I'm probably going to have to grab the whole frame. The scan doubling sounds interesting, but it will be displayed on an LCD monitor. What I think I'm going to do is make the bitmap 1024x768, double the video input and center it vertically in the bitmap. This shouldn't be too hard and I'll need to add extra memory anyway, so that's not an issue. I'll just plot two pixels instead of one and plot two scanlines at a time. This will double the resolution which will be necessary anyway (512x342 is tiny on a large monitor) and make the bitmap a resolution that is more or less standard. Compatibility with different types of monitors is key here. I want to be able to plug in any type of contemporary VGA monitor and have this work. The best way to guarantee this is by displaying a bitmap. This, of course, comes with a speed and memory penalty, but it's not that bad. The amount of memory I'll need is cheap and even if I can only get 30 FPS, it is still more than fast enough.
No, after doing quite a bit of research, it looked like I was getting in way over my head and I dropped the project. I'd like to play around with an FPGA at some point, but this isn't the project to learn FPGA programming. This project is the FPGA project simplified.