When you have made a selection in an image, you can create an alpha channel from the selection. An alpha channel preserves the shape and opacity of a selection, so you can use it to make the same selection in the image again.
You can think of an alpha channel as a mask that selects some areas and protects other areas from painting and image editing. Because alpha channels make precise selections of varying intensities, alpha channels let you control which areas in an image will be affected by painting tools and filters, and the intensity of the effects.
When you view an alpha channel, you see a grayscale image. When the channel is made from a selection, it contains white areas representing fully selected pixels, black areas representing unselected pixels, and gray areas representing pixels that are partially selected, with the gray lightness values corresponding to the selection opacity.
This section describes how to save a selection in a channel, and how to load a channel to make a selection in an image, using dialog boxes. You can also use shortcuts in the Channels palette, as described under Channels Palette Options.
You can also click the Selection button in the Channels palette to save the current selection in a new channel.
Removes any current selections and creates a new selection
Add to Selection
Preserves the current selection and selects additional pixels based on the channel
Subtract from Selection
Removes pixels from the current selection based on the channel
Intersect with Selection
Creates a new selection composed of pixels that appear in both the current selection and the channel you are loading
Canvas X Draw image channels are compatible with the alpha channels used in other image-editing programs, including Adobe Photoshop. However, you must use the correct procedure to preserve alpha channels when you export an image into Photoshop format.
If you use the Save As command and choose Photoshop format, the resulting file will not contain the alpha channels associated with the image.
Canvas X Draw uses up to 24 channels to store the digital information that makes up the image in a paint object.
There are three types of channels: color or image channels, alpha channels, and channel masks. All images have at least one channel. An image can also contain one or more alpha channels and a single channel mask, if you create them.
Choose Image | Show Channels.
Vector objects and text objects do not have color or image channels. However, you can create a channel mask for any object. If an object has a channel mask, you can place the channel mask in Edit mode, and use the Channels palette to work with the channel mask and create alpha channels. Alpha channels are stored with an object as long as the object has a channel mask.
Images in RGB Color, CMYK Color, and LAB Color mode have separate color channels. A color channel stores one component of the image; e.g., in CMYK Color mode, the Magenta channel stores the magenta parts of the image. This channel contains the image that would appear on the magenta plate if you output color separations.
A paint object’s image mode determines the number of color channels. RGB Color images have Red, Green, and Blue color channels. CMYK Color images have Cyan, Magenta, Yellow, and Black channels. LAB Color images have Lightness, A, and B channels.
In the Channels palette, a composite channel appears above the color channels. The composite channel represents the complete image — the composite of the image’s color channels. The composite channel is labeled RGB, CMYK, or LAB, depending on the image mode.
Other image modes do not have separate color channels. Images in Black & White, Duotone, Indexed, and Grayscale mode have a single image channel.
After you make a selection in an image, you can save the selection in an alpha channel. Later, you can load the channel to make the same selection.
An alpha channel is a grayscale channel that is the same size and resolution as the paint object in which it is stored. Pixels in alpha channels can range in lightness from 0 (black) to 255 (white). The lightness levels of pixels in an alpha channel correspond to a range of selection levels.
Black pixels in an alpha channel correspond to masked, or non-selected, pixels in an image. White pixels correspond to selected pixels. Gray pixels correspond to various levels of selection, with lighter grays corresponding to greater selection than darker grays.
If you want black pixels to correspond to selected, rather than masked pixels, click Selected Area in the New Channel or Channel Options dialog box.