Click an arbitrary position on the image.
A circular window centered on that position will appear.
Move the control points of the circle on the image to adjust the position and size of the circular mask.
Click [UNDO] to cancel the previous operation.
Click [DRAW].
The circular mask is drawn and filled in.
If you click [ERASE], the circular mask is erased.
Click [OK]. The mask setup is complete.
Clicking [CANCEL] will cancel the setup.