Question

I am creating a captcha image (please do not suggest premade ones).

It gives you the ability to call

$captcha = new captcha();
$captcha->size(30)->getImage();

which sets the font size to 30 and then generates the captcha.

my issue is with sizing.

How can I work out how wide the image should be?
the captcha has 6 characters.
I thought I could just do ($size*6)+10 to make the image wide enough + give it a 5px padding on each side (text is centered), but apparently not.

---

Code

<?php

session_start();

class captcha {

    private $font = 'monofont.ttf';
    private $characters = '23456789bcdfghjkmnpqrstvwxyz';
    private $size = 30;
    private $count = 6;
    private $colors = array(
        'b' => array('r' => 0,   'g' => 0,   'b' => 0),
        't' => array('r' => 200, 'g' => 200, 'b' => 200),
        'n' => array('r' => 127, 'g' => 127, 'b' => 127)
    );


    function count($count) {
        $this->count = $count;
        return $this;
    }

    function size($size){
        $this->size = $size;
        return $this;
    }

    function characters($characters) {
        $this->characters = $characters;
        return $this;
    }

    function backgroundColor($red, $green, $blue){
        $this->colors['b']['r'] = $red;
        $this->colors['b']['g'] = $green;
        $this->colors['b']['b'] = $blue;
        return $this;
    }

    function noiseColor($red, $green, $blue){
        $this->colors['n']['r'] = $red;
        $this->colors['n']['g'] = $green;
        $this->colors['n']['b'] = $blue;
        return $this;
    }

    function textColor($red, $green, $blue){
        $this->colors['t']['r'] = $red;
        $this->colors['t']['g'] = $green;
        $this->colors['t']['b'] = $blue;
        return $this;
    }

    function generateCode() {
        $code = '';
        $i = 0;
        while ($i < $this->count) {
            $code .= strtoupper(substr($this->characters, mt_rand(0, strlen($this->characters) - 1), 1));
            $i++;
        }
        return $code;
    }

    function getWidth() {
        return ($this->count * $this->size);
    }

    function getHeight() {
        return $this->size+10;
    }


    function getCaptcha() {

        $code = $this->generateCode();
        $this->width = $this->getWidth();
        $this->height = $this->getHeight();

        $image = @imagecreate($this->width, $this->height) or die('Cannot initialize new GD image stream');

        //define colors
        $tColor = imagecolorallocate($image, $this->colors['t']['r'], $this->colors['t']['g'], $this->colors['t']['b']);
        $nColor = imagecolorallocate($image, $this->colors['n']['r'], $this->colors['n']['g'], $this->colors['n']['b']);
        $bColor = imagecolorallocate($image, $this->colors['b']['r'], $this->colors['b']['g'], $this->colors['b']['b']);

        //fill image
        imagefill($image, 0, 0, $bColor);

        /* generate random dots in background */

        for ($i = 0; $i < ($this->width * $this->height) / 3; $i++) {

            imagefilledellipse($image, mt_rand(0, $this->width), mt_rand(0, $this->height), 1, 1, $nColor);

        }

        /* generate random lines in background */

        for ($i = 0; $i < ($this->width * $this->height) / 150; $i++) {

            imageline($image, mt_rand(0, $this->width), mt_rand(0, $this->height), mt_rand(0, $this->width), mt_rand(0, $this->height), $nColor);
        }

        /* create textbox and add text */

        $textbox = imagettfbbox($this->size, 0, $this->font, $code) or die('Error in imagettfbbox function');

        $x = ($this->width - $textbox[4]) / 2;

        $y = ($this->height - $textbox[5]) / 2;

        imagettftext($image, $this->size, 0, $x, $y, $tColor, $this->font, $code) or die('Error in imagettftext function');
        //imagefilter($image, IMG_FILTER_NEGATE);
        imagefilter($image, IMG_FILTER_SMOOTH, 1);

        /* output captcha image to browser */

        header('Content-Type: image/jpeg');

        header('Cache-Control: no-cache, must-revalidate');

        imagejpeg($image);

        imagedestroy($image);

        $_SESSION['security_code'] = $code;

    }
}

$captcha = new captcha();
$captcha->size(30)->count(6)->getCaptcha();

?>

Answer 1

You can use imagegetttfbbox() to determine the bounding box which can contain your string. But since you’re generating a captcha, which means the characters are highly distored, I don’t think it’d particularly accurate. It’s intended for regular un-altered text.