The filling process works first by determining which pixels should be filled and then tracing those pixels to produce a vectorized path. The tracing process has limited precision which can result in inaccuracies in the filled region. You can improve the filling accuracy several ways. The first way is to zoom in on the region you are filling. Zooming in increases the number of screen pixels in the filled region which results in a more accurate tracing.

The moon man was given a gray fill using the Strumento Riempimento while the zoom level was 25% on the left and 200% on the right.

The second way to improve the filling accuracy is to expand the fill region slightly in a process akin to «trapping» that printers use to account for small misalignments in their printing plates. This works especially well for cartoons where the fills can be put on a separate Livello beneath a Livello containing the black lines. The amount of expansion is controlled by the Grow/shrink by parameter. As the name suggests, one can both expand and reduce the fill area.

The moon man was given a gray fill using the Strumento Riempimento while the zoom level was 200%. On the left, no «trapping» was used. On the right the Grow/shrink by parameter was set to 1 pixel. The fill was then moved behind the line drawing. On both sides, the region around the eye has been expanded by a factor of four.