In a previous entry I described how I was using ffmpeg to do screen captures for demos. I wanted to share a few new tricks with that. I wanted to make it easier to shoot demos, so I created a special wallpaper when I'm doing one. My screen dimensions are 1920x1200. So, I created a background of that size using GIMP and a graphic I found on the web. I haven't fully vetted this for copyright, so if this belongs to you and you are upset, let me know and I'll redo it.
Within that background I drew a 1280x720 box (the resolution for HD video) and bordered it with a yellow line. Now I know where to put all of my frames when I do the demo. Anything outside of that box will not be recorded.
Finally, I altered my capture command by changing -i :0.0 to -i :0.0+139,152. This tells ffmpeg to offset the capture by 139 pixels on the x axis and 152 pixels on the y axis.
It is highly unlikely that you will be able to use my wallpaper as-is, though you are welcomed to try. I'm sharing it freely (provided that it is freely available from the originator!). You will probably need to make your own for your situation. Now, when I'm doing a demo, I call this up. The video gets a black background (which I could easily repaint any time I wish) and I can run other things around my capture without having to edit it out.
Let me know if you have any tricks like this.