An image forming apparatus is configured to control the density of an output image based on an inherent gamma characteristic of the apparatus. The apparatus includes a patch image forming unit that forms patch images having multiple gradation values, a measuring unit that measures the density or lightness of each patch image, a sample point arrangement unit that arranges sample points representative of measured density or lightness values in a descending or ascending order of the gradation values, an inflection point region detecting unit that detects any specific inflection point region, a straight line region detecting unit that detects any specific straight line region, an interpolating unit that interpolates by adding and/or deleting a specific sample point depending upon whether (i) an inflection point region is detected, (ii) a straight line region is detected, or (iii) neither an inflection point region nor a straight line region is detected.