title: How can I capture images from the screen and compare them to known images in C++?
body: <p>In an effort to see if it is possible to easily break very simple CAPTCHAs, I am attempting to write a program (as simple and small as possible). This program, which I hope to write in C++, should do the following:</p>
  
  <ol>
  <li><p>Make a partial screenshot of a known area of the screen. (Assuming that the CAPTCHA is always in the exact same place - for instance: Pixels 500-600 x, pixels 300-400 y).</p></li>
  <li><p>Automatically dissect the CAPTCHA into individual letters. (The CAPTCHAS I will create for testing will all have only a few white letters, always on a black background, spaced well apart, to make things easy on me.)</p></li>
  <li><p>The program then compares the "cut" letters against an array of "known" images of letters (which look similar to the letters used in the CAPTCHA), which contains 26 elements, each holding an image of a single letter of the English alphabet.</p></li>
  <li><p>The program takes the letter associates with the image that the comparison mapped to, and sends that key to the console (via std::cout)</p></li>
  </ol>
  
  <p>My question is: Is there an easy-to-use library (I am only a beginner at programming), which can handle tasks 1-3 (The 4. is rather easy)? Especially the third point is something I haven't found pretty much anything worthwhile on. What would be ideal is if this library had a "score" function, using a float to indicate how similar the images are. Then, the one with the highest score is the best hit. (I.e: 100.0 means the images are identical, 29.56 means they are very different, etc.)</p>