TEACH SEEPICTURE                                  Aaron Sloman, Nov 1981

-- INTRODUCTION TO THE SEEPICTURE PACKAGE -----------------------------

Before working through this file, you should have done TEACH * TURTLE to
find out how the turtle program works.  This is about making the pop11
turtle 'draw' pictures and running programs which analyse the pictures.
You should also be familiar with the POP-11 DATABASE. (TEACH * DATABASE).

Try the following commands.  (Mark the range and then use CONTROL D to
load it.  If you do not know how to do this see TEACH * LMR)

    turtle();
    newpicture(20,15);
    draw(8);
    turn(90);
    draw(8);
    display();

The command 'TURTLE();' caused the computer to invoke the POP11 turtle,
which crawls around an imaginary picture leaving a trail of asterisks. The
command 'newpicture(20,15)' makes the picture a convenient size for the
screen. The command DISPLAY causes the picture drawn by the other commands
to be printed on the terminal. The computer cannot see what is on the
terminal. However, if you type

    lib seepicture;

This will make available a collection of programs which can analyse the
picture as it is stored in the computer, in the form of a two-dimensional
array. The SEEPICTURE program will produce descriptions of the picture in
the POP11 database. You should therefore have worked through TEACH
DATABASE before going on.

The SEEPICTURE procedure uses three procedures FINDLINES, FINDJUNCS and
PAINTPICTURE. The first looks for lines in the turtle picture and puts a
description in the database. The second finds where lines meet and where
the ends are, and produces a description of junctions and ends in the
database. The third alters the turtle picture to show what lines,
junctions and ends have been found. NB These procedures work only on lines
which are horizontal, vertical or diagonal (45 degrees). Very short lines
will not be found, for reasons which will become clear later.

-- findlines ----------------------------------------------------------

We now have a look at what FINDLINES does. It is assumed that you have
already made a picture, as above. The following commands cause the
computer to examine the picture, store a description in the database, and
print out the result:

    findlines();
    database ==>

try that.

The FINDLINES program distinguishes horizontal and vertical lines. Now
clear the picture and start again, with a more complex picture:

    turtle();
    repeat 4 times
        draw(10); turn(90);
    endrepeat;
    display();

Now see what the computer can see in that:

    findlines();
    database ==>

-- findjuncs ----------------------------------------------------------

You can ask the computer to examine the line description and build up a
description of the junctions, recording, for each junction, its type, its
location, and the locations of the junctions connected to it:

    findjuncs();
    database ==>

You can make the picture more complex, and have it re-examined:

    jumpto(1,5);
    drawto(15,5);
    display();

Now the computer should find more junctions, including a CROSS a TEE and a
free END.  Try:

    findlines();
    findjuncs();
    database ==>

Notice that, for convenience, ENDS of lines are treated like junctions,
i.e. there are entries in the database of the form

    [junc end <location> <nextpoint>]

Note that CRS means 'CROSS'.

-- paintpicture -------------------------------------------------------
There is also a program called paintpicture, which uses the information
put in the database by findlines and findjuncs to re-draw the picture.
Try this:

    paintpicture();
    display();

The computer will mark the orientations of the lines, and the locations
of the different types of junctions.

-- seepicture ---------------------------------------------------------
Instead of calling first the procedure FINDLINES, then the procedure
FINDJUNCS, then PAINTPICTURE, you can use the single procedure SEEPICTURE,
which does the lot. Try:

    turtle();
    draw(5);
    turn(90);
    draw(10);
    turn(-90);
    draw(5);
    display();

Now let SEEPICTURE do its work, showing us how it calls the other
procedures:

    trace findlines findjuncs paintpicture;
    seepicture();
    database ==>

See how the picture has been repainted:

    display();

The turtle is best at drawing horizontal and vertical lines. Some lines
will work at 45 degrees, but things can go wrong. You can, if you wish,
now experiment with different pictures, using commands like:

    turtle();   ;;; to clear the picture (it is 15 along and 10 up)
    jump(5);    ;;; to jump 5 steps in the current direction
    draw(8);    ;;; to draw 8 steps in the current direction
    turn(90);   ;;; to turn the current direction 90 degrees
                ;;; anti-clockwise
    turn(-90)   ;;; ditto, but clockwise.
    seepicture();   ;;; To analyse the picture
    database ==>    ;;; To print out the database description.
    display();      ;;; To print out the picture

If you want to learn more about the turtle see TEACH * TURTLE;

Having produced a database description of a picture, you can then use
PRESENT, LOOKUP, ALLPRESENT and FOREACH in procedures which try to make
use of the database information to interpret the picture, for instance,
by recognising objects. For example, could you define a procedure which
will recognise a square.

For a summary of the facilities described here see
HELP * SEEPICTURE

For addition work on analysing pictures see the following TEACH files
TEACH PICTURES    Exercises on analysing straight line 'turtle' drawings
TEACH REGIONS   Use of LIB regions for region finding in turtle pictures
TEACH LABELLING   Introduction to line-labelling in a lamina world
TEACH WALTZ       Introduction to Waltz-filtering

-----<Copyright University of Sussex 1987.  All rights reserved.>-------