6. Graphics

Berkeley Logo provides traditional Logo turtle graphics with one turtle. Multiple turtles, dynamic turtles, and collision detection are 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 to make 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 [--100 --100] and [100 100] 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 not be 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 and operating system environment. 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 black background and white pen.

6.1 Turtle Motion
6.2 Turtle Motion Queries
6.3 Turtle and Window Control
6.4 Turtle and Window Queries
6.5 Pen and Background Control
6.6 Pen Queries
6.7 saving and loading pictures

6.1 Turtle Motion

forward
back
left
right
setpos
setxy
setx
sety
setheading
home
arc

forward

FORWARD dist FD dist

moves the turtle forward, in the direction that it's facing, by the specified distance (measured in turtle steps).

back

BACK dist BK dist

moves the turtle backward, i.e., exactly opposite to the direction that it's facing, by the specified distance. (The heading of the turtle does not change.)

left

LEFT degrees LT degrees

turns the turtle counterclockwise by the specified angle, measured in degrees (1/360 of a circle).

right

RIGHT degrees RT degrees

turns the turtle clockwise by the specified angle, measured in degrees (1/360 of a circle).

setpos

SETPOS pos

moves the turtle to an absolute screen position. The input is a list of two numbers, the X and Y coordinates.

setxy

SETXY xcor ycor

moves the turtle to an absolute screen position. The two inputs are numbers, the X and Y coordinates.

setx

SETX xcor

moves the turtle horizontally from its old position to a new absolute horizontal coordinate. The input is the new X coordinate.

sety

SETY ycor

moves the turtle vertically from its old position to a new absolute vertical coordinate. The input is the new Y coordinate.

setheading

SETHEADING degrees SETH degrees

turns the turtle to a new absolute heading. The input is a number, the heading in degrees clockwise from the positive Y axis.

home

HOME

moves the turtle to the center of the screen. Equivalent to SETPOS [0 0] SETHEADING 0.

See section setpos , See section setheading .

arc

ARC angle radius

draws an arc of a circle, with the turtle at the center, with the specified radius, starting at the turtle's heading and extending clockwise through the specified angle. The turtle does not move.

6.2 Turtle Motion Queries

pos
xcor
ycor
heading
towards
scrunch

pos

POS

outputs the turtle's current position, as a list of two numbers, the X and Y coordinates.

xcor

XCOR (library procedure)

outputs a number, the turtle's X coordinate.

ycor

YCOR (library procedure)

outputs a number, the turtle's Y coordinate.

heading

HEADING

outputs a number, the turtle's heading in degrees.

towards

TOWARDS pos

outputs a number, the heading at which the turtle should be facing so that it would point from its current position to the position given as the input.

scrunch

SCRUNCH

outputs a list containing two numbers, the X and Y scrunch factors, as used by SETSCRUNCH. (But note that SETSCRUNCH takes two numbers as inputs, not one list of numbers.)

See section setscrunch .

6.3 Turtle and Window Control

showturtle
hideturtle
clean
clearscreen
wrap
window
fence
fill
label
textscreen
fullscreen
splitscreen
setscrunch
refresh
norefresh

showturtle

SHOWTURTLE ST

makes the turtle visible.

hideturtle

HIDETURTLE HT

makes the turtle invisible. It's a good idea to do this while you're in the middle of a complicated drawing, because hiding the turtle speeds up the drawing substantially.

clean

CLEAN

erases all lines that the turtle has drawn on the graphics window. The turtle's state (position, heading, pen mode, etc.) is not changed.

clearscreen

CLEARSCREEN CS

erases the graphics window and sends the turtle to its initial position and heading. Like HOME and CLEAN together.

See section home .

wrap

WRAP

tells the turtle to enter wrap mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will "wrap around" and reappear at the opposite edge of the window. The top edge wraps to the bottom edge, while the left edge wraps to the right edge. (So the window is topologically equivalent to a torus.) This is the turtle's initial mode. Compare WINDOW and FENCE.

See section fence .

window

WINDOW

tells the turtle to enter window mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will move offscreen. The visible graphics window is considered as just part of an infinite graphics plane; the turtle can be anywhere on the plane. (If you lose the turtle, HOME will bring it back to the center of the window.) Compare WRAP and FENCE.

See section home .

fence

FENCE

tells the turtle to enter fence mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will move as far as it can and then stop at the edge with an "out of bounds" error message. Compare WRAP and WINDOW.

See section wrap .

fill

FILL

fills in a region of the graphics window containing the turtle and bounded by lines that have been drawn earlier. This is not portable; it doesn't work for all machines, and may not work exactly the same way on different machines.

label

LABEL text

takes a word or list as input, and prints the input on the graphics window, starting at the turtle's position.

textscreen

TEXTSCREEN TS

rearranges the size and position of windows to maximize the space available in the text window (the window used for interaction with Logo). The details differ among machines. Compare SPLITSCREEN and FULLSCREEN.

See section splitscreen .

fullscreen

FULLSCREEN FS

rearranges the size and position of windows to maximize the space available in the graphics window. The details differ among machines. Compare SPLITSCREEN and TEXTSCREEN.

In the DOS version, switching from fullscreen to splitscreen loses the part of the picture that's hidden by the text window. Also, since there must be a text window to allow printing (including the printing of the Logo prompt), Logo automatically switches from fullscreen to splitscreen whenever anything is printed. [This design decision follows from the scarcity of memory, so that the extra memory to remember an invisible part of a drawing seems too expensive.]

splitscreen

SPLITSCREEN SS

rearranges the size and position of windows to allow some room for text interaction while also keeping most of the graphics window visible. The details differ among machines. Compare TEXTSCREEN and FULLSCREEN.

See section textscreen .

setscrunch

SETSCRUNCH xscale yscale

adjusts the aspect ratio and scaling of the graphics display. After this command is used, all further turtle motion will be adjusted by multiplying the horizontal and vertical extent of the motion by the two numbers given as inputs. For example, after the instruction SETSCRUNCH 2 1 motion at a heading of 45 degrees will move twice as far horizontally as vertically. If your squares don't come out square, try this. (Alternatively, you can deliberately misadjust the aspect ratio to draw an ellipse.)

For Unix machines and Macintoshes, both scale factors are initially 1. For DOS machines, the scale factors are initially set according to what the hardware claims the aspect ratio is, but the hardware sometimes lies. The values set by SETSCRUNCH are remembered in a file (called SCRUNCH.DAT) and are automatically put into effect when a Logo session begins.

refresh

REFRESH

tells Logo to remember the turtle's motions so that they can be reconstructed in case the graphics window is overlayed. The effectiveness of this command may depend on the machine used.

norefresh

NOREFRESH

tells Logo not to remember the turtle's motions. This will make drawing faster, but prevents recovery if the window is overlayed.

6.4 Turtle and Window Queries

shownp
screenmode
turtlemode

shownp

SHOWNP SHOWN?

outputs TRUE if the turtle is shown (visible), FALSE if the turtle is hidden. See SHOWTURTLE and HIDETURTLE.

See section showturtle , hideturtle .

screenmode

SCREENMODE

outputs the word TEXTSCREEN, SPLITSCREEN, or FULLSCREEN depending on the current screen mode.

turtlemode

TURTLEMODE

outputs the word WRAP, FENCE, or WINDOW depending on the current turtle mode.

6.5 Pen and Background Control

The turtle carries a pen that can draw pictures. At any time the pen can be UP (in which case moving the turtle does not change what's on the graphics screen) or DOWN (in which case the turtle leaves a trace). If the pen is down, it can operate in one of three modes: PAINT (so that it draws lines when the turtle moves), ERASE (so that it erases any lines that might have been drawn on or through that path earlier), or REVERSE (so that it inverts the status of each point along the turtle's path).

pendown
penup
penpaint
penerase
penreverse
setpencolor
setpalette
setpensize
setpenpattern
setpen
setbackground

pendown

PENDOWN PD

sets the pen's position to DOWN, without changing its mode.

penup

PENUP PU

sets the pen's position to UP, without changing its mode.

penpaint

PENPAINT PPT

sets the pen's position to DOWN and mode to PAINT.

penerase

PENERASE PE

sets the pen's position to DOWN and mode to ERASE.

See section erase .

penreverse

PENREVERSE PX

sets the pen's position to DOWN and mode to REVERSE. (This may interact in hardware-dependent ways with use of color.)

See section reverse .

setpencolor

SETPENCOLOR colornumber SETPC colornumber

sets the pen color to the given number, which must be a nonnegative integer. Color 0 is always black; color 7 is always white. Other colors may or may not be consistent between machines.

setpalette

SETPALETTE colornumber rgblist

sets the actual color corresponding to a given number, if allowed by the hardware and operating system. Colornumber must be an integer greater than or equal to 8. (Logo tries to keep the first 8 colors constant.) The second input is a list of three nonnegative integers less than 64K (65536) specifying the amount of red, green, and blue in the desired color. The actual color resolution on any screen is probably less than 64K, but Logo scales as needed.

setpensize

SETPENSIZE size SETPENPATTERN pattern

set hardware-dependent pen characteristics. These commands are not guaranteed compatible between implementations on different machines.

setpenpattern

SETPENSIZE size SETPENPATTERN pattern

set hardware-dependent pen characteristics. These commands are not guaranteed compatible between implementations on different machines.

setpen

SETPEN list (library procedure)

sets the pen's position, mode, and hardware-dependent characteristics according to the information in the input list, which should be taken from an earlier invocation of PEN.

See section pen .

setbackground

SETBACKGROUND color SETBG color

set the screen background color.

6.6 Pen Queries

pendownp
penmode
pencolor
palette
pensize
pen
background

pendownp

PENDOWNP PENDOWN?

outputs TRUE if the pen is down, FALSE if it's up.

penmode

PENMODE

outputs one of the words PAINT, ERASE, or REVERSE according to the current pen mode.

See section erase , reverse .

pencolor

PENCOLOR PC

outputs a color number, a nonnegative integer that is associated with a particular color by the hardware and operating system.

palette

PALETTE colornumber

outputs a list of three integers, each in the range 0-65535, representing the amount of red, green, and blue in the color associated with the given number.

pensize

PENSIZE PENPATTERN

output hardware-specific pen information.

pen

PEN (library procedure)

outputs a list containing the pen's position, mode, and hardware-specific characteristics, for use by SETPEN.

See section setpen .

background

BACKGROUND BG

outputs the graphics background color.

6.7 saving and loading pictures

savepict
loadpict
epspict

savepict

SAVEPICT filename

command. Writes a file with the specified name containing the state of the graphics window, including any nonstandard color palette settings, in Logo's internal format. This picture can be restored to the screen using LOADPICT. The format is not portable between platforms, nor is it readable by other programs. epspict to export Logo graphics for other programs.

loadpict

LOADPICT filename

command. Reads the specified file, which must have been written by a SAVEPICT command, and restores the graphics window and color palette settings to the values stored in the file. Any drawing previously on the screen is cleared.

See section savepict .

epspict

EPSPICT filename

command. Writes a file with the specified name, containing an Encapsulated Postscript (EPS) representation of the state of the graphics window. This file can be imported into other programs that understand EPS format. Restrictions: the drawing cannot use ARC, FILL, PENERASE, or PENREVERSE; any such instructions will be ignored in the translation to Postscript form.

See section arc , See section fill , See section penerase , See section penreverse .