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This paper presents a new method, based on gray-scale mathematical morphology, called bitwise decomposition. Aimed at visual prosthetics, the design is compact and portable. The use of the proposed gray-scale erosion allows for on-pixel image processing and a higher fill factor. The design is based on digital pixel sensors, where analog-to-digital conversion is carried out next to the photodiode. The advantages are faster readout and better reliability. Implemented in 0.13 mum CMOS technology, the design achieves a fill factor of 22%. The pixel array needs only a minimum of external control and processing circuitry to produce an edge image.