How OpenGL and Direct3D play along with other widgets in a window?

I'd like to know how a Direct3D Device or OpenGL context will know in which portion of the window to render its contents.

In the case of Direct3D, if I create a viewport midscreen, leaving bor开发者_运维百科ders outside, will the rendering interfere with the rest of the widgets around it?

How does it work with OpenGL?

As a bonus, why was it so hard to use D3D with Qt, for example, if D3D respected its viewport space and Qt were informed not to render in the viewport area? I know now it's not hard to get those to play along together anymore, but I recall it was tricky sometime ago.

Anyone?

OpenGL is normally used with an openGL widget which handles all the window interactions.

There used to be a D3D interface to Qt but it was dropped around Qt4.5 because it's not cross platform. It's relatively easy to paint D3D into a Qt widget but menus etc over the top can be tricky.