title: getting window screenshot windows API
body: <p>I am trying to make a program to work on top of an existing GUI to annotate it and provide extra calculations and statistical information. I want to do this using image recognition, as I have learned a fair amount about this in University using Matlab and similar things. I can get a handle to the window I want to perform image recognition on, but I don't know how to turn that handle into an image of that window, and all its visible child windows. I suppose I am looking for something like the screenshot function, but restricted to a single window.</p>
  
  <p>How would I go about doing this? I suppose I'd need something like a .bmp to mess about with. Also, it would have to be efficient enough that I could call it several times a second without my PC grinding to a halt.</p>
  
  <p>Hopefully this isn't an obvious question, I typed some things into google but didn't get anything related.</p>