A button is available on Edit Tools to toggle Masking on/off (or type the / key).  When Mask is enabled, the current screen image will be combined with a Background according to parameters set via the Mask Options screen.  The Mask Background can be either a solid color or an image.  The Background area is defined by either a pre-defined Shape or a Stencil, which is usually a grayscale image.  The static Mask Background will appear in the darker portions of the Shape or Stencil, while the changeable image shows through the lighter areas.  (This is reversed when the Negative Mask option is set.)  The Mode parameter determines how the Background will be merged into the display; either On/Off to cause distinct areas, or Blend to mix the Background translucently with the display image.  The Strength value controls the relative contrast between areas.

The Set Mask Background Image option can be used at any time to capture the current screen display as the Mask Background Image.  Then when Mask is enabled, the Background portions of the screen will be filled with that static image, while the rest of the screen can be altered with Liquib manipulations, or perhaps different source images are loaded.  Essentially, this allows two completely different images to be merged, according to the current Mask Mode, either On/Off or Blend.

Set Mask Stencil/Background and Set Mask Stencil/Background Negative options are also available to simultaneously:  1) Convert the current screen display into a Stencil, based on its light/dark areas, and then set that Stencil as the Mask.  and  2) Also set the current screen as the Mask Background Image (the same as using the Set Mask Background Image option described above).  The effect of these actions is to cause portions of the image to appear frozen and floating over the variable, changing, non-background areas of the screen.  This can produce some very interesting results.  The Set Mask Stencil Background choice will freeze the darker portions of the display, while the Negative version will freeze the lighter areas.

NOTE:  The Create Mask screen provides more flexible options for building a Mask based on the current screen display, as well as other images.  It also makes it possible to save the resulting Masks permanently as Stencil Images.

Mask actions can be captured in Scripts, and later revised via the Script Control and Editing screen.  Random Masking can be applied during Automatic Effects, according to options set in the Automatic Effects Configuration screen.