1. An image processing apparatus comprising: a selection unit configured to select at least a pixel that is different from a pixel of interest in an image based on a similarity between a pixel value of the pixel different from the pixel of interest and a pixel value of the pixel of interest; a pixel number setting unit configured to set a number of pixels to be selected by the selection unit; a determination unit configured to determine, from a pixel selected by the selection unit and based on whether a number of pixels selected by the selection unit has reached the number of pixels set by the pixel number setting unit, at least a pixel to be used in predetermined processing; and a processing unit configured to perform the predetermined processing on the pixel of interest using at least a pixel value of a pixel different from the pixel of interest, wherein the processing unit performs the predetermined processing on the pixel of interest using a pixel value of the pixel determined by the determination unit.