Off-screen rendering

Good morning,

OpenGL provided off-screen-rendering. I was able to take a picture of View in OpenCascade (the window was hidden) with the function WNT_PixMap::Dump. But I dont want to take a picture, I would like to have an array with pixel's value. How can I do it ? Is it possible to use an OpenGL function glReadPixels or aomething in the art ?