Image scanner

The present invention relates generally to an image scanner with improved image processing speed, and more specifically to an image scanner in which a double-side read mode for reading image data on the right and reverse sides of a document with improved resolution through the optimum signal processing is provided to read image data on the right and reverse sides of a double-sided manuscript simultaneously and the read image data is transmitted to a host computer at high speed; the image data obtained being subjected to white level control. An enhancement and interpolation circuit forms enhanced image data by performing enhancement and interpolation processing on image data. A floating slice circuit obtains a floating slice level for said image data. A binarization and density conversion circuit binarizes enhanced image data using the floating slice level to form binarized image data. In the double-side read mode, simultaneously read image data on the right and reverse sides of a document are transferred at a first rate. In the one-side read mode, image data on the right side of a document is transferred at a second rate that is slower than the first rate by data transfer means. An image discriminating circuit forms an image discriminating instruction for read image data. A white level signal generating circuit forms a new white level signal by subjecting said image data and a current white level signal in accordance with an operation designation instruction corresponding to an image discriminating instruction. This new white level signal is stored as the next white level signal.