Page "Flood fill" Paragraph 56

A mark is used for the first 2-pixel boundary that is encountered to remember where the passage started and in what direction the painter was moving.

If the mark is encountered again and the painter is traveling in the same direction, then the painter knows that it is safe to paint the square with the mark and to continue in the same direction.

This is because ( through some unknown path ) the pixels on the other side of the mark can be reached and painted in the future.

The mark is removed for future use.