From viewport to billboard

My OpenGL program is going to a trade show next month and my boss wants to have a 3-foot by 6-foot billboard made showing a sample screen. I’ve saved screens to bitmap files before but he’s asking for 300 dpi resolution.

Has anyone been through this already?

I have What we did when we needed monstrously huge screenshots (so that they yielded very high dpi) was to break up the final display view into sub-views, by adjusting the camera for each sub-view and rendering the same scene with each camera iteration. For example, if the screen was normally 640x480, and we wanted to double the resolution, we would render four 640x480 screens of the same scene, but adjusting the camera for each screen (top-left, top-right, bottom-left, bottom-right, each being 640x480). Then the 4 screens combined to a hi-res version of the standard display. I hope that makes sense