ios - Capture 60fps in iPhone app

Question

I am working on a project where we will be using iPhones as cameras for capturing a scene. When recording we need to record @60fps and not 30fps (as natively supported). So I am working on an app to do this as the iPhone 4S hardware supports 720p@60fps (if you jailbreak your phone you can achieve this).

Does anybody know how to do this in Objective-C on iOS? Today I saw an app out there (slopro) that can record 60fps on non jailbroken phones. Any advice or tips is much appreciated.

Answer 1

After some tinkering, this answer has split into two parts:

How to capture frames at 60fps

The AVCaptureSessionPreset1280x720 on the iPhone4s/5.1, with frame durations set to:

connection.videoMinFrameDuration = CMTimeMake(1, 60);*
connection.videoMaxFrameDuration = CMTimeMake(1, 60);

gives you a stable, super smooth capture.

How to capture frames into a file @60fps
Capturing frames is all very well, but presumably you want to keep them.
As Brad noted in the comments, writing those frames to a file is another story. Sadly, no matter what configurations I tried, encoding the frames via an AVAssetWriter caused the capture rate to drop to the observed ~37fps and no amount of fiddling with alwaysDiscardsLateVideoFrames could change it. However, in this approach every single frame is copied from AVFoundation to your app, and then back again, which is quite pointless and very wearing for the bus. Luckily, AVFoundation has a class that removes this round trip: AVCaptureMovieFileOutput.

If you let AVFoundation do the writing for you then the iPhone4S can capture and encode frames + audio to a .mov file at 60fps* without breaking a sweat (~25% CPU).

While 60fps video capture is great feature, I can't help but feel a little disappointed as AVCaptureMovieFileOutput rules out a lot of fun things (e.g. realtime effects with GL shaders, recording start/stop without frame loss).

cake/eat it

*59 if you're still running iOS 5.0.1