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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