Finally, it should repaint with the original background color. The graphics operations on MathLAN workstations are so fast that a test run of your procedure will probably appear as nothing more than a quick flicker. You can slow your procedure down by inserting, in the body of your kernel, a call to the DrScheme procedure sleep . This procedure takes a positive real number as argument and causes DrScheme to suspend its processing for the specified number of seconds. Pens and brushes To draw a foreground figure, such as a rectangle, in a color other than black, we change the drawing context's pen .
Pens are objects.
Most pens use the 'solid style, which draws a continuous line. A device context accepts messages for drawing points, straight lines, circular and elliptical arcs, polygons, rectangles with rounded corners, and splines. You can read about all of these in the MrEd Graphical Toolbox Manual , starting at the page dealing with device contexts . Exercise 3 Define a pen that draws solid lines, two pixels wide, in dark blue. Use it to draw a circle in the middle of drawing-canvas .