Image reading apparatus and method for performing shading correction using reference data generated by reading a moving surface of reference plate and positioning that plate based on sheet size

A method for generating image data, comprising:

• receiving an input of a sheet size;
• determining whether the input sheet size is a non-standard size or a standard size;
• generating, with an image reading unit, first or second reference data and controlling movement of a reference plate relative to the image reading unit, the reference plate including a reference surface for generating reference data for shading correction of the image reading unit and a background surface for background of a sheet to be read to generate image data, based on whether the input sheet size is the non-standard size or the standard size so that:
• if the input sheet size is determined to be the non-standard size, the image reading unit generates the first reference data by reading the reference surface of the reference plate while the reference surface is incrementally moved relative to the image reading unit so that, after the first reference data is generated, the reference surface is positioned at a reading position of the image reading unit, and
• if the input sheet size is determined to be the standard size, the image reading unit generates the second reference data by reading the reference surface of the reference plate while incrementally moving the reference surface relative to the image reading unit so that, after the second reference data is generated, the background surface is positioned at the reading position of the image reading unit;
• and
• performing the shading correction in the image reading unit based on the generated first or second reference data.