It gives a circular or a rectangular representation of your image with all the possible intermediates between both.
The result of your settings will appear in the Preview without affecting the image until you click on
.Slider and input box allow you to set the "circularity" of the transformation, from rectangle (0%) to circle (100%).
This option controls the angle the drawing will start from (0 - 359°), and so turns it around the circle center.
When this option is checked, the drawing will start from the right instead of the left.
If unchecked, the mapping will put the bottom row in the middle and the top row on the outside. If checked, it will be the opposite.
If unchecked, the image will be circularly mapped into a rectangle (odd effect). If checked, the image will be mapped into a circle.