Gydion: In your Nvidia control panel choose to scale on the GPU keeping the aspect.
jermungand: Where in the control panel do I find that option?
Under Display is an option for "Adjust desktop size and position." In the window on the right under #2 ("Apply the following settings") are two tabs. Select "Scaling." There is an option called "Perform scaling on:" When you click the drop down, it gives you two options: Display and GPU. Select GPU and go from there.
Obviously, I have an Nvidia card as well. I am unfamiliar with how this process works, though, and have my own difficulties (mostly with MM7 rather than MM6 - see my thread on MM7 window problem) that may or may not be fixable through this control panel.
My MM6 issue is that while I can play in a window, I cannot actually move the window; it is stuck in the upper left corner of my main screen. It still works, but would be nice if I could move the window to the center of my screen when in window as opposed to full screen mode. Anyone have any ideas on that?
Edit: did not see the prior response with attached pic when I posted. The pic is perfect.
