The Clipping transformation can block off the part of an image either inside or outside one of a small number of shapes specified by parameters in the Mapping tab.

If "Inside" is selected as the Clipping Region, the Clipping transformation will block out the area "inside" the Clipping Shape. If "Outside" is selected, the Clipping Region constitutes a frame blocked out "outside" the Clipping Shape.

One can specify the coordinates of the Clipping Shape in a "Screen-Relative" coordinate system attached to the Fractal Window by checking the "Screen-Relative" check box. Otherwise, the coordinates are "absolute", referring to the coordinate system in which the fractal calculations are performed.

"Screen-Relative" coordinates are useful to define a picure frame for your image: one can "zoom" and "pan" the image and not cause the "screen-relative" picture frame to zoom and pan with them. They're also useful for me when I'm defining an "outside" clipping area to show only a certain part of an object and want to "zoom" and "pan" the image behind the Clipping Shape to find the portion I want to show without moving the blocking Clipping Shape in the process. This process is like moving a picture around behind a porthole to find the part one wants to show through.

I tend to use "Screen-Relative" coordinates, which I find it very stabilizing.

When using "absolute" coordinates for the clipping, one can right-click on various defining parameters in the Mapping tab for the Clipping Shape and, in the popup menu that appears, click on "Eyedropper". The eyedropper thus accessed can be used to click on areas within the image in the Fractal Window to specify location coordinates for the Clipping Shape. If "Screen-Relative" is clicked, one cannot use the eyedropper, and must either calculate the coordinate values or determine them by experimentation.

I determine the "screen-relative" coordinates of my clipping shapes through a process of trial-and-error, in which I start with a set of coordinates I have already used and then painstakingly change them bit-by-bit until the shape becomes exactly what I wish it to be exactly where I wish it to be in terms of shape, size, location, and rotation.