1. An image processing apparatus comprising: a processor; and a memory coupled to the processor and containing instructions that, when executed by the processor, cause the processor to perform operations comprising: performing transformation processing, the transformation processing including keystone correction or curved surface transformation; identifying transformation information related to amounts of transformation respectively between a first plurality of coordinates on a transformed image obtained by performing the transformation processing on an input image and a second plurality of coordinates on the input image corresponding to the first plurality of coordinates; determining, based on the identified transformation information, a smoothing parameter related to an intensity of smoothing processing for reducing high-frequency components of an image for each region of the transformed image; performing the smoothing processing on an image based on the input image, based on the determined smoothing parameter for each region; and outputting an output image obtained by performing the transformation processing on the image based on the input image after having been smoothened through the smoothing processing.