Transparency Effects with Channel Masks

Channel masks let you add transparency to images without altering them permanently. A channel mask creates transparency without changing any pixels in an image. You can remove a channel mask to eliminate the transparency effect at any time.

To Make an Image Transparent:

To make parts of an image transparent—to eliminate the background in a photograph, for example—you can create a channel mask from a selection. This procedure explains how to transfer a selection into a channel mask to make selected areas transparent.

1. Choose Image | Show Channels to open the Channels palette. Double-click the paint object to put it into Edit mode. You can use various techniques to select the areas you want to be transparent:
   • You can click the Wand tool to select similar colors throughout the image. If a photograph has a colored background, for example, click the background to select it. You can also use the Color Range command to make a selection.
   • To soften the edges of the selection, you can use the Image | Select | Feather command.
2. Click the Selection button in the Channels palette to save the selection in a new alpha channel. In the alpha channel, white pixels correspond to the selection. (A partial selection produces gray pixels in the channel). Black pixels in the channel correspond to unselected areas of the image. To create transparent areas from the selection, you need to invert the channel.
3. Press Esc to deselect the selection in the image. Then, click the alpha channel in the Channels palette to make it active.
4. Choose Image | Adjust | Invert. This reverses the white and black areas in the channel.
5. Drag the alpha channel into the Channel Mask slot in the Channels palette. Black areas in the channel mask produce transparent areas in the image. White areas in the channel mask produce opaque areas in the image.
6. Press Esc to exit Image Edit mode. The selection you made is now transparent. If you place the paint object on a background in your document, the background will be visible through the transparent areas of the image.

To Create a Transparency Fade:

Create a transparent fade effect using a channel mask. This procedure explains how to create a blend in an alpha channel, and then create a channel mask to make an image fade to transparency.

1. Choose Image | Show Channels to open the Channels palette. Double-click a paint object to put it in Edit mode.
2. Click the New Channel button in the Channels palette. A new alpha channel appears in the palette. Click the channel to make it active.
3. Select the Blend tool. With the foreground color set to white and the background color set to black, drag vertically from top to bottom in the channel. This creates a blend from white to black.
   • You can change the distance that you drag the Blend tool in the channel to adjust the length and position of the fade to transparency.
   • You might need to use the Blend settings in the Properties bar to select the Linear option for the Blend tool before you create the blend in the alpha channel. You can also set other options for the Blend tool to fine-tune blends.
4. Drag the alpha channel into the channel mask slot in the Channels palette. White pixels at the top of the channel produce opaque areas. Black pixels produce completely transparent areas. Gray pixels in the channel mask correspond with partially transparent areas in the image.

To Create a Channel Mask by Rendering:

Canvas X Draw can create a channel mask when you render a vector, group, paint, or text object.

To create the mask, choose the Mask and Transparency options in the Render dialog box. Canvas X Draw will create a channel mask that makes blank areas around and inside the objects transparent.

When you make a new paint object by pasting a non-rectangular selection copied from an image, Canvas X Draw makes a channel mask to hide white pixels surrounding the selection.

Preserving Transparency in Images

Select the Preserve Visibility checkbox in the Channels palette to preserve transparency when you paint or apply filters to an image. The Preserve Visibility checkbox is available when you edit a paint object that has a visibility mask. A paint object has a visibility mask if it was created with a transparent background, or if you applied a visibility mask with the Add Visibility Mask command.

If a paint object has an opaque background, the Preserve Visibility checkbox is disabled.

To Preserve Transparency While Editing an Image:

When a paint object is in Edit mode, select Preserve Visibility in the Channels palette. When selected, you can paint and edit the image without affecting clear areas or reducing the transparency of partially transparent pixels.

The Preserve Visibility option affects all aspects of image editing. When Preserve Visibility is selected, pasted selections do not affect clear areas. Also, a pasted selection will match the transparency of the existing pixels when you defloat the selection.

If an image is completely clear (contains no colored pixels), you cannot alter the image when Preserve Visibility is selected.

You must deselect Preserve Visibility to paint in clear areas of an image. Then, if you want to edit the painted areas, select Preserve Visibility again.

For example, you can deselect Preserve Visibility and paint airbrush strokes in a clear image. Then, select Preserve Visibility and you can paint over the airbrush strokes to change their color, without losing the soft edge or “spilling” color into clear areas.

When Preserve Visibility is selected and you use the Eraser tool, pixels you “erase” are painted with the current background color; they are not erased to clear. Also, when you use any painting tool to apply color, you cannot make pixels more or less transparent. This is why you cannot apply any color in clear areas when Preserve Visibility is selected.

When you select Preserve Visibility, you can paint and apply filters to modify pixels that are less than 100% transparent. You can change the hue, saturation, and intensity of pixels but can’t change their transparency.

To Paint or Apply Filters to an Entire Image:

Deselect Preserve Visibility. This turns off the visibility mask and lets painting tools and filters affect the entire image.