Abstract: A start point (x'.sub.0,y'.sub.0) and an end point (x'.sub.m,y'.sub.m) of a sub-line are obtained from a start point (x.sub.0,y.sub.0) and an end point (x.sub.m,y.sub.m) of a main line, and coordinate data of the start point (x'.sub.0,y'.sub.0) and the end point (x'.sub.m,y'.sub.m) are supplied to a DDA, so that a point array of the sub-line can be generated from the DDA. The main line is shifted along the x- or y-axis by one dot to obtain the sub-line. Intensity information from an intensity modulation circuit is noninverted/inverted in accordance with the main line/sub-line mode so as to provide a complementary relationship between the main line and the sub-line.A vector direction from the start point (x.sub.0,y.sub.0) to the end point (x.sub.m,y.sub.m) is detected to belong to one of the four areas obtained by dividing the x-y coordinate plane by lines y=x and y=-x when the start point (x.sub.0,y.sub.0) is defined as the origin of the coordinate system.