12. Apparatus according to claim 11, wherein the means for modifying the address words that define locations of the input scene having said selected spatial relationship with said predetermined location comprise a register containing the offset word, a switch controlled by the comparison means for selecting the register in the event that the location defined by the address word has said selected spatial relationship with said predetermined location, and an arithmetic unit connected to the switch for receiving the offset word contained in the register and combining it with the address word.
12. Apparatus according to claim 11, wherein the means for modifying the address words that define locations of the input scene having said selected spatial relationship with said predetermined location comprise a register containing the offset word, a switch controlled by the comparison means for selecting the register in the event that the location defined by the address word has said selected spatial relationship with said predetermined location, and an arithmetic unit connected to the switch for receiving the offset word contained in the register and combining it with the address word. 13. Apparatus according to claim 12, comprising a second register containing a second offset word, and wherein the comparison means causes the switch to select the second register when the address word defines a location of the input scene having a second selected spatial relationship with said predetermined location, said second selected spatial relationship and the first-mentioned selected spatial relationship being mutually exclusive, so that the arithmetic unit receives the second offset word when the address word defines a location having said second selected spatial relationship with said predetermined location and combines the address word with the second offset word.