From left to right: 1) according to the stellar inclination and to the spot’s latitude and longitude, the xyz position of the nrho points on the spot’s circumference are calculated in 3-D. 2) The maximum and minimum y and z determine a smaller area where the spot is. 3) These {yz} positions (in 3D) are projected on the y-z plane, which gives (p_y, p_z) positions. 4) From these values, a smaller grid is defined. Each cell of this grid is scanned. A reverse to that performed in step 1 rotation is performed to establish if the cell is inside the spot or not.