Assuming that an original pixel adjacent to a first original pixel and opposite to a second original pixel is a third original pixel, and an original pixel adjacent to the second original pixel and opposite to the first original pixel is a fourth original pixel, and letting d1 be pixel data on the first original pixel, d2 be pixel data on the second original pixel, d3 be pixel data on the third original pixel, and d4 be pixel data on the fourth original pixel, a first edge component E is calculated on the basis of an equation for operation E=-d3+d1+d2-d4 in the first step, and a second edge component Es is calculated on the basis of an equation for operation Es=|d3+d1-d2-d4|.