captcha open font error

there is a problem when add font_path value.
With out "font_path" Everything went fine i need to increase font size and add bold character.
So I had Arial Bold Font.
working config

PHP Code:

$vals = [
            'img_path' => './captcha/',
            'img_url' => base_url('captcha/'),
            'img_width' => 260,
            'img_height' => 45,
            'expiration' => 900,
            'word_length' => 6,
            'font_size' => 20,
            'img_id' => 'Imageid',
            'pool' => 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',

            // White background and border, black text and red grid
            'colors' => array(
                'background' => array(255, 255, 255),
                'border' => array(255, 255, 255),
                'text' => array(0, 0, 0),
                'grid' => array(255, 40, 40)
            )
        ]; 


Are Fine
when I Had

PHP Code:

$vals = [
            'img_path' => './captcha/',
            'img_url' => base_url('captcha/'),
            'font_path' => './dist/fonts/arialbd.ttf', // font line from server
            'img_width' => 260,
            'img_height' => 45,
            'expiration' => 900,
            'word_length' => 6,
            'font_size' => 20,
            'img_id' => 'Imageid',
            'pool' => 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ',

            // White background and border, black text and red grid
            'colors' => array(
                'background' => array(255, 255, 255),
                'border' => array(255, 255, 255),
                'text' => array(0, 0, 0),
                'grid' => array(255, 40, 40)
            )
        ]; 


I got a error  that :
Severity: Warning
Message: imagettftext(): Could not find/open font
Filename: helpers/captcha_helper.php
Line Number: 272