On covering a digital disc with concentric circles in Z2Z2

Digital circles and digital discs satisfy many bizarre anisotropic properties, understanding of which is essential for solving various problems in image analysis and computer graphics. In this paper we study the underlying properties of absentee pixels that appear while covering a digital disc with concentric digital circles. We present, for the first time, a mathematical characterization of these pixels based on number theory and digital geometry. Interestingly, the absentees occur in multitude, and we show that their count varies quadratically with the radius. The notion of infimum parabola and supremum parabola has been used to derive the count of these absentees. Using this parabolic characterization, we derive a necessary and sufficient condition for a pixel to be a disc absentee, and obtain the geometric properties of the absentees. An algorithm to locate the absentees is presented. We show that the ratio of the absentee pixels to the total number of disc pixels approaches a constant with increasing radius. Test results have been furnished to substantiate our theoretical findings.