GPUImageMovieWriter - occasional black frames at either ends of the recorded video

Question

I have recording app implementation where user can tap the "record" button to start/stop recording. I achieve this with a basic GPUImageVideoCamera with output set to a GPUImageView as well as a GPUImageMovieWriter.

50% of the time, the recorded clip ends up with a couple (or a single) black frame at either ends, sometimes both. The implementation is fairly straightforward, but here is it anyway.

gpuImageView = [[GPUImageView alloc] initWithFrame:cameraView.frame];
gpuImageView.fillMode = kGPUImageFillModePreserveAspectRatioAndFill;

[cameraView addSubview:gpuImageView];

videoCamera = [[GPUImageVideoCamera alloc] initWithSessionPreset:AVCaptureSessionPresetHigh cameraPosition:(usingBackCamera) ? AVCaptureDevicePositionBack : AVCaptureDevicePositionFront];

[videoCamera addTarget:gpuImageView];
[videoCamera addTarget:movieWriter];
videoCamera.audioEncodingTarget = movieWriter;
[videoCamera startCameraCapture];

double delayToStartRecording = 0.5;
dispatch_time_t startTime = dispatch_time(DISPATCH_TIME_NOW, delayToStartRecording * NSEC_PER_SEC);
        dispatch_after(startTime, dispatch_get_main_queue(), ^(void){
            NSLog(@"Start recording");
            [movieWriter startRecording];
        });

And then stop the recording with the following (while the live camera continues to show up on GPUImageView.

[movieWriter finishRecording];

Has anyone else experienced this and/or found a solution to avoid black frames? I cannot pause/resume camera capture so to ensure seamless user experience.

Answer 1

Typically when this happens it's because the queue is writing audio to the file before it gets any video frames. Hard to tell from your implementation, but if you make sure the first thing you write to the file is a video frame and don't write any audio until after the first video frame is written, you won't have any black frames.