Abstract: A computer generates coordinates determining a window of display data to be displayed on a monitor. One of these coordinates may indicate the starting pixel number in a line where the window starts. The computer further generates reference information indicating a time of occurrence and a corresponding running number of a first predetermined pixel, and a time of occurrence and a corresponding running number of a second predetermined pixel of the display data. It is possible to determine, from this reference information, the instant of occurrence of the starting pixel for every running number of this pixel of the window. This has the advantage that the reference information needs to be available once only, while the instants of occurrence of several windows are determined from the coordinates of the windows and this single reference information.