= Mask off areas of an image =

Most compositing nodes support a _mask_. The mask confines the operation
of the node to specific areas of the input image.

* Pixels of the input image corresponding to black pixels in the mask
  are not affected at all by the operation.

* Pixels of the input image corresponding to white pixels in the mask
  show the full effect of the operation.

* Gray pixels in the mask blend between the input and the manipulated
  output (so, antialiased edges in the mask create antialiased edges
  between affected and masked areas of the output image).

* If you have a mask image with opposite values (that is, it has white
  in the areas you want to mask off), or a different size, you don't
  need to manipulate the image directly to use it as a mask. You can use
  the __Resize mask to fit image__ and __Invert mask__ options on the
  filter node's __Mask__ tab to fix the mask there.

tasks>>
    Mask an operation with a separate image:
        # Set up the compositing node that contains the mask image.

        # Connect the mask node to the bottom (mask) input of the node
          you want to mask.

        # You may need to go to the masked node's __Mask__ tab and set
          __Operation mask__ to "Mask input".

    Mask an operation using a plane (such as Alpha) from the input:
        # Go to the filter node's __Mask__ tab and set __Operation
          mask__ to "First input".

        # Use the pop-up menu next to "First input" to choose the plane
          or component to use as the mask.

          Houdini displays the name of the masking plane next tot the
          connection in the  [network
          editor|/ref/panes/network] .

    Scale the effect of an operation (with or without a mask):
        * Go to the filter node's __Mask__ tab and change the __Effect
          amount__ parameter.

    Turn off masking of a node:
        * Go to the node's __Mask__ tab and set __Operation mask__ to
          "None".