If you want to export every a frame every 3 seconds then simply drop your footage in the new composition icon at the bottom of the Project Panel, select your layer, enable time remapping, go to frame 10, then drag the last keyframe in time remapping to the left until the value in time remapping for frame 10 is 30 seconds. Then check frame 1, it should read 3 seconds.
EZ as pie....
One thing I forgot to mention, make sure that you open up footage interpretation and have the timecode from the footage set to start at 0. This makes calculations work out just fine.
Here's a screenshot showing frame 5 and 15 seconds on the time remapping value.
I have a method that is easier than what Todd and Rick have suggested, assuming you don't want to export specific frames as Todd's referenced script would do.
- Add your comp to the Render Queue.
- Open the Render Settings dialog.
- In the Frame Rate box, choose Use This Frame Rate. Set the value to your desired interval. If you want 1 frame per second, enter 1. If you want 1 frame every 4 seconds, enter 0.25. (You can also simply enter 1/4. Easter egg: AE can do math in numerical input fields.)
- OK the Render Settings dialog.
- Change the Output Module settings as desired.*
- Start the render.
*I don't recommend using QuickTime if your frame rate value is below 1. Use an image sequence format like PNG or TIFF instead. When I tested this method, rendering the QuickTime file succeeded but importing it back into AE caused AE to start giving errors when trying to use QuickTime files (the file caused the QT32 Server process to crash). Sigh. I'll file the bug tomorrow.**
**UPDATE 2013-09-23: After isolating and filing the bug, it's not as bad as I initially found. The problem is isolated to frame rates of 0.49fps or less, and not all QuickTime codecs exhibit the problem. Animation and ProRes are among those that are affected.
Regardless, for this workflow it's best to use image sequence formats.
I agree with Tim's recommendation for outputting at exactly regular intervals. I thought that you were wanting to export chosen representative images from throughout the movie, not necessarily at regular intervals.
Thanks to all who replied, but especially thanks to Tim! This was exactly what I needed.
I wanted to be able to create a log of visuals that are in a series of videos that were made. Now I can export stills from the videos, complete with a time code stamp to let me know where the visual can be found. (After that, I will use Adobe Bridge to assign keywords to the pics. For example, I will type in "teapot" and see all the occurrences of a teapot in the videos. Or a doorway. Or a crazy guy with a sock puppet. You get the idea.)