this obtained value of the checksum is compared with the already stored value of the checksum of the shading data if the values of both checksums are found to coincide as a result of the comparison image processing using the stored shading data is executed if the values of both checksums do not coincide however image processing using shading data having a fixed value is executed application_number US 61008996 A description an embodiment of the present invention will be described with reference to the drawings fig1 is a block diagram showing the construction of an image processing apparatus according to an embodiment of the present invention as shown in fig1 the image processing apparatus of this embodiment has a cs sensor 1 as means for reading a document the cs sensor mainly improves the performance of the sensor which reads at a magnification of 1 u00d7 a driver circuit 2 supplies a clock signal for driving the cs sensor 1 and an amplifier circuit 3 amplifies the output of the cs sensor 1 to an appropriate level the cs sensor 1 produces an output signal bit by bit pixel by pixel a sample and hold circuit 4 extracts from this output signal only an output portion corresponding to the document image the signal sampled and held by the sample and hold circuit 4 is applied to a dc regenerator circuit 5 which.