About bitmap masks

Fireworks bitmap masks are like Photoshop layer masks in that the pixels of the mask object affect the visibility of underlying objects.

Original objects and a bitmap mask applied using its grayscale appearance

You can apply bitmap masks in two ways:

• Using an existing object to mask other objects. This technique is like applying a vector mask.

• Creating an empty mask. Empty masks start out as either transparent or opaque. A transparent (or white) mask shows the masked object in its entirety, and an opaque (or black) mask hides the masked object completely. You can use the bitmap tools to draw on or modify the mask object, revealing, or hiding the underlying objects.

  When you create a bitmap mask, the Property inspector displays information about how the mask is applied. If you select a bitmap tool when a bitmap mask is selected, the Property inspector displays the properties of the mask and options for the selected tool.

  By default, most bitmap masks are applied using their grayscale appearance, but you can also apply them using their alpha channel.

 community help (adobe.com)