Binarizing water sensitive papers - how to assess the coverage area properly?

Water sensitive paper is a convenient tool used to assess the coverage of an area with liquid in many various applications. It is useful for visualization and evaluation of potential spray deposits on plants. This article attempts to guide readers in the quantification of the coverage area of the water sensitive paper with liquid. It summarises a comparison of 15 global thresholding and binarization algorithms for their ability to quantify the coverage area accurately. We used 40 water sensitive papers obtained from two field experiments, in which the quality of spraying potatoes and rape was tested. Obtained results show that when binarizing water sensitive papers, careful choice of thresholding method is necessary, as differences in the results obtained with the use of various thresholding algorithms were observed and they were quite substantial in some cases. In relation to expert assessment, the algorithms proposed by Huang and Wang (1995), Ridler and Calvard (1978) and Otsu (1979), provided the best results.