LOGO+instructions

code GRAPHICS

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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.

TURTLE MOTION -

FORWARD dist FD dist

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

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 degrees LT degrees

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

RIGHT degrees RT degrees

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

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.

SETXY xcor ycor

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

SETX xcor

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

SETY ycor

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

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

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

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.

TURTLE MOTION QUERIES -

POS

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

XCOR							(library procedure)

outputs a number, the turtle's X coordinate.

YCOR							(library procedure)

outputs a number, the turtle's Y coordinate.

HEADING

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

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

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.)

TURTLE AND WINDOW CONTROL -

SHOWTURTLE ST

makes the turtle visible.

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

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 CS

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

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.

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.

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.

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.

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.

LABEL text

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

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.

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.

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.]

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.

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.)

In wxWidgets only, SETSCRUNCH also changes the size of the text font used for the LABEL command to try to keep the height of characters scaled with the vertical turtle step size.

For all modern computers 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.

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.

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.

TURTLE AND WINDOW QUERIES -

SHOWNP SHOWN?

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

SCREENMODE

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

TURTLEMODE

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

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.

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 PD

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

PENUP PU

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

PENPAINT PPT

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

PENERASE PE

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

PENREVERSE PX

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

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).

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.

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.

SETPENPATTERN pattern

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

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.

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

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

PEN QUERIES ---

PENDOWNP PENDOWN?

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

PENMODE

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

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.

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.

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.

PEN							(library procedure)

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

BACKGROUND BG

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

SAVING AND LOADING PICTURES ---

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. See EPSPICT to export Logo graphics for other programs.

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.

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