print <<'--';

   (x1,y1)                   (x2,y2)
      *-------------------------*

--
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Abigail

Nowadays, most graphics toolkits already supply you with a function line_to or line, which draws the line for you, so I recommend that you read through the documentation of your graphics toolkit and find what the name and the usage of the corresponding function is.

If your graphics toolkit only has a set_pixel function and no line function, then using Bresenhams algorithm for drawing lines is most likely one of the fast methods to draw lines. Understanding how the algorithm works is also very rewarding, as the algorithm operates without floating point numbers.

If you are more looking for the general concept of drawing a line between two points, the drawing of a line between two points can be interpreted as finding all points (x,y) on the line passing through (x1,y1);(x2,y2) where x1 <= x < x2.

For lazy people, here's a perl implementation of this algorithm:

        my($x1,$y1, $x2,$y2) = @_;
        use integer;
        my($dx, $dy, $incr1, $incr2, $d, $x, $y, $xend, $yend, $xdirfl
+ag, $ydirflag);
        $dx = abs($x2-$x1);
        $dy = abs($y2-$y1);
        if ($dy <= $dx) {
        $d = 2*$dy - $dx;
        $incr1 = 2*$dy;
        $incr2 = 2 * ($dy - $dx);
        if ($x1 > $x2) {
            $x = $x2;
            $y = $y2;
            $ydirflag = (-1);
            $xend = $x1;
        } else {
            $x = $x1;
            $y = $y1;
            $ydirflag = 1;
            $xend = $x2;
        }
        set_point($x,$y);
        if ((($y2 - $y1) * $ydirflag) > 0) {
            while ($x < $xend) {
            $x++;
            if ($d <0) {
                $d+=$incr1;
            } else {
                $y++;
                $d+=$incr2;
            }
            _set_point($x,$y);
            }
        } else {
            while ($x < $xend) {
            $x++;
            if ($d <0) {
                $d+=$incr1;
            } else {
                $y--;
                $d+=$incr2;
            }
            set_point($x,$y);
            }
        }
        } else {
        $d = 2*$dx - $dy;
        $incr1 = 2 * $dx;
        $incr2 = 2 * ($dx - $dy);
        if ($y1 > $y2) {
            $y = $y2;
            $x = $x2;
            $yend = $y1;
            $xdirflag = (-1);
        } else {
            $y = $y1;
            $x = $x1;
            $yend = $y2;
            $xdirflag = 1;
        }
        set_point($x,$y);
        if ((($x2 - $x1) * $xdirflag) > 0) {
            while ($y < $yend) {
            $y++;
            if ($d < 0) {
                $d+=$incr1;
            } else {
                $x++;
                $d+=$incr2;
            }
            set_point($x,$y);
            }
        } else {
            while ($y < $yend) {
            $y++;
            if ($d < 0) {
                $d+=$incr1;
            } else {
                $x--;
                $d+=$incr2;
            }
            set_point($x,$y);
            }
        }
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%!
%% Example of simple line drawing

/inch {72 mul} def      % Define an inch->point conversion function

newpath                 % Start a new path

1 inch 1 inch moveto    % Set the current point an inch in and up

2 inch 1 inch lineto    % Create an inch long line segment to the righ
+t

stroke                  % Draw the line

showpage                % Paint and eject the page
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Nice idea!
There is a CPAN-module which makes it easy to create a postscript (I never used it, but the description says it is simple :-))

use Tk;
my ($x1,$y1,$x2,$y2) = (100,100,130,130);
my $top = MainWindow->new;
my $can = $top->Canvas()->pack;
$can->createLine($x1,$y1,$x2,$y2);
MainLoop;
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use strict;
use esp;
esp::make_me_a_magical_line(x1,y1,x2,y2);
esp::thank_you;
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