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# 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.

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). However, the turtle is drawn one step behind its actual position, so that the display of the base of the turtle's triangle does not obscure a line drawn perpendicular to it (as would happen after drawing a square).

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.

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## 6.1 Turtle Motion

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### forward

    FORWARD dist
    FD dist

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

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### 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.)

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### left

    LEFT degrees
    LT degrees

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

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### right

    RIGHT degrees
    RT degrees

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

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### setpos

    SETPOS pos

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

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### setxy

    SETXY xcor ycor

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

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### setx

    SETX xcor

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

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### sety

    SETY ycor

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

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### 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.

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### home

    HOME

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

See section setpos , See section setheading .

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### 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.

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## 6.2 Turtle Motion Queries

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### pos

    POS

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

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### xcor

    XCOR						(library procedure)

outputs a number, the turtle's X coordinate.

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### ycor

    YCOR						(library procedure)

outputs a number, the turtle's Y coordinate.

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### heading

    HEADING

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

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### 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.

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### 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 .

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## 6.3 Turtle and Window Control

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### showturtle

    SHOWTURTLE
    ST

makes the turtle visible.

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### 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.

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### 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.

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### 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 .

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### 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 .

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### 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 .

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### 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 .

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### 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.

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### filled

    FILLED color instructions

runs the instructions, remembering all points visited by turtle motion commands, starting and ending with the turtle's initial position. Then draws (ignoring penmode) the resulting polygon, in the current pen color, filling the polygon with the given color, which can be a color number or an RGB list. The instruction list cannot include another FILLED invocation. (wxWidgets only)

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### label

    LABEL text

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

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### setlabelheight

    SETLABELHEIGHT height

command (wxWidgets only). Takes a positive integer argument and tries to set the font size so that the character height (including descenders) is that many turtle steps. This will be different from the number of screen pixels if `SETSCRUNCH` has been used. Also, note that `SETSCRUNCH` changes the font size to try to preserve this height in turtle steps. Note that the query operation corresponding to this command is `LABELSIZE`, not `LABELHEIGHT`, because it tells you the width as well as the height of characters in the current font.

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### 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 .

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### 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`.

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.

In the DOS version, switching from fullscreen to splitscreen loses the part of the picture that's hidden by the text window. [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.]

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### 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 .

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### 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 all modern computers, 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. For DOS, the values set by `SETSCRUNCH` are remembered in a file (called ‘`scrunch.dat`’) and are automatically put into effect when a Logo session begins.

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### refresh

    REFRESH

(command) tells Logo to remember the turtle's motions so that they can be used for high-resolution printing (wxWidgets) or to refresh the graphics window if it is moved, resized, or overlayed (non-wxWidgets). This is the default.

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### norefresh

    NOREFRESH

(command) tells Logo not to remember the turtle's motions, which may be useful to save time and memory if your program is interactive or animated, rather than drawing a static picture you'll want to print later (wxWidgets). In non-wxWidgets versions, using NOREFRESH may prevent Logo from restoring the graphics image after the window is moved, resized, or overlayed.

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## 6.4 Turtle and Window Queries

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### 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 .

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### screenmode

    SCREENMODE

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

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### turtlemode

    TURTLEMODE

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

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### labelsize

    LABELSIZE

(wxWidgets only) outputs a list of two positive integers, the width and height of characters displayed by `LABEL` measured in turtle steps (which will be different from screen pixels if `SETSCRUNCH` has been used). There is no `SETLABELSIZE` because the width and height of a font are not separately controllable, so the inverse of this operation is `SETLABELHEIGHT`, which takes just one number for the desired height.

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## 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).

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### pendown

    PENDOWN
    PD

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

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### penup

    PENUP
    PU

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

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### penpaint

    PENPAINT
    PPT

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

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### penerase

    PENERASE
    PE

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

See section erase .

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### penreverse

    PENREVERSE
    PX

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

See section reverse .

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### setpencolor

    SETPENCOLOR colornumber.or.rgblist
    SETPC colornumber.or.rgblist

sets the pen color to the given number, which must be a nonnegative integer. There are initial assignments for the first 16 colors:

     0  black	 1  blue	 2  green	 3  cyan
     4  red		 5  magenta	 6  yellow	 7 white
     8  brown	 9  tan		10  forest	11  aqua
    12  salmon	13  purple	14  orange	15  grey

but other colors can be assigned to numbers by the `PALETTE` command. Alternatively, sets the pen color to the given RGB values (a list of three nonnegative numbers less than 100 specifying the percent saturation of red, green, and blue in the desired color).

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### 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 numbers less than 100 specifying the percent saturation of red, green, and blue in the desired color.

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### setpensize

    SETPENSIZE size

sets the thickness of the pen. The input is either a single positive integer or a list of two positive integers (for horizontal and vertical thickness). Some versions pay no attention to the second number, but always have a square pen.

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### setpenpattern

    SETPENPATTERN pattern

sets hardware-dependent pen characteristics. This command is not guaranteed compatible between implementations on different machines.

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### setpen

    SETPEN list					(library procedure)

sets the pen's position, mode, thickness, 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 .

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### setbackground

    SETBACKGROUND colornumber.or.rgblist
    SETBG colornumber.or.rgblist

set the screen background color by slot number or RGB values. See `SETPENCOLOR` for details.

See section setpencolor .

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## 6.6 Pen Queries

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### pendownp

    PENDOWNP
    PENDOWN?

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

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### penmode

    PENMODE

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

See section erase , reverse .

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### pencolor

    PENCOLOR
    PC

outputs a color number, a nonnegative integer that is associated with a particular color, or a list of RGB values if such a list was used as the most recent input to `SETPENCOLOR`. There are initial assignments for the first 16 colors:

     0  black        1  blue         2  green        3  cyan
     4  red          5  magenta      6  yellow       7 white
     8  brown        9  tan         10  forest      11  aqua
    12  salmon      13  purple      14  orange      15  grey

but other colors can be assigned to numbers by the `PALETTE` command.

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### palette

    PALETTE colornumber

outputs a list of three nonnegative numbers less than 100 specifying the percent saturation of red, green, and blue in the color associated with the given number.

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### pensize

    PENSIZE

outputs a list of two positive integers, specifying the horizontal and vertical thickness of the turtle pen. (In some implementations, including wxWidgets, the two numbers are always equal.)

    PENPATTERN

outputs system-specific pen information.

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### pen

    PEN						(library procedure)

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

See section setpen .

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### background

    BACKGROUND
    BG

outputs the graphics background color, either as a slot number or as an RGB list, whichever way it was set. (See `PENCOLOR`.)

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## 6.7 Saving and Loading Pictures

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### 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.

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### 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 .

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### 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 `FILL`, `PENERASE`, or `PENREVERSE`; any such instructions will be ignored in the translation to Postscript form.

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

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## 6.8 Mouse Queries

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### mousepos

    MOUSEPOS

outputs the coordinates of the mouse, provided that it's within the graphics window, in turtle coordinates. If the mouse is outside the graphics window, then the last position within the window is returned. Exception: If a mouse button is pressed within the graphics window and held while the mouse is dragged outside the window, the mouse's position is returned as if the window were big enough to include it.

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### clickpos

    CLICKPOS

outputs the coordinates that the mouse was at when a mouse button was most recently pushed, provided that that position was within the graphics window, in turtle coordinates. (wxWidgets only)

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### buttonp

    BUTTONP
    BUTTON?

outputs `TRUE` if a mouse button is down and the mouse is over the graphics window. Once the button is down, `BUTTONP` remains true until the button is released, even if the mouse is dragged out of the graphics window.

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### button

    BUTTON

outputs 0 if no mouse button has been pushed inside the Logo window since the last call to `BUTTON`. Otherwise, it outputs an integer between 1 and 3 indicating which button was most recently pressed. Ordinarily 1 means left, 2 means right, and 3 means center, but operating systems may reconfigure these.

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