selection in the active image. You cannot apply the Calculations command to composite channels.

To load a saved selection into an image:

1. Choose Select > Load Selection. For Document, the active filename is selected.
2. For Channel, choose the channel containing the selection you want to load.
3. Click Invert to make the nonselected areas selected and vice versa.
4. If the destination image already has a selection, indicate how to combine the selections. (For information on these options, see “Saving a mask  selection”
5. Click OK.

To load a selection from another image (Photoshop):

1. Open the two images you want to use. Note: The images must have identical pixel dimensions.
2. Make the destination image active, and choose Select > Load Selection.
3. For Document, choose the source image.
4. For Channel, choose the channel containing the selection you want to use as a mask.
5. Click Invert if you want to make the nonselected areas selected and vice versa.
6. If the destination image already has a selection, indicate how to combine the selections. (For information on these options, see “Saving a mask selection”
7. Click OK.

Merging channels

Multiple grayscale images can be combined into a single image. Some grayscale scanners let you scan a color image through a red filter, a green filter, and a blue filter to generate red, green, and blue images. Merging lets you combine the separate scans into a single, color image.

Splitting channels into separate images

You can split the channels of a flattened image into separate images. The original file is closed, and the individual channels appear in separate grayscale image windows. The title bars in the new windows show the original filename plus the channel abbreviation (Windows) or full name (Mac OS). Any changes since the last save are retained in the new images and lost in the original.

Mixing color channels

The Channel Mixer command lets you modify a color channel using a mix of the current color channels. With this command, you can do the following:

• Make creative color adjustments not easily done with the other color adjustment tools.
• Create high-quality grayscale images by choosing the percentage contribution from each color channel.
• Create high-quality sepia-tone or other tinted images.
• Convert images to and from some alternative color spaces, such as YCbCr.
• Swap or duplicate channels.