You can limit the sections of the source that are painted onto the canvas using the source's matte. To limit painting on the canvas using the matte. See Restricting Brush Strokes.

To limit source brush strokes using a source matte:

1. In the Paint Modes box, select Reveal (E).
2. In the Sources list, select a source.
3. From the source controls, select an option in the Matte Source box.

   Select:	To:
   Matte Off	Paint anywhere on the canvas.
   Matte On	Limit painting to areas in the source matte.
   Matte Invert	Invert the source matte and limit painting to areas outside the source matte.

4. Draw strokes.

   Brush strokes only use areas of the selected source that are delimited by its source matte.