1. An image forming apparatus comprising an image interpolation unit configured to compute a correct pixel value of a target pixel subject to interpolation of a halftone image, wherein the image interpolation unit includes: a base pattern setting unit configured to set a base pattern including the target pixel subject to interpolation; a reference pattern setting unit configured to set at least one reference pattern in a region peripheral to the target pixel subject to interpolation, the reference pattern having a size equal to or larger than a size of the base pattern; an analogous pattern acquisition unit configured to acquire at least one analogous pattern analogous to the base pattern from the reference pattern; a high-resolution pattern creating unit configured to create a high-resolution pattern having a predetermined resolution or higher by synthesizing the acquired one or more analogous patterns; a pixel value estimating unit configured to compute an estimated pixel value of the target pixel subject to interpolation based on the created high-resolution pattern; and a pixel value determination unit configured to determine the correct pixel value of the target pixel subject to interpolation based on the computed estimated pixel value of the target pixel subject to interpolation, and wherein the image interpolation unit further includes a halftone dot size estimation unit configured to estimate at least one of a width and a height of a halftone dot as a size of the halftone dot, and wherein the base pattern setting unit determines the size of the base pattern based on the estimated size of the halftone dot.