iShowU offers many compression options (more than I'll ever likely need). You can easily set the capture window to the whole screen or a smaller portion of the screen. It encodes the video while you're working, so when you're done the video is ready instantly.
Below is a video I created showing how to add velocity vectors to a video in LoggerPro. I did the voice over in iMovie so I could redo the pieces I screwed up. But you can also record using the microphone, system sound, or a combination while capturing the video.
The youtube video doesn't do iShowU justice. The quality of the video I captured was perfect. The low quality of my youtube video is due to my not really knowing what I'm doing in iMovie.
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