Adding noise to captcha

[risoknop]

I have made a simple captcha class today, it's already functional but I'm thinking of some clever way to add some random geometrical shapes in order to create a noise.

Should I just flood the captcha with some random lines?

Also, I suppose the captcha text should be stored in the session, am I right?

Here is what I have so far:

<?php

/*
 @desc Creates captcha verification image
 @author Richard Knop
*/
class Captcha
{
    private $length;

    private $string = array();
    
    private $image;
    
    public function generateString($length, $base = '1234567890qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM')
    {
        $this->length = $length;
        for ($i = 0, $max = strlen($base) - 1; $i < $length; $i++)
        {
            $this->string[] = $base[rand(0,$max)];  
        }
    }
    
    public function draw($fontSize)
    {
        header("Content-type: image/png");
        
        $width = $fontSize * 2 * $this->length;
        $height = 3 * $fontSize;
        
        $this->image = imagecreatetruecolor($width, $height);
        
        $r = rand(0, 127);
        $g = rand(0, 127);
        $b = rand(0, 127);
        $bgColor = imagecolorallocate($this->image, $r, $g, $b);
        imagefill($this->image, 0, 0, $bgColor);
        
        $font = 'arial.ttf';
        $partialWidth = $width / $this->length;
        for ($i = 0; $i < $this->length; $i++)
        {
            $angle = rand(-45, 45);
            $bbox = imagettfbbox($fontSize, $angle, $font, $this->string[$i]);
            $w = $bbox[2] - $bbox[0];
            $h = $bbox[1] - $bbox[7];

            $x = $i * $partialWidth + ($partialWidth - $w) / 2;
            $y = $height - ($height - $h) / 2;
            $color = imagecolorallocate($this->image, rand(192, 255), rand(192, 255), rand(192, 255));
            imagettftext($this->image, $fontSize, $angle, $x, $y, $color, $font, $this->string[$i]);
        }
        
        $lineColor = imagecolorallocate($this->image, 255 - $r, 255 - $g, 255 - $b);
        for ($increment = $height / 20, $i = $increment; $i < $height; $i += $increment)
        {
            imageline($this->image, 0, $i, $width, $i, $lineColor);
        }
        
        imagepng($this->image);
        imagedestroy($this->image);
    }
    
    public function getString()
    {
        return implode('', $this->string);
    }
}

$captcha = new Captcha();
$captcha->generateString(5);
$captcha->draw(20);

Just copy and paste the code to test it, if interested.

[risoknop]

Added some basic noise (random lines) but it's not exactly very brilliant... any ideas?