Image processing apparatus, image processing method, and storage medium

A non-transitory computer readable medium storing a program causing a computer to provide functions for image processing, the functions comprising:

• a first detection unit that detects boundaries from an image;
• an extraction unit that extracts straight-line segments from the boundaries detected by the first detection unit;
• a second detection unit that detects a region in which differences between pixel values of near pixels located across the straight-line segments extracted by the extraction unit is larger than a predetermined first value, or equal to or larger than the predetermined first value;
• a classification unit that classifies the straight-line segments in the region detected by the second detection unit into four sides of quadrangles;
• a third detection unit that detects colors or densities in outer areas of the four sides classified by the classification unit;
• a selection unit that selects combinations that differences between the colors or the densities corresponding to the straight-line segments in the combinations are equal to or smaller than a predetermined second value, or smaller than the predetermined second value, from the combinations of the four sides that possibly form the quadrangles;
• a fourth detection unit that detects coordinates of four vertexes obtained when the straight-line segments in the combinations are extended, and a combination that an area of a quadrangle formed by the corresponding four coordinates satisfies a predetermined condition, from the combinations selected by the selection unit;
• and
• a correction unit that corrects the quadrangle formed by the combination detected by the fourth detection unit to a rectangle.

An image processing method, comprising:

• controlling at least one processor to execute operations comprising:
• detecting boundaries from an image;
• extracting straight-line segments from the boundaries;
• detecting a region in which differences between pixel values of near pixels located across the straight-line segments are larger than a predetermined first value, or equal to or larger than the predetermined first value;
• classifying the straight-line segments in the region into four sides of quadrangles;
• detecting colors or densities in outer areas of the four sides;
• selecting combinations that differences between the colors or the densities and corresponding to the straight-line segments in the combinations are equal to or smaller than a predetermined second value, or smaller than the predetermined second value, from the combinations of four sides that possibly form the quadrangles;
• detecting a combination that coordinates of four vertexes obtained when the straight-line segments in the combination are extended, and an area of a quadrangle formed by the corresponding four coordinates satisfies a predetermined condition, from the combinations;
• and
• correcting the quadrangle formed by the combination to a rectangle.