1. An image processing apparatus that generates an output image by a deformation process including a coordinate transformation on an input image, comprising a computer configured to implement: an input unit configured to input a deformation parameter; a divisor calculation unit configured to calculate a minimum value of a divisor for the coordinate transformation based on the deformation parameter on the input image, in accordance with a divisor used in a division operation for the coordinate transformation based on the deformation parameter on a maximum coordinate value and a minimum coordinate value among a plurality of coordinate values in the input image; a coefficient calculation unit configured to, based on a relation between a divisor and an error in a division operation, obtain a range of the divisor in a way that a result of the division operation becomes equal to or smaller than an allowable error, and calculate a ratio by dividing a minimum value of the range by the minimum value of the divisor calculated by the divisor calculation unit; a parameter adjustment unit configured to calculate an adjustment parameter, used in the coordinate transformation on the input image, based on a result of multiplying the deformation parameter input by the input unit by the ratio calculated by the coefficient calculation unit, and execute round processing on the adjustment parameter; an image deformation processing unit configured to perform the deformation process including the coordinate transformation of the plurality of coordinate values of the input image using the rounded adjustment parameter; and a display control unit configured to control a display unit for displaying an output image based on image data obtained by the deformation process on the input image by the image deformation processing unit.