Instead of giving the exact or absolute position of a point on the screen, its

distance from another point can be given -- this is the relative position of the

point:

The point A in the figure is at 500,200. The point B is 200 to the left and 300

above A, so its relative position is -200,300. This program draws a rectangle

using A and B as the positions of the two comers:

10 MODE 129

20 MOVE 500,200

30 PLOT 97,-200,300

Now change line 20 and RUN the program again:

20 MOVE 300, 100

PLOT 97 tells the computer to draw a rectangle using the two points given,

with the second point being relative to the first point. This means that the

computer always draws the same size rectangle, wherever the first point is

placed. Relative positioning is very useful if a drawing needs to be moved

around on the screen.

The PLOT commands are very versatile and provide a great deal of control over

how images are drawn. Lines or figures can be drawn absolutely or relatively,

solid or dotted, in the foreground or background colour. Figures can be drawn

in outline or as solid blocks of colour. A full list of the PLOT commands is given

in Appendix 8.

C 18