Computer-readable storage medium storing image processing program and image processing apparatus

An image processing apparatus comprising:

• a memory configured to store a source picture having a feature area in which pixel pairs are defined;
• and
• a processor coupled to the memory and configured to execute a process including:
• calculating luminance differences of the individual pixel pairs in the feature area, the luminance differences each being a difference in luminance between pixels constituting a corresponding one of the pixel pairs;
• and
• calculating a local feature value of the feature area, based on the calculated luminance differences of the individual pixel pairs, the local feature value being a collection of bit values respectively corresponding to the individual pixel pairs,
• wherein the calculating of the local feature value includes:
• comparing a specific luminance difference with a specified range between a lower bound and an upper bound, the specified range including a zero point of luminance difference, the specific luminance difference having been calculated for a specific pixel pair corresponding to a specific bit value in the local feature value,
• assigning a first value to the specific bit value when the specific luminance difference is greater than the upper bound of the specified range,
• assigning a second value to the specific bit value when the specific luminance difference is smaller than the lower bound of the specified range, and
• assigning a predetermined one of the first and second values to the specific bit value when the specific luminance difference falls in the specified range.