getting started with photoshop - Channels & its features (2)

Deleting channels

You may want to delete spot or alpha channels you no longer need before saving an image. Complex alpha channels can substantially increase the disk space required for an image.

To delete a channel (Photoshop):

1. Select the channel in the Channels palette.

2. Do one of the following:

• Alt-click (Windows) or Option-click (Mac OS) the Trash button.
• Drag the channel name in the palette to the Trash button.
• Choose Delete Channel from the Channels palette menu.
• Click the Trash button at the bottom of the palette. Then click Yes.

To delete a channel (ImageReady):

Choose Select > Delete Channel, and choose the channel from the submenu.

Note: When deleting a channel from a file with layers, visible layers are flattened and hidden layers are discarded. This is done because removing a color channel converts the image to Multichannel mode, which does not support layers.

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 alternative color spaces, such as YCbCr, which is used in digital video.
• Swap or duplicate channels.

To mix color channels:

1. In the Channels palette, select the composite color channel.

2. Choose Image > Adjustments > Channel Mixer.

3. For Output Channel, choose the channel in which to blend one or more existing (or source) channels.

4. Drag any source channel's slider to the left to decrease the channel's contribution to the output channel or to the right to increase it, or enter a value between -200% and +200% in the text box. Using a negative value inverts the source channel before adding it to the output channel.

5. Drag the slider or enter a value for the Constant option. This option adds a black or white channel of varying opacity--negative values act as a black channel, positive values act as a white channel.

6. Select Monochrome to apply the same settings to all the output channels, creating a color image that contains only gray values.

Use the Channel Mixer with the Monochrome option applied to control the amount of detail and contrast in the images you plan to convert to grayscale.

If you select and then deselect the Monochrome option, you can modify the blend of each channel separately, creating a handtinted appearance.

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.)

If you are planning to print an image with spot colors, you need to create spot channels to store the colors. To export spot channels, save the file in DCS 2.0 format or PDF.

Note: ImageReady supports Photoshop spot color channels as alpha channels.

Creating spot channels

You can create a new spot channel or convert an existing alpha channel to a spot channel.

To create a new spot channel:

1. Choose Window > Channels to display the Channels palette.

2. To fill a selected area with a spot color, make or load a selection.

3. Do one of the following to create a channel:

• Ctrl-click (Windows) or Command-click (Mac OS) the New Channel button in the Channels palette.
• Choose New Spot Channel from the Channels palette menu.

If you made a selection, that area is filled with the currently specified spot color.

4. Click the color box, and choose a color.

If you select a custom color, your print service provider can more easily provide the proper ink to reproduce the image.

5. 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); 0% simulates a transparent ink that completely reveals the inks beneath (such as a clear varnish). You can also use this option to see where an otherwise transparent spot color (such as a varnish) will appear.

Note: The Solidity and color choice options affect only the on-screen preview and the composite print. They have no effect on the printed separations.

6. To enter a name for the spot channel, choose a custom color in step 4, and the channel automatically takes the name of that color.

Be sure to name spot colors so they'll be recognized by other applications reading your file. Otherwise the file might not print.

To convert an alpha channel to a spot channel:

1. Do one of the following:

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

2. If needed, rename the channel.

3. Select Spot Color.

4. Click the color box, choose a color in the Color Picker dialog box, and click OK.

5. Click OK. The areas of the channel containing grayscale values are converted to spot color.

6. Choose Image > Adjustments > Invert to apply the color to the selected area of the channel.