A display control apparatus for processing an input image to be displayed partially overlapping another image as part of a projection system, the input image signal (Lsig) having a portion that corresponds to a display area that does overlap another image and a portion that corresponds to a display area that does not overlap another image, said apparatus comprising: a timing unit configured to generate an offset addition timing based on designated coordinates and a synchronisation signal; a designation unit configured to designate a first offset correction value (Dblk) for the gradation of the input image signal (LSig) at which black is displayed; a control means configured to determine a threshold of gradation (Lset) by multiplying the first offset correction value (Dblk) by a predetermined coefficient value α, where α is equal to or greater than 1, and configured to generate, according to the offset addition timing, a second offset correction value (Doffset) for gradation levels in the portion of the input image signal that corresponds to a display area that does not overlap another image, wherein in the case that the gradation level of the input image signal is less than or equal to the threshold of gradation, the control means is configured to generate the second offset correction value according to: Doffset = Dblk − Dblk × Lsig / Lset where Doffset denotes the second offset correction value, Dblk denotes the first offset correction value, Lset denotes the threshold of gradation, and Lsig denotes a gradation level of the input image signal; and wherein in the case that the gradation level of the input image signal is greater than the threshold of gradation, the control means is configured to generate the second offset correction value according to: Doffset = 0; and adding means configured to add the second offset correction value (Doffset) to the input image signal. a timing unit configured to generate an offset addition timing based on designated coordinates and a synchronisation signal; a designation unit configured to designate a first offset correction value (Dblk) for the gradation of the input image signal (LSig) at which black is displayed; a control means configured to determine a threshold of gradation (Lset) by multiplying the first offset correction value (Dblk) by a predetermined coefficient value α, where α is equal to or greater than 1, and configured to generate, according to the offset addition timing, a second offset correction value (Doffset) for gradation levels in the portion of the input image signal that corresponds to a display area that does not overlap another image, wherein in the case that the gradation level of the input image signal is less than or equal to the threshold of gradation, the control means is configured to generate the second offset correction value according to: Doffset = Dblk − Dblk × Lsig / Lset where Doffset denotes the second offset correction value, Dblk denotes the first offset correction value, Lset denotes the threshold of gradation, and Lsig denotes a gradation level of the input image signal; and wherein in the case that the gradation level of the input image signal is greater than the threshold of gradation, the control means is configured to generate the second offset correction value according to: Doffset = 0; and adding means configured to add the second offset correction value (Doffset) to the input image signal. Doffset = 0; and adding means configured to add the second offset correction value (Doffset) to the input image signal.