How to get game images when they are not currently active window

[sunlru]

I made a fishing robot and got it by taking screen shots, but now I want to do something else while fishing on this computer, so I want to keep the game window in the background but still be able to fishing. I try to use Win32 API to capture game images, I can capture normal GUI software images, but can not get game images, only obtained a black image, whether there is a way to achieve. Thank you for your answer.