Texture analysis of grey-tone images by mathematical morphology: A nondestructive tool for the quantitative assessment of stone decay

Both a low-cost and easily handled nondestructive methodology and its valid ation criterion are proposed. The methodology is based on image analysis by mathematical morphology for the assessment of decayed stone surfaces in hi storic limestone buildings. It is adapted to follow the evolution, at macro scopic time and space scales, of stone materials used in art pieces and mon ument building stones. This methodology is applied to the quantitative anal ysis of textures of static grey-tone CCD video camera images representative of flat stone structures that cannot be handled. These structure line the walls inside an important church, Basilica da Estrela, built in the 18th ce ntury and located in the city of Lisbon. Detailed visual/tactile observatio n shows that these flat vertical structures are more or less damaged depend ing on their position inside the church. The damage is possibly associated with different environment conditions both from the atmosphere inside the c hurch and from contact with the walls of the church. The presence of these pathologies breaks down the order and organization of sound rock textures i ntroducing changes in the topographical and optical characteristics of the texture of the surfaces. This new methodology is based on the granulometry and covariance analysis of grey-stone images corresponding to the structure s studied. The validation criterion allows the results of the proposed meth odology to be compared to the results of a previous qualitative study made by experts in the field using visual assessment and monument mapping as a v alid methodology to assess the degree of decay. This procedure is based on optical and topographical characteristics and type of decay of the stone su rface, and the quantity of material lost from the surface of the panels dur ing the year. As discussed in this paper, in the context of cultural herita ge, the proposed methodology was validated and its results can be considere d an improvement on and complementary to expert visual analysis and also to other nondestructive techniques.