Urban open spaces have been defined as the empty spaces left apart the built elements. It comprises urban public spaces such as streets, city squares and the environment around monuments, a variety of green spaces, remnants of natural systems such as along watercourses, etc. All these spaces typically constitute the actual arena of urban life, for instance, in terms of meeting places. Their amenity value includes a contribution to the quality of life and aesthetic enjoyment. That is to say they should be truly considered as basic urban components rather than as the ‘space left over after development’. Awareness of this phenomenon has brought a renewed interest for the open spaces as such amongst most urban stakeholders (land planners, urban designers, political representatives). Nevertheless, this implies a necessity of designing new representation and analysis methods. This paper introduces some new methods based on well known techniques introduced either by architects and computer scientists in the field of visibility assessment. These are based on spherical projections of the urban built environment and mathematical analysis of the image of the sky on these projections. Several indicators can be directly computed from these analyses, considering either the outline, the surface or the skeleton of the sky image. They may help to identify proper qualities of urban open spaces rather than qualities of their surrounding buildings. Moreover, the variations of these parameters along urban route may help to categorize homogeneous parts of the urban open spaces, and transitions between them. The main advantages of the developed methods lie in the fact that they consider the third dimension and the Gibsonian motion perspective, i.e. the gradual changes in the visual field in the rate of displacement.