1. An image processing apparatus comprising: an image signal extraction unit to extract at least a part of an image signal from image data based on signal characteristics of the image signal; a threshold calculation unit to calculate a threshold as a function of one or more of a height of an image represented by the image data or edge strength of the image; an edge detection unit to detect an edge portion in the image data from the extracted image signal by applying high-pass filter processing to only a luminance value of the image signal, and to compare a result of the high-pass filter processing to the threshold to identify a high-frequency component as the edge portion; and a color shift amount detection unit to detect a color shift amount by calculating a correlation of at least two color components contained in the image signal according to a distance from a reference position at the edge portion in the image data.