Gradient direction calculation device

A gradient direction calculation device that calculates a gradient direction of a pixel, including:

• an element gradient vector calculation unit that calculates two mutually perpendicular element gradient vectors with respect to a pixel of interest by using a difference in pixel values in the vicinity of the pixel of interest;
• a quadrant determination unit that determines, based on a combination of positive/negative signs of each of the two element gradient vectors, a quadrant among the four quadrants defined by the two element gradient vectors that contains a gradient direction of the pixel of interest;
• a ratio comparison unit that determines comparison results by comparing each of the ratios of magnitudes of the two element gradient vectors with a predetermined threshold;
• and
• a gradient direction determination unit that determines a gradient direction corresponding to a combination of the positive/negative signs and the comparison results.