Image processing apparatus, image processing method, and image processing program

A program storage medium storing an image processing program that causes a computer to execute:

• first gradation-distribution calculation processing for dividing the gradation range of an input image signal into a plurality of gradation sections and calculating a first histogram showing the frequency of pixel values in each gradation section;
• decision processing for detecting a feature component signal related to the contrast of an image from the input image signal and for determining, on the basis of this feature component signal, whether the image contains a low-contrast region whose contrast is lower than a prescribed threshold;
• second gradation-distribution calculation processing for calculating, using the first histogram, a second histogram in which the gradation range of the input image signal is divided into a greater number of sections than the number of gradation sections in the first histogram;
• and
• gradation correction processing for performing gradation correction processing using the second histogram when it is determined in the decision processing that the image contains a low-contrast region, and for performing gradation correction processing using the first histogram when it is determined that the image does not contain a low-contrast region.

An image processing method comprising:

• a first gradation-distribution calculation step of dividing the gradation range of an input image signal into a plurality of gradation sections and calculating a first histogram showing the frequency of pixel values in each gradation section;
• a decision step of detecting a feature component signal related to the contrast of an image from the input image signal and determining, on the basis of this feature component signal, whether the image contains a low-contrast region whose contrast is lower than a prescribed threshold;
• a second gradation-distribution calculation step of calculating, using the first histogram, a second histogram in which the gradation range of the input image signal is divided into a greater number of sections than the number of gradation sections in the first histogram;
• and
• a gradation correction step of performing gradation correction processing using the second histogram when it is determined in the decision processing that the image contains a low-contrast region, and of performing gradation correction processing using the first histogram when it is determined that the image does not contain a low-contrast region.