Influence of the Number and Pattern of Geometrical Entities in the Image upon PNG Format Image Size

The research is focused on the study of the impact of the number and the pattern of geometrical entities and colour models in map like drawings upon the file size of PNG format. The main outputs originate from the exploration of an extended sample set of PNG images generated by Web Map Services of selected European servers. Original images were subsequently transformed to different colour models of PNG (RGBA, RGB, Palette, interlaced, non-interlaced). The images were analysed according to the number of entities, style and size of lines and the number of used RGBA values. We found that the number of geometrical entities can be replaced by the ratio of foreground pixels in an image. The file size grows with higher image density according to different functions which are driven by colour models and line widths, partly also by patterns. In order to minimise files sizes it is recommended to transform images into appropriate palette models and to avoid anti-aliasing changes of transparency.