Jpeg-compressed file creating method

A JPEG-compression-file generation method for generating a JPEG compression file from image data, using a second quantization table obtained based on a quantization data coefficient value and on a first quantization table as a reference, the method comprising:

• (a) a step of deriving using a computer, from the image data before JPEG compression, image feature value;
• (b) a step of deriving using a computer the quantization data coefficient value by, with respect to a relational expression, which includes the quantization data coefficient value and the image feature value, for expressing the file size of a JPEG compression file, designating a desired file size as the file size of the JPEG compression file, and
• the step (a) comprises:
• (a-1) a step of dividing the image data into a plurality of unit blocks;
• (a-2) a step of calculating for each of the unit blocks a pixel-value difference between pixels in predetermined positions;
• and
• (a-3) a step of deriving the image feature value by summing each pixel-value difference calculated in the step (a-2);
• and
• the step (a-2) comprises:
• (a-2-1) a step of deriving a first of the image feature values using a pixel-value difference between two pixels at a first distance range;
• and
• (a-2-2) a step of deriving a second of the image feature values using a pixel-value difference between two pixels at a second distance range.