Image data processing apparatus and method

An image data processing apparatus, comprising:

• a reader that reads image data included an image file from an external storage medium, the image data being variable-length coded data of image blocks obtained by dividing an image in a main scanning direction and a sub scanning direction and arranged in an order of the main scanning direction and the sub scanning direction;
• a storage that temporarily saves the image data read by the reader;
• a printing unit that prints the image corresponding to the image file on a recording material based on print data in response to a print instruction from a user;
• and
• a data processor that requests image data to the reader and performs a specified data processing to the image data fed from the reader in response to the request to generate the print data,
• wherein:
• the reader performs a first reading processing for reading the image data in an arrangement order thereof from the storage medium and the data processor performs a boundary information derivation processing based on the image data for obtaining boundary information representing a boundary in a data arrangement in the storage medium between the image data belonging to two image blocks located adjacent in the data arrangement in the storage medium and at mutually different positions in the sub scanning direction, whereas the reader performs a second reading processing for reading the image data from the storage medium in an order determined based on the boundary information and saving them in the storage;
• and
• at least one of the boundary information derivation processing and the second reading processing is started before receiving the print instruction from the user.