Open Radar

 
Summary:
Our current image processing application, we would like to be able to do the following
- On devices that support it (iPhone 5s / 6 / 6+), use a AVCaptureSession to capture high framerate (>= 120fps) CMSampleBuffer from the rear camera
- Process those frames at native (high) framerate in RAM and/or encode them to a video file on disk
- Display a feedback of the original video from the capture session to the user *at 30fps* using an AVCaptureVideoPreviewLayer

Rationale:  videos > 30fps produce an unpleasant visual effect when played in 1x. While we recognise the value of having more frames for video processing and slo-mo playback, having the preview run at at least 60fps does not have any benefit but instead introduce unpleasant visual effects.

Steps to Reproduce:
1. Look at AVCaptureVideoPreviewLayer class documentation
2. Note that there is no way of limiting the displayed framerate of the preview to 30fps

Expected Results:
There should be such a way. 

Actual Results:
To this day there is no way of limiting the framerate of the preview, which, since 240 and 120 are divided by 30, could be a simple drop frames (display 1 frame, drop the next 3, display 1 frame, etc.)

Version:
All versions prior to iOS 8.1 (included)

Notes:


Configuration:
iPhone 5s, 6, 6+