Measuring Degradation of Colour Pigments on Cave Walls
M. Mirmehdi, A. Chalmers, L. Barham, and L. Griffiths.
Department of Computer Science and Department of Archaeology
University of Bristol, Woodland Rd, Bristol BS8 1UB
The absence of Upper Palaeolithic cave art in Britain may be the result of the accelerated weathering of limestone caves during the Holocene. To test this proposition, it is intended to place mineral-based stencils in three Mendip caves and record monthly changes in pigment colour and adherence, and compare to seasonal variations in local climate. Conventional photography has proved to be an inadequate medium for recording changes to the pigments. A high-resolution digital camera would facilitate digital image processing of the data and improve the accuracy and precision of the monitoring and evaluation. This paper reports on the proposed image analysis techniques which have been tested on pseudo-real data generated specifically for this project using a simple digital camera and stencils specifically drawn on cave walls. The analysis uses techniques such as edge detection to locate a colour chart. A circle detecting Hough transform is then applied to locate the circular stencils. Methods are employed to normalise the colour data to incorporate any environmental changes from one image capture session to another. After normalisation and colour data correction, the stencilled areas are segmented using k-means clustering and their colour and shape statistics are measured. These measurements can then be compared with the master image or data to record changes.