Extension Commands

Extension commands are primitives that are available in logopy, but unlikely to be available in other logo implementations. Further, some extensions may only be compatible with certain turtle backends.

Extensions provide interesting features at the expense of portability. All logopy extension commands begin with “EXT.” to make it clear to the programmer that this is an extension that is probably not available on a different Logo interpreter.


to EXT.ELLIPSE :major :minor [:angle 360] [:clockwise true]

The EXT.ELLIPSE command draws an ellipse with its major and minor axes set to lengths major and minor. The major axis will bisect the ellipse at an angle equal to the current turtle heading. The minor axis bisects the ellipse at an angle perpendicular to the major axis. The major axis may be shorter than the minor axis.

The center of the ellipse will be located to the right of the turtle if the ellipse is drawn clockwise. If the ellipse is drawn anti-clockwise, the center will be to the left of the turtle. In both cases, the center of the ellipse will be half the length of the minor axis from the turtle.

An optional parameter may be specified to indicate the angle that should be swept out from the center of the ellipse when drawing its perimeter. In this manner, an elliptical arc can be drawn instead of a complete ellipse. Drawing starts at the turtle’s starting position, and continues throughout angle degrees around the perimeter of the ellipse.

The optional parameter clockwise determines whether the turtle will trace out the ellipse in a clockwise or anti-clockwise direction.

The turtle moves as it draws the ellipse. If a complete ellipse is drawn, the turtle will end up at its original starting position and heading. Otherwise, the turtle will move to the end of the elliptical arc and have a heading perpendicular to the curve at that point.


to EXT.UNFILLED :instrlist

The EXT.UNFILLED command behaves specially when used with the SVG turtle backend. It can be used inside a FILLED instruction to indicate that an enclosed shape should not be filled.

When used with the TK GUI back end, the Logo instructions contained in instrlist are processed normally by the interpreter.

When used with the SVG turtle backend, a number of special rules apply. First, the extension can only be used within an enclosing FILLED command. When used within a FILLED command, the path(s) traversed are masked. The mask will prevent those areas from being filled.