Horák J., Ardielli J., Růžička J., Novák J., Szturcová D. Optimization of PNG output provided by WMS European geoportals

The goal of the study is to provide recommendations how to optimize PNG outputs - minimize file sizes respecting the information content which may be significant i.e. for mobile devices.

The sample set of PNG images generated by 9 selected European geoportals was investigated to evaluate default settings of WMS services and influence of the number and pattern of features to the image size. 52% of WMS services utilise RGBA model as default settings even though their images seldom contain more than 256 colours. Recommendations may be to change the default setting to 8bit-palette model or to specify in all WMS requests an option for generating PNG image with the colour model appropriate to the data type. Original images were subsequently transformed to different colour models (RGBA, RGB, Palette, interlaced, non-interlaced) and analysed according to the number of entities, style and size of lines and a frequency of applied RGBA values. The number of geometrical entities may be replaced by the ratio of foreground pixels in the image. The results show 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.