1. An image correction apparatus comprising: a feature-value calculating unit that calculates a feature value indicative of average brightness of an input image from pixel values of pixels of the input image; a boundary determining unit that determines a pixel value at a boundary for correcting each pixel of the input image to make the pixel brighter or darker by using the feature value of the input image calculated by the feature-value calculating unit; and a correction-amount calculating unit that calculates, with respect to the pixel value of each pixel of the input image, an area for determining a contrast correction amount for a pixel value smaller than the pixel value at the boundary determined by the boundary determining unit such that a pixel is to be made darker, and an area for determining a contrast correction amount for a pixel value larger than the pixel value at the boundary determined by the boundary determining unit such that a pixel is to be made brighter, wherein the feature-value calculating unit calculates an average lightness or a mode as the feature value indicative of the average brightness of the input image from the pixel value of each pixel of the input image, the boundary determining unit determines an inflection point of a correction curve line as the pixel value at the boundary for correcting each pixel of the input image to make the pixel brighter or darker by using the average lightness or the mode of the input image calculated by the feature-value calculating unit, and the correction-amount calculating unit calculates, with respect to the pixel value of each pixel of the input image, a correction curve line as an area for determining a contrast correction amount for a pixel value that is below the inflection point at the boundary determined by the boundary determining unit such that a pixel is to be made darker and for a pixel value that is above the inflection point at the boundary determined by the boundary determining unit such that a pixel is to be made brighter.