The paper deals with a problem arisen in developing a system for the aided virtual recomposition of fragmented frescos (in particular the S. Mathew's fresco of the S. Francis Upper Church in Assisi). The goal is to expand the capabilities of the operators which remains responsible of the whole process. A core functionality is the automatic evaluation of similarity between images of fragments in a consistent way with evaluations made by humans using their visual perception: a critical property for working in tight cooperation with the operators. This requires a color representation close to human color matching.
S-CIELAB, a spatial extension of the CIELAB color representation, is a space whose metrics closely reproduces, through the Euclidean norm, the color distances perceived by a human observer and accounts for the effects of the spatial distribution of colors.
S-CIELAB extends CIELAB by incorporating factors related to the pattern-color sensitivity of the human eye. The system ascribes to the fragment pattern-color characteristics according to the visual perception the human operator has of the fragment; the use of automatic tools for color evaluation avoids the inconsistent results due to different operators and to fatigue of the same person over time.
This paper shows the results of the application of S-CIELAB colour metric to digital images of the fragments coming from the S. Mathew’s fresco, in the S. Francis Upper Church in Assisi, broken during the earthquake of September 1997. S-CIELAB, a spatial extension to the CIELAB colour representation, is a colour space whose metrics closely reproduce, through Euclidean norm, the colour distances perceived by a human observer. S-CIELAB accounts for the effects due to the spatial distribution of colours into the image. It is based on the application of a suitable spatial filtering to the colour image data, aimed to simulate the spatial blurring measured on the human visual system. In the system for aided virtual recomposition of fragmented frescos we are developing, the interaction between the operators and the huge collection of fragments is grounded on a ‘query-by-example’ paradigm. On the base of a set of images of fragments provided as examples, the system browses through the whole collection and retrieves all fragments similar in terms of their colour and texture contents. The application of S-CIELAB colour metric allows the system to ascribe to each fragment its pattern-colour characteristics according to the visual perception of the human operators. The close approximation of human perception allows a meaningful co-operation, increasing the system’s efficiency in the retrieval of fragments and reducing the workload of the restorer.