aUCBLogo provides traditional Logo turtle graphics with one or more turtles. Collision detection is not supported. This is the most hardware-dependent part of Logo; some features may exist on some machines but not others. Nevertheless, the goal has been tomake Logo programs as portable as possible, rather than to take fullest advantage of the capabilities of each machine. In particular, Logo attempts to scale the screen so that turtle coordinates [-400 -300] and [400 300] fit on the graphics window, and so that the aspect ratio is 1:1, although some PC screens have nonstandard aspect ratios.
The center of the graphics window (which may or may notbe the entire screen, depending on the machine used) is turtle location [0 0]. Positive X is to the right; positive Y is up. Headings (angles) are measured in degrees clockwise from the positive Y axis. (This differs from the common mathematical convention of measuring angles counterclockwise from the positive X axis.) The turtle is represented as an isoceles triangle; the actual turtle position is at the midpoint of the base (the short side).
Colors are, of course, hardware-dependent. However, Logo provides partial hardware independence by interpreting color numbers 0 through 7 uniformly on all computers:
0 black 1 blue 2 green 3 cyan
4 red 5 magenta 6 yellow 7 white
Where possible, Logo provides additional user-settable colors; how many are available depends on the hardware andoperating systemenvironment. If at least 16 colors are available, Logo tries to provide uniform initial settings for the colors 8-15:
8 brown 9 tan 10 forest 11 aqua
12 salmon 13 purple 14 orange 15 grey
Logo begins with a white background and black pen.