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.

To...	Do this
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 .
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”.