PURPOSE:To perform the coordinate conversion of the picture element position between the polar frame and the X-Y frame with a memory circuit of comparatively small capacity by employing the first and second position designating circuits which store specified data. CONSTITUTION:The first and second position designating circuits 10 and 11 are used to convert the picture element position between the X-Y frame and the polar frame. The first position designating circuit 10 stores the picture element position on the X-Y frame corresponding to the reference position of the scanning lines on the polar frame. On the other hand, the second position designating circuit 11 stores the changing amount of the picture element position of the X-Y frame following the scanning of the picture element on the polar frame. Because the number of the digits for the changing amount of the picture element position is one over several to one over ten odd of that for the picture element position, the capacity of the second position designating circuit 11 can be small.
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