• @Gordon / @MaBe,

    to illustrate a bit more in detail my findings in #22 (zoom in a bit to see the dark grey better):

    1. drawing just the outlines, then redrawing the outlines with a different value... obviously the 'traversing' or ladder step lines that connect the out with the inner arc and vice-versa stay (1)
    2. keeping the four points that define the the traversing lines for filling a polygon in (black) background color before redrawing the outlines with a new value still leave pieces of the previous traversing lines behind. (2a) shows increasing values and (2b) decreasing ones.
    3. Changing the clean-up color from (black) background color to red shows what is really drawn by the cleanup polygon - again - (3a) with increasing values and (3b) with decreasing values. Even though the very same points are used and should cover the previous traverse lines, the fillPoly does it different...
    4. Just redrawing the traverse lines with (black) background color before drawing the new outlines fixes the issue... ;-)