Capturing Images from AVCaptureSession

Question

I am learning about AVCaptureSession and how to capture multiple images with its delegate method

- (void)captureOutput:(AVCaptureOutput *)captureOutput 
     didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer 
     fromConnection:(AVCaptureConnection *)connection

My goal is to capture 1 or many images with a predefined rate per second. For example, 1 or 2 images per 1 second. So I set

 AVCaptureVideoDataOutput *captureOutput = [[AVCaptureVideoDataOutput alloc] init];
 captureOutput.alwaysDiscardsLateVideoFrames = YES; 
 captureOutput.minFrameDuration = CMTimeMake(1, 1);

When [self.captureSession startRunning]; is started my log file shows delegate is being called 20 times a second. Where is it coming from and how to capture images with my intended intervals?

Solution

You can use the function given below and if you want to capture at specific intervals, then set a timer and call that function again.

-(IBAction)captureNow
    {

    AVCaptureConnection *videoConnection = nil;
    for (AVCaptureConnection *connection in [stillImageOutput connections])
    {
        for (AVCaptureInputPort *port in [connection inputPorts])
        {
            if ([[port mediaType] isEqual:AVMediaTypeVideo] )
            {
                videoConnection = connection;
                break;
            }
        }
        if (videoConnection) 
        {
            break;
        }
    }

    NSLog(@"About to request a capture from: %@", stillImageOutput);
    [stillImageOutput captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler: ^(CMSampleBufferRef imageSampleBuffer, NSError *error)
    {

        CFDictionaryRef exifAttachments = CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
        if (exifAttachments)
        {
            // Do something with the attachments.
            NSLog(@"Attachments: %@", exifAttachments);
        }
        else
        { 
            NSLog(@"No attachments found.");
        }

        NSData *imageData = [AVCaptureStillImageOutput jpegStillImageNSDataRepresentation:imageSampleBuffer];
        UIImage *image = [[UIImage alloc] initWithData:imageData];
        [[self vImage] setImage:image];

    }];
}

For more reference you can see iOS4: Take photos with live video preview using AVFoundation.