How to improve the image quality and generate random string image in the plugin validates_captcha

Validates captcha is a good pluging to implement captcha in your rails application.
However i found that there is repetition of the string of the image and the quality of image is not that good. To get a good quality image and random string replace the code of the file /vendor/plugns/validates_captcha/lib/captcha_challenge.rb with the following code...

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