Customizing alpha channels

You can change an alpha channel’s name, color indication, and mask tint opacity in the Channel Options dialog box. By default, Canvas numbers alpha channels, sets the mask tint opacity to 50%, and assigns a mask color.

Canvas provides the mask color and opacity settings in the Channel Options dialog box as visual aids only. These settings do not affect the original image or channel.

To specify channel options:

1. With a paint object in edit mode, choose Image | Show Channels to open the Channels palette.
2. Click an alpha channel and select Channel Options in the pop-up menu, or double-click the channel you want to edit to open the Channel Options dialog box.
3. Enter a new name in the Name field to rename the channel.
   • Color Indicates: These options control whether white or black pixels in the channel will select pixels in the image when you load the channel. If you want white pixels in the channel to indicate selected pixels, choose Masked Area. If you choose Selected Area, the normal operation of the channel will be inverted, so that black pixels in the channel will select pixels when the channel is loaded. If you use this option, keep in mind that the channel will make selections that are the inverse of normal channel selections.
   • To change the mask tint color for the channel: Select a color from the Color pop-up menu. Canvas displays the tint when an alpha channel and at least one other channel are visible.
   • To change the opacity of the tint color: Enter a value from 1 to 100% in the Opacity text box.
4. Click OK after entering the settings you want.

Activating channels

To edit a channel:

Click the channel name in the Channels palette to make it active. Canvas uses shading to indicate that a channel is active.

• You can make more than one channel active by Shift-clicking the names of the channels in the palette.
• To make a channel visible but not active, click the left column to make an eye icon appear.
• You can make more than one channel visible by dragging in the left column in the Channels palette.

Painting tools and filters affect active channels only.

To make all color channels in an image both visible and active:

Click the composite channel in the Channels palette. The composite channel always appears first at the top of the Channels palette.

Editing an alpha channel

You can apply painting tools, filters, and effects to the image in an alpha channel. By editing the image, you can adjust what the channel will select when you load it as a selection.

1. With an image in edit mode, choose Image | Show Channels.
2. Create a new channel to use as a selection mask by doing one of the following:
   • If you have a selection in the image that you want to customize by editing in a channel, use the Save command to create a channel from the selection (see To save a selection in a channel:).
   • To start with a “blank” alpha channel, use the New Channel command. Be sure to choose the Masked Area option under Color Indicates in the New Channel dialog box.
3. In the Channels palette, click the new channel to activate it. The channel appears shaded in the palette and the image changes to show only the channel. Now click in the left column of the first (composite) channel. The original image appears with a transparent colored “mask” on the image. The color mask indicates the areas that will be masked — not selected — by the channel.
4. Use painting tools or filters to edit the image in the channel. The changes you make affect the active channel only.
5. Load the channel by choosing Image | Select | Load to make a selection with the channel.
6. Choose the channel name in the pop-up menu and then click OK.