getting started with photoshop - Channels & its features (3)

Modifying spot channels

You can edit a spot channel to add or remove color in it, change a spot channel's color or on-screen color solidity, and merge a spot channel with the image's color channels.

To edit a spot channel:

1. Select the spot channel in the Channels palette.

2. Use a painting or editing tool to paint in the image. Paint with black to add more spot color at 100% opacity; paint with gray to add spot color with lower opacity.

Note: Unlike the Solidity option in the Spot Channel Options dialog box, the Opacity option in the painting or editing tool's options determines the actual density of ink used in the printed output.

To change a spot channel's options:

1. Do one of the following:

• Double-click the spot channel thumbnail 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.

By selecting a custom color, your print service can more easily provide the proper ink to reproduce the image.

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); 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.

To merge spot channels:

1. Select the spot channel in the Channels palette.

2. Choose Merge Spot Channel from the palette menu.

The spot color is converted to and merged with the color channels. The spot channel is deleted from the palette.

Merging spot channels flattens layered images. The merged composite reflects the preview spot color information, including the Solidity settings. For example, a spot channel with a solidity of 50% will produce different merged results than the same channel with a solidity of 100%.

In addition, the resulting merged spot channels usually don't reproduce the same colors as the original spot channels, because CMYK inks can't represent the range of colors available from spot color inks.

Adjusting overlapping spot colors

To prevent overlapping spot colors from either printing over or knocking out the underlying spot color, remove one of the spot colors where they overlap.

Use a printed sample of the overprinted inks to adjust your screen display to help you predict how colors will look when printed.

Note: In some cases, such as varnish and bump plates, you may want colors to overprint.

To adjust overlapping spot colors:

1. In the Channels palette, select the spot channel with the color you want to print.

2. Choose Select > Load Selection.

To quickly select an image in a channel, hold down Ctrl (Windows) or Command (Mac OS), and click the channel in the Channels palette.

3. For Channel, choose the spot channel from step 1, and click OK.

4. To create a trap when knocking out the underlying color, choose Select > Modify > Expand or Contract, depending on whether the overlapping spot color is darker or lighter than the spot color beneath it.

5. In the Channels palette, select the underlying spot channel that contains areas you want to knock out. Press Backspace (Windows) or Delete (Mac OS).

This method can be used to knock out areas from any channels under a spot color, such as the CMYK channels.

6. If a spot color in one channel overlaps more than one other spot color, repeat this process for each channel that contains areas you want removed.

Using channel calculations to blend layers and channels

You can use the blending effects associated with layers to combine channels within and between images into new images using the Apply Image command (on single and composite channels) and the Calculations command (on single channels). These commands offer two additional blending modes not available in the Layers palette--Add and Subtract. Although it's possible to create new combinations of channels by copying channels into layers in the Layers palette, you may find it quicker to use the calculation commands to blend channel information.

The calculation commands perform mathematical operations on the corresponding pixels of two channels (the pixels with identical locations on the image) and then combine the results in a single channel. Two concepts are fundamental to understanding how the calculation commands work.

• Each pixel in a channel has a brightness value from 0 (off or black) to 255 (on or white). The Calculations and Apply Image commands manipulate these values to produce the resulting composite pixels.
• These commands overlay the pixels in two or more channels. Thus, the images used for calculations must have the same pixel dimensions.