1. An image processing apparatus comprising: a first storage unit configured to store a plurality of images having the same contents with different resolutions therein; an obtaining unit configured to obtain a size of a region to be processed in which a feature parameter is to be calculated and which is a portion of one of the plurality of images; a determination unit configured to determine a number of pixels which is required for obtaining the feature parameter, the number of pixels required depending on a type of feature parameter; a resolution calculation unit configured to calculate a resolution suitable for the size of the region to be processed obtained using the obtaining unit, from the number of required pixels; and a feature parameter calculation unit configured to select an image having the resolution calculated using the resolution calculation unit from among stored images, and to obtain the feature parameter from a portion of the selected image to be processed corresponding to the region to be processed.