From photography to drawing 323 Archeologia e Calcolatori 12, 2001, 323-334 FROM PHOTOGRAPHY TO DRAWING: A SEGMENTATION TECHNIQUE FOR MURAL PAINTINGS 1. PRESENTATION OF THE PROBLEM Diverse research areas require precise graphic representations of mural paintings, i.e. exact copies of the originals, which depict the graphic elements and, in some cases, the condition of the original mural, including its deterioration. This is useful to observe the figures represented by the artist as well as their components, size, relative position of their parts, and the inclusion or omission of outlines and their thickness. A precise representation also allows the study of the painting contents and the methodology used by the artist, for example the use of geometrical lines, from which his way of thinking and mathematical knowledge can be deduced. On the one hand, the registration of mural paintings represents an important step for the restoration process of assets. These include the necessity to locate the degraded sections of a painting and the selection of the correct procedure to repair it in order to stop the deterioration process. Furthermore, recording all this information allows the observation of the conservation evolution of the masterpiece and reveals if the deterioration process continues and whether the techniques applied were satisfactory. The easiest way to record a mural painting is by using a photograph, which provides a representation of all the information a picture can capture, and encompasses the whole visible spectrum. It gives even more information when a special film such as infrared film is used. However, in order to register the most important information used by the restorers, it is often convenient to have a sketch of the painting, due to the manipulation and reproduction needs and because this way it is possible to include additional information such as graphics and texts. For purposes of proportion and composition studies, information about colour and tone changes can be a problem and can obstruct the observation of important information since it includes visual noise. It is also not helpful to transform it to a grey tone image since this can make the painting confusing and difficult to understand. Therefore, photography does not satisfy the entire mural painting register needs. One of the techniques sometimes used by restorers to obtain an exact drawing of the mural consists in a footprint making. A plastic cover is placed over the mural to trace all the important elements – i.e. graphic representations, chipped walls, dampness, loosening of the mural coating, etc. The result is a copy with a scale of 1:1. In the specific case of Mesoamerican mural paintings,