Channel Masks

A channel mask is a special channel that defines transparency in an object. While channels are typically associated with paint objects, you can apply a channel mask to any type of object.

Like an alpha channel in a paint object, a channel mask is basically a grayscale image. Channel masks can be edited using image-editing tools and techniques, similar to alpha channels.

You can think of a channel mask as a template for transparency. The channel mask is the same size as the masked object and is aligned with it. In the case of a paint object, the channel mask has the same resolution and number of pixels as the paint object.

Channel masks, like grayscale images, contain pixels that are assigned 256 possible lightness levels, or luminance values. Luminance values in a channel can range from 0 (black) to 255 (white).

In an alpha channel, luminance corresponds to selection intensity. In a channel mask, luminance values correspond to 256 levels of transparency, from 100% to 0% transparency, in the masked object. Black pixels (0 luminance) produce 100% transparency, while white pixels (255 luminance) produce 0% transparency.

Therefore, when you paint in a channel mask, painting with black produces clear areas and painting with white produces opaque areas in the masked object. Painting with gray produces partial transparency relative to the gray value, with darker grays producing greater transparency than lighter grays.

Black in a channel mask produces 100% transparency. A feathered edge produces partial transparency.

Keep in mind that an object’s appearance can be affected by its transfer mode, scope setting, and opacity, in addition to a channel mask. Changing the transfer mode can completely change the appearance of an object that has a channel mask. (See Using Transfer Modes.)

Creating Channel Masks

You can create a blank channel mask, or create a channel mask from a paint object. You create a channel mask when you render objects. (See Rendering Objects and Images.)

You can use the Sprite tool, or the Channels palette to create blank channel masks. (See Channel Masks for more information about the Channels palette.)

To Create a Channel Mask:

This procedure creates a blank channel mask for an object.

1. Select a text, paint, vector, or group object to mask.
2. Select Object | SpriteLayers | New Channel Mask.
   • If the object to be masked is not a paint object, a dialog box asks you to set the resolution of the mask. Enter a resolution from 1 to 2,540 ppi and click OK.
   • If the object is a paint object, the channel mask’s resolution will be the same as the paint object’s resolution.
3. The object appears in Channel Mask Edit mode, with the object visible and the channel mask selected. You can edit the channel mask with painting tools. (See Editing Channel Masks.)
4. Press Esc to leave Edit mode when you finish.

When you create a new channel mask, the channel is filled with white pixels. At this point, the channel mask produces no transparency because white pixels in the channel mask produce 0% transparency in the masked object. As you edit the channel, painting with gray produces partial transparency, and painting with black produces 100% transparency.

To Create a Channel Mask with the Sprite Tool:

1. Use the Sprite tool to apply a channel mask to a text, vector, or image object.
2. Select the object to which you want to apply the mask.

3. Select the Sprite tool and click on the selected object. 

   The object switches to Image Edit mode.

   You can identify whether you are in Image Edit mode by the Status bar.

4. Once masked, apply transparency effects to the mask with Painting tools.

To Set the Channel Mask Scope:

When you apply a channel mask to a vector object, the channel mask affects the vector object’s fill ink or fill ink and stroke (pen ink). To change the effect, change the Scope option in the Properties bar. (See Controlling the Scope of Transparency Effects.)

Masking with a Paint Object

You can create a channel mask by attaching a paint object to another object. If you have an existing paint object that you want to use as a channel mask, it’s quicker to use this procedure than to use the Channels palette to place the paint object in a channel mask.

To Attach a Channel Mask:

1. Place a paint object to use as a mask in front of the object to be masked. The two objects do not have to overlap or touch, but the paint object must be in front of the other object in the stacking order.
2. Select both objects.
3. Choose Object | SpriteLayers | Attach Mask. Canvas X Draw creates a channel mask and both objects remain selected. The original paint object is not changed.

If the paint object and the object to be masked aren’t the same size, Canvas X Draw scales the image of the paint object to fit the masked object.

A channel mask created from a paint object is the same as any channel mask. You can edit it the same as if you created a blank channel mask.

Like any other channel mask, a channel mask created from a paint object produces transparency relative to its gray values. If the channel mask is solid white, it creates no transparency; if it is solid black, it creates 100% transparency and makes the masked object invisible.

Editing Channel Masks

You can use painting tools, filters, and image-editing commands to modify the effect of a channel mask.

To Edit a Channel Mask:

Place the masked object in Channel Mask Edit mode.

Editing a channel mask is similar to editing a channel in a paint object. You can paint in the channel mask with shades of gray. You can make selections with selection tools, commands, and alpha channels. You can apply image-adjustment commands and filters to the entire channel mask or to just the selected areas.

Options for Channel Mask Editing

You can enter Channel Mask Edit mode using any of the following methods.

• With the mouse: Press Command and double-click a masked object.

If the object doesn’t have a channel mask, this creates a channel mask and puts the object in Channel Mask Edit mode.

• In the Channels palette: When a paint object is in Edit mode, click the channel mask to select it for editing. When any other object is in Channel Mask Edit mode, the channel mask is the only channel that can be selected.

To Edit a Channel Mask:

1. Select the masked object.
2. Choose Object | SpriteLayers | Edit Channel Mask. The masked object appears in Channel Mask Edit mode.

In this mode, the channel mask is active and the object is also visible. You can paint in the channel or modify it to change the transparency of the underlying object.

3. Press Esc to leave Edit mode when you finish editing. The object remains selected.

To Remove a Channel Mask:

Removing a channel mask from an object removes the transparency effect produced by the channel mask.

1. Select the masked object.
2. Choose Object | SpriteLayers | Detach Mask. Canvas X Draw removes the channel mask from the selected object.

When you detach a channel mask, Canvas X Draw converts the channel mask to a paint object and places it in the document. The paint object includes the alpha channels from the masked object if it had alpha channels.

See Also:

• Controlling the Scope of Transparency Effects
• Rendering Objects and Images
• Vector Masks