Image forming apparatus, method of gamma correction and storage medium storing gamma correction program

A non-transitory storage medium storing a gamma correction program used for output density control of an image forming apparatus which outputs image data based on an inherent gamma characteristic of the image forming apparatus, wherein the gamma correction program causes a computer which constitutes the image forming apparatus to function as:

• a patch image forming unit that forms a plurality of patch images having multiple gradation values on a specific medium;
• a measuring unit that measures the density or lightness of each of the patch images formed on the medium;
• a sample point arrangement unit that arranges sample points representative of measured density or lightness values in a descending or ascending order of the gradation values;
• an inflection point region detecting unit that detects any specific inflection point region from among regions defined between the consecutively arranged sample points;
• a straight line region detecting unit that detects any specific straight line region from among the regions defined between the consecutively arranged sample points;
• an interpolating operation unit that performs interpolating operation by adding and/or deleting a specific sample point depending on whether (i) an inflection point region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3, (ii) a straight line region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3, or (iii) neither an inflection point region nor a straight line region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3;
• (i) in case the above condition (i) is met,
• (ia) the interpolating operation unit adds a midpoint of a line segment Sn+1Sn+2 as a new sample point and deletes the sample points Sn+1 and Sn+2;
• or
• (ib) the interpolating operation unit adds a center of gravity of a triangle formed by the sample points Sn, Sn+1 and Sn+2 and a center of gravity of a triangle formed by the sample points Sn+1, Sn+2 and Sn+3 as new sample points and deletes the sample points Sn+1 and Sn+2;
• (ii) in case the above condition (ii) is met,
• the interpolating operation unit adds the midpoint of the line segment Sn+1Sn+2 as a new sample point;
• (iii) in case that the above condition (iii) is met,
• the interpolating operation unit adds a center of gravity of a triangle formed by the sample points Sn+1 Sn+2 and an intersection point of a straight line containing a line segment SnSn+1 and a straight line containing a line segment Sn+2Sn+3 as a new sample point, or
• the interpolating operation unit adds an incenter of a triangle formed by the sample points Sn+1 and Sn+2 and an intersection point of a straight line containing a line segment SnSn+1 and a straight line containing a line segment Sn+2Sn+3 as a new sample point;
• a characteristic acquisition unit that acquires a gradation characteristic by the interpolating operation;
• and
• a characteristic correction unit that corrects the gamma characteristic based on the acquired gradation characteristic.

A method of performing gamma correction for output density control of an image forming apparatus which outputs image data based on an inherent gamma characteristic of the image forming apparatus, the method comprising the steps of:

• forming a plurality of patch images having multiple gradation values on a specific medium;
• measuring the density or lightness of each of the patch images formed on the medium;
• arranging sample points representative of measured density or lightness values in a descending or ascending order of the gradation values;
• detecting any specific inflection point region from among regions defined between the consecutively arranged sample points;
• detecting any specific straight line region from among the regions defined between the consecutively arranged sample points;
• performing interpolating operation by adding and/or deleting a specific sample point depending on whether (i) an inflection point region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3, (ii) a straight line region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3, or (iii) neither an inflection point region nor a straight line region is detected among regions defined by four consecutively arranged sample points Sn to Sn+3;
• (i) in case the above condition (i) is met,
• (ia) adding a midpoint of a line segment Sn+1Sn+2 as a new sample point and deletes the sample points Sn+1 and Sn+2;
• or
• (ib) adding a center of gravity of a triangle formed by the sample points Sn, Sn+1 and Sn+2 and a center of gravity of a triangle formed by the sample points Sn+1, Sn+2 and Sn+3 as new sample points and deletes the sample points Sn+1 and Sn+2;
• (ii) in case the above condition (ii) is met,
• adding the midpoint of the line segment Sn+1Sn+2 as a new sample point;
• (iii) in case that the above condition (iii) is met,
• adding a center of gravity of a triangle formed by the sample points Sn+1 and Sn+2 and an intersection point of a straight line containing a line segment SnSn+1 and a straight line containing a line segment Sn+2Sn+3 as a new sample point, or
• adding an incenter of a triangle formed by the sample points Sn+1 and Sn+2 and an intersection point of a straight line containing a line segment SnSn+1 and a straight line containing a line segment Sn+2Sn+3 as a new sample point;
• acquiring a gradation characteristic by the interpolating operation;
• and
• correcting the gamma characteristic based on the acquired gradation characteristic.