Example run of the diffusion algorithm. The green and blue squares represent the fields in a patch. The blue ones mark the base row. The red line segments depict the progress of the algorithm as it traverses all the fields. Panel a: diffusion after fours iterations, extending the threads upwards. The remaining panels illustrate several points of the diffusion process: in panel b two threads skip across the hole left by a bright star, while two other threads merge, in panel c two threads merge while the remaining ones reach the top, in panel d the threads are extended two rows downwards, and panel e shows the final sequence, after the linking of adjacent threads. The plots are in elliptic coordinates.