Measure brightness with the iOS front camera.

MeasureBrightness.m

/*	Assumes your class has the following instance variables:

    AVCaptureSession            *_videoSession;
    AVCaptureDevice             *_cameraDevice;
    AVCaptureStillImageOutput   *_imageOutput;
    AVCaptureConnection         *_avConnection;
    
*/

-(void) setupStillVideoCaptureSession
{
    //  Set up the video capture session.
    _videoSession = nil;
    _cameraDevice = nil;
    _imageOutput = nil;
    _avConnection = nil;
    NSError *err = nil;
    
    _videoSession = [[AVCaptureSession alloc] init];
    if ([_videoSession canSetSessionPreset:AVCaptureSessionPresetLow]) {
        _videoSession.sessionPreset = AVCaptureSessionPresetLow;
    } else {
        NSLog(@"Could not set AVCaptureSession preset.");
        return;
    }
    //  Find the front camera device.
    
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices) {
        if ([device position] == AVCaptureDevicePositionFront) {
            _cameraDevice = device;
        }
        
        NSLog(@"Found device %@", device);
        NSLog(@"Name: %@", [device localizedName]);
        NSLog(@"Position: %@", ([device position] == AVCaptureDevicePositionBack ? @"back" : @"front"));
    }
    //  Add the front camera device to the session.
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:_cameraDevice error:&err];
    if (err) {
        NSLog(@"Error instantiating capture input device: %@", err.description);
        return;
    }
    if ([_videoSession canAddInput:input]) {
        [_videoSession addInput:input];
    } else {
        NSLog(@"Could not add input device to capture session.");
    }
    //  Create and add the still image output device to the session.
    _imageOutput = [[AVCaptureStillImageOutput alloc] init];
    if ([_videoSession canAddOutput:_imageOutput]){
        [_videoSession addOutput:_imageOutput];
    } else {
        NSLog(@"Could not add output device to capture session.");
    }
    //  Find the AV connection.
    _avConnection = nil;
    for (AVCaptureConnection *connection in _imageOutput.connections) {
        for (AVCaptureInputPort *port in [connection inputPorts]) {
            if ([[port mediaType] isEqual:AVMediaTypeVideo]) {
                _avConnection = connection;
                break;
            }
            if (_avConnection) break;
        }
    }
    if (!_avConnection) {
        NSLog(@"Could not find AVCaptureConnection for capture session.");
        return;
    }
    [_videoSession startRunning];
}


-(void)sampleBrightness
{
    if (_imageOutput) {
        [_imageOutput captureStillImageAsynchronouslyFromConnection:_avConnection completionHandler:^(CMSampleBufferRef imageSampleBuffer, NSError *error) {
            if (error) {
                NSLog(@"Error taking picture: %@", error.description);
                return;
            }
            CFDictionaryRef exifAttachments = CMGetAttachment(imageSampleBuffer, kCGImagePropertyExifDictionary, NULL);
            if (exifAttachments) {
                // Do something with the attachments.
                bool containsBrightnessKey = CFDictionaryContainsKey(exifAttachments, kCGImagePropertyExifBrightnessValue);
                if (!containsBrightnessKey) {
                    NSLog(@"EXIF dictionary doesn't contain brightness key");
                    return;
                }
                CFStringRef brightness = CFDictionaryGetValue(exifAttachments, kCGImagePropertyExifBrightnessValue);
                NSLog(@"@Brightness: %@", (__bridge NSString*)brightness);
                
            }
            //  If you want to do more with the sample, you may continue as follows:
            //
            //            CMItemCount count = CMSampleBufferGetNumSamples(imageSampleBuffer);
            //            NSLog(@"Got %ld samples", count);
            //
            //            CMBlockBufferRef buffer = CMSampleBufferGetDataBuffer(imageSampleBuffer);
            //            if (buffer) {
            //                size_t len = CMBlockBufferGetDataLength(buffer);
            //                char *data = NULL;
            //                CMBlockBufferGetDataPointer(buffer, 0, NULL, &len, &data);
            //            }
            
        }];
    }

}