There is provided a controller configured to execute: obtaining of color values of picture elements, respectively, of P pieces of target picture element columns; identifying of a blank picture element, indicating a white color as a brightest color; and identifying a candidate picture element line as a blank line which is a picture element line indicating the white color as the brightest color, the candidate picture element line being constructed of T pieces of picture elements arranged side by side in a second direction and including the P pieces of target picture elements located at a same position in the first direction, in a case that the candidate picture element line satisfies a first blank line condition, wherein the P is a integer in a range of not less than 2 to less than the T.