Image scanner

An image scanner comprising:

• a light source configured to emit light of each of three colors of red, blue, and green;
• a light source controller configured to control the light source to sequentially emit light of each of the three colors in a turn-on sequence;
• a line sensor configured to read an image, on a document, of one line along a first direction;
• a position changer configured to change a reading position in a second direction perpendicular to the first direction, the reading position being a position of the image of one line that is read by the line sensor;
• a reader configured to, while changing the reading position by the position changer and emitting light from the light source to the document in the turn-on sequence, cause the line sensor to read reflection light of the emitted light, thereby acquiring gradation image data including gradation values of the three colors for each pixel in one line;
• a top color shift determiner configured to determine whether a top color shift occurs in the gradation image data at a determination position that is a position of a particular pixel in one line, the top color shift being caused by a change of an image to be read, in the second direction, from a white image to a black image;
• a top replacing unit configured to, in response to determining that the top color shift occurs at the determination position, replace gradation image data at the determination position with black image data;
• a bottom color shift determiner configured to determine whether a bottom color shift occurs in the gradation image data at the determination position, the bottom color shift being caused by a change of an image to be read, in the second direction, from a black image to a white image;
• and
• a bottom replacing unit configured to, in response to determining that the bottom color shift occurs at the determination position, replace gradation image data at the determination position with white image data.