A paint object is an opaque rectangular area, and the image it contains will block any other objects placed behind it. However, Canvas now lets you use a special channel mask to make parts of the image appear to be transparent, as if they had been cut out.
In a channel used for this type of masking, the pixels that are 50% black or darker (lightness level of 127 or less) designate the areas that will be masked (made to appear transparent) in the paint object.
You can use painting tools and commands in this special channel to edit the mask. By adding black or white pixels, you can alter the shape of the mask. However, in the mask channel all pixels are treated as either black or white; gray pixels do not indicate partial selection, as in other alpha channels. Therefore, a mask created by a channel is hard-edged.
Creating channel masks
Canvas can create a channel mask when you rasterize a vector object or text (or when you Save As a document in the Photoshop file format). To tell Canvas to create the mask, select the new Create Mask option in the Render dialog box. The channel mask that Canvas creates will have black pixels corresponding to blank areas in the image.
When you make a new paint object by pasting a non-rectangular selection copied from an image, Canvas now automatically makes a channel mask to hide white pixels surrounding the selection.
This new option in the Render dialog box replaces the "Transparency" option.
Using an existing channel as a channel mask
In the Image Channels palette, you can designate an existing channel to be a channel mask for a paint object.
1) In the pop-up menu in the Image Channels palette, choose Channel Mask. The Channel Mask dialog box appears.
2) In the Channel Mask dialog box, choose the channel you want to use as a mask in the pop-up menu and click OK.
To remove a mask from an image: With a paint object in edit mode, choose Channel Mask from the pop-up menu in the Image Channels palette. In the dialog box, select "None" and click OK.
Notes:
You can no longer apply a Null Fill to an image. This feature has been replaced by using alpha channels as a mask. This is a more powerful method of making areas of image objects transparent and is supported by PostScript.
Canvas 5 renders all the objects in a document when you save it in a non-Canvas 5 raster format. If you have any alpha channels (including channel masks) created for any paint images in the document this information will be lost when you Save As due to this rendering. If you wish to transfer an image into another application and maintain alpha channel information, use the Image>Export command.
