Real time segmentation of a scene into objects and background is really important and represents an initial step of objects tracking. Starting from the codebook method we propose some modifications which show significant improvements in most of the normal and also difficult conditions. We also propose an evaluation method in order to objectively compare several segmentation techniques, based on receiver operating characteristic (ROC) analysis and on precision and recall method. We propose to summarize the quality factor of a method by a single value, based either on a weighted Euclidian Distance or on a harmonic mean between two related characteristics.