Mask of values giving angular position relative to center. The angle is defined according to ISO standards in which the angle is measured counter- clockwise from the x axis, measured in a normal coordinate system with y- axis pointing up and x axis pointing right.
In other words: for increasing angle, the coordinate moves counterclockwise around the feature center starting on the right side.
However, in most images, the y-axis will point down so that the coordinate will appear to move clockwise around the feature center.