This effect causes rings to appear as filled areas instead of lines. For each ring, the area bounded by it and the previous ring is filled, using the same color that would have been used to draw the line. A ring can be both filled and drawn as a line; see Outline.
To enable/disable fill for new rings only, use Image/Fill or F6. Since existing rings are unaffected, the change occurs gradually. To enable/disable fill for both new and existing rings, use Image/Invert/Fill or Shift+F6.
Fill increases graphics overhead and may reduce performance.