Top 200 Programs/Top 200 Programs.iso/Bob8/THOMPSON/LIBERTY/PRODUCT/TUTORIAL.EXE/WK5CM1.TXT

Text File | 1996-06-06 | 12KB | 366 lines

Week 5 course material
Liberty BASIC programming course
Copyright 1996 Shoptalk Systems
All Rights Reserved

An Introduction to Programming Graphics
==============================================================================

One of the most interesting and useful things that can computers can do is
create graphics.  These can be useful diagrams, page layouts, animations,
beautiful works of art, etc.


Kinds of Graphics Controls
==============================================================================

Liberty BASIC programs can display graphics using two different types of
graphics controls.  The first is a graphics window.  We create a graphics
window in our programs using an OPEN statement in the same fashion we create
other kinds of windows:

    open "My Drawing Area" for graphics as #graphWin

We can also add one or more graphicbox controls to other windows, like so:

    graphicbox #main.graph, 10, 10, 150, 200
    open "My Window" for window as #main

Both kinds of graphics controls understand the same set of graphics commands.
Using the second approach can provide more than one drawing area, or it can
be used to create a graphics area with other controls in a window or dialog
box.


Turtle Graphics
==============================================================================

In many computer graphics systems there is a way to draw using a pen and
paper metaphor.  Because some versions of this system used real paper, and
also a pen attached to a small robot that looked like a turtle (a
hemisphere), this way of drawing is often referred to as turtle graphics.

The idea is that you send the turtle commands like:

   -raise the pen up from the paper
   -lower the pen down to the paper
   -move forward a specified distance, drawing if the pen is down
   -turn a specific number of degrees

By stringing these kinds of drawing commands together, useful and meaningful
things can be drawn.

Liberty BASIC includes support for this kind of drawing.  Here is a short
list of the Liberty BASIC turtle graphics commands:

    up    - lift the pen up (don't draw)
    down  - lower the pen down (draw)
    home  - place the pen in the center of the graphics area
    go    - move foreward in the current direction
    goto  - go to a specified position
    place - go to a specified position but don't draw
    turn  - turn the turtle clockwise a specified number of degrees
    north - cause the turtle to point north (straight up)
    posxy - tell us what the location of the turtle is

Let's draw a box in the center of a graphics control:

    'draw a box
    open "Draw A Box" for graphics as #draw
    print #draw, "up"
    print #draw, "home"
    print #draw, "north"
    print #draw, "go 50"
    print #draw, "turn 90"
    print #draw, "go 50"
    print #draw, "down"
    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"
    input r$

Run the program and look at the box we've drawn.  Now maximize the window.
See how the box doesn't get redrawn?  This is because we haven't told Liberty
BASIC that we're done drawing a sequence.  To make what we've drawn 'stick',
we must send a FLUSH command.  Insert the following line before our INPUT
statement:

    print #draw, "flush"

Now rerun the program and maximize the window.  You will see that the box is
redrawn now whenever the window is resized or redrawn.


Doing more with less
------------------------------------------------------------------------------

For what our box drawing program does it is quite long.  There are a couple
of things we can do to shorten the program.  One is to print more than one
command on a line.  Look at this:

    print #draw, "up"
    print #draw, "home"
    print #draw, "north"
    print #draw, "go 50"
    print #draw, "turn 90"
    print #draw, "go 50"
    print #draw, "down"

An equivalent line is:

    print #draw, "up ; home ; north ; go 50 ; turn 90 ; go 50 ; down"

See how we can print more than one command per line by inserting a semicolon
in between each command.

Now we turn the turtle 90 degrees and go 100 pixels.  This happens four
times:

    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"
    print #draw, "turn 90"
    print #draw, "go 100"

With this example we can combine the TURN and GO commands into one line with
a semicolon, and we can use a loop to perform the action four times, like
so:

    for x = 1 to 4
        print #draw, "turn 90 ; go 100"
    next x

Here is the complete program rewritten as above:

    'draw a box
    open "Draw A Box" for graphics as #draw
    print #draw, "up ; home ; north ; go 50 ; turn 90 ; go 50 ; down"
    for x = 1 to 4
        print #draw, "turn 90 ; go 100"
    next x
    print #draw, "flush"

    input r$

For fun, here is a short program that draws a spiral:

    'draw a spiral
    open "Draw A Spiral" for graphics as #draw
    print #draw, " home ; down"
    for x = 1 to 100
        print #draw, "turn 91 ; go "; x * 2
    next x
    print #draw, "flush"

    input r$

See how we take the value of x in our FOR/NEXT loop and use it in our GO
command.  We do this to make each next line longer than the last, and this
is what makes our spiral get bigger as we draw it.

IMPORTANT: Spaces are important in any command that we print to a graphics
control.  If we remove the spaces, the command will not work, and it will
not return an error.  For example, the following line will NOT work:

        print #draw, "turn 91 ; go"; x * 2

See how the space was removed after the GO command?  This small change is
very important.


More about FLUSH
-----------------------------------------------------------------------------

FLUSH doesn't only make drawn items 'stick' so that they redraw.  It gives
all the items drawn before that FLUSH a common number.  This is useful
so that a collection of drawing elements can be referred to together.

Here is an example of how we might use this feature:

    'demonstrate the usefulness of flush
    open "Draw A Circle" for graphics as #draw
    print #draw, " home ; north ; down"
    for x = 1 to 30
        print #draw, "turn 12 ; go 10"
        print #draw, "flush"
    next x

    for x = 1 to 29 step 2
        print #draw, "delsegment "; x
    next x

    print #draw, "redraw"

    input r$

Notice how we flush after each TURN and GO command.  This give each line
its own segment number.  Then we loop through the odd numbers 1, 3,
5, ... 29 so we can delete every other segment.  When we print the REDRAW
command, it only redraws the segments we haven't deleted and we see a
dashed effect.

This segmented graphics capability is used a lot in the FreeForm program
(FF12.BAS) that is included with Liberty BASIC.

We can get the current segment number at any time while drawing by using
the SEGMENT command, like so:

    'demonstrate the usefulness of flush
    open "Draw A Circle" for graphics as #draw
    print #draw, " home ; north ; down"
    for x = 1 to 30
        print #draw, "turn 12 ; go 10"
        print #draw, "flush"
        'get the current segment number and print it
        print #draw, "segment"
        input #draw, segId
        print segId
    next x

    for x = 1 to 29 step 2
        print #draw, "delsegment "; x
    next x

    print #draw, "redraw"

    input r$

The SEGMENT command does NOT get the number of the last segment flushed.
Subtract 1 to get that number.


Color Graphics
-----------------------------------------------------------------------------

We can dress up our graphics by adding some color to them.  Let's look at
two commands for doing this:

    'draw a spiral
    open "Draw A Spiral" for graphics as #draw
    print #draw, "fill blue"
    print #draw, " home ; down ; color white"
    for x = 1 to 100
        print #draw, "turn 91 ; go "; x * 2
    next x
    print #draw, "flush"

    input r$

The FILL command (the line after our OPEN statement) causes our graphic
control to be filled with the color blue.  Then our spiral is drawn as
white by setting the turtle's pen to white in the next line.

Users running 65,000 or more color drivers on their machine can specify
RGB values for colors.  Each color can be given a value from 0 to 255 with
0 being darkest and 255 being lightest.  Try this example:

    'draw a spiral
    open "Draw A Spiral" for graphics as #draw
    print #draw, "fill darkgray"
    print #draw, " home ; down"
    for x = 1 to 100
        print #draw, "color "; x * 2 + 55; " 0 " ; 200 - x
        print #draw, "turn 91 ; go "; x * 2
    next x
    print #draw, "flush"

    input r$

Here is a list of valid colors:

    black, blue, brown, cyan, darkblue, darkcyan, darkgray,
    darkgreen, darkpink, darkred, green, lightgray, palegray,
    pink, red, white, yellow



Line Thickness
------------------------------------------------------------------------------

So far everything we've drawn has been razor-thin.  Drawing with a bold line
is simple matter.  The SIZE command is all that's needed.  Try this:

    'draw a box
    open "Draw A Box" for graphics as #draw
    print #draw, "up ; home ; north ; go 50 ; turn 90 ; go 50 ; down"
    'use a big fat pen
    print #draw, "size 10"
    for x = 1 to 4
        print #draw, "turn 90 ; go 100"
    next x
    print #draw, "flush"

    input r$

If no size is specified, the default is 1.


Drawing Text with Graphics
------------------------------------------------------------------------------

Drawing text in a graphics control is simple.  Position the pen as desired
and print the text with an backslash in front.  Take a look:

    'display some text
    open "Text example" for graphics as #draw
    print #draw, "place 50 50"
    print #draw, "\Hello world!"
    print #draw, "flush"

    input r$

Each backslash in printed text causes a new line to be printed.  So if you
want to display text that has a backslash in it (like a file path), then
begin your text with a vertical bar '|' like so:

    'display some text
    open "Text example" for graphics as #draw
    print #draw, "place 50 50"
    print #draw, "|c:\config.sys"
    print #draw, "flush"

    input r$

You can use the COLOR command to change the color of printed text.  Watch
what happens when you also fill the window with some other color.

    'display some text
    open "Text example" for graphics as #draw
    print #draw, "fill red ; color blue"
    print #draw, "place 50 50"
    print #draw, "|c:\config.sys"
    print #draw, "flush"

    input r$

To change this white backdrop to the be the same color as our FILL of red,
we can use the BACKCOLOR command, like this:

    'display some text
    open "Text example" for graphics as #draw
    print #draw, "fill red ; color blue ; backcolor red"
    print #draw, "place 50 50"
    print #draw, "|c:\config.sys"
    print #draw, "flush"

    input r$


Challenge Exercise
------------------------------------------------------------------------------

Create a program named TIME.BAS that gets the current time from the TIME$()
function and then draws a clock with an hour, minute and second hand.  The
display should have a background color, and each hand should have its own
color.  A button on the clock should cause the display to redraw with the
current time when clicked on.  The program doesn't have to run continuously.