1. An image generating apparatus, comprising: a glass plate configured to hold a document placed upside down on the glass plate; a calibration pattern placed on the glass plate; a plurality of image sensors arranged under the glass plate, wherein each of the plurality of image sensors is configured to capture and generate a raw image, the raw image covers a portion of the document and a portion of the calibration pattern through the glass plate; and a processing unit capable of combining all raw images generated by the plurality of image sensors based on the portion of the calibration pattern in the raw image captured by each of the plurality of image sensors, and the processing unit is further configured to generate a complete image of the document.