The CAPTCHA system tested so far was relatively straightforward to solve -- the black font color meant that the text could easily be distinguished from the background, and additionally, the text was level and did not need to be rotated for Tesseract to interpret it accurately. Often, you will find websites using simple custom CAPTCHA systems similar to this, and in these cases, an OCR solution is practical. However, if a website uses a more complex system, such as Google's reCAPTCHA, OCR will take a lot more effort and may become impractical.
In these examples, the text is placed at different angles and with different fonts and colors, so plenty more work needs to be done to clean and preprocess the image before OCR is accurate. These advanced CAPTCHAs can sometimes even be difficult for people to interpret, making it that much more difficult to do so with a simple script.
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