HERE is a short and interesting paper describing how free software was used to extract a self-portrait of Leonardo da Vinci from one of his notebooks.
A self-portrait of young Leonardo.
Amelia Carolina Sparavigna
Dipartimento di Fisica
Politecnico di Torino
"One of the most famous drawings by Leonardo da Vinci is a self-portrait in red chalk, where he looks quite old. In fact, there is a sketch in one of his notebooks, partially covered by written notes, that can be a self-portrait of the artist when he was young. The use of image processing, to remove the handwritten text and improve the image, allows a comparison of the two portraits."