AVCaptureDevice adjustingExposure is False but captured image is dark

Question

The Mac OS X app I'm coding is taking a photo capture using the macbook built-in facetime camera.

On MacBookAir3,2, MacBookPro8,2 and MacBookPro10,2 it works fine but on new macbooks it takes "dark" photos. I understand it's because of auto exposure but I have trouble to get it working. The AVCaptureDevice adjustingExposure is set to NO but the captured photo is still completely dark.

The code: setupCamera is called once during the app launch

-(void) setupCamera
{
    session = [[AVCaptureSession alloc] init];
    session.sessionPreset = AVCaptureSessionPresetPhoto;

    sessionInitialized = YES;

    device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

    [device lockForConfiguration:NULL];
    if ([device isExposureModeSupported:AVCaptureExposureModeContinuousAutoExposure])
        [device setExposureMode:AVCaptureExposureModeContinuousAutoExposure];

    if ([device isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus])
        [device setFocusMode:AVCaptureFocusModeContinuousAutoFocus];

    if ([device isWhiteBalanceModeSupported:AVCaptureWhiteBalanceModeContinuousAutoWhiteBalance])
        [device setWhiteBalanceMode:AVCaptureWhiteBalanceModeContinuousAutoWhiteBalance];

    [device unlockForConfiguration];


    NSError *error = nil;
    AVCaptureDeviceInput *input = [AVCaptureDeviceInput deviceInputWithDevice:device error:&error];
    if(error != nil) {
        // ...
    }

    if([session canAddInput:input]) {
        [session addInput:input];
    } else {
        // ...
    }

    output = [[AVCaptureStillImageOutput alloc] init];
    NSDictionary *outputSettings = @{ AVVideoCodecKey : AVVideoCodecJPEG };
    [output setOutputSettings:outputSettings];

    if([session canAddOutput:output]) {
        [session addOutput:output];
    } else {
        // ...
    }
}

... then each click on the snap button in the UI calls the shootPhoto function:

-(void) shootPhoto
{
    [session startRunning];

    if([device lockForConfiguration:NULL]) {
        if ([device isExposureModeSupported:AVCaptureExposureModeContinuousAutoExposure])
            [device setExposureMode:AVCaptureExposureModeContinuousAutoExposure];

        if ([device isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus])
            [device setFocusMode:AVCaptureFocusModeContinuousAutoFocus];

        if ([device isWhiteBalanceModeSupported:AVCaptureWhiteBalanceModeContinuousAutoWhiteBalance])
            [device setWhiteBalanceMode:AVCaptureWhiteBalanceModeContinuousAutoWhiteBalance];

        [device unlockForConfiguration];
    }

    if(device.adjustingFocus == NO && device.adjustingExposure == NO && device.adjustingWhiteBalance == NO) {
        [self actuallyCapture];
    } else {
        [device addObserver:self forKeyPath:@"adjustingExposure" options:NSKeyValueObservingOptionNew context:MyAdjustingExposureObservationContext];
        [device addObserver:self forKeyPath:@"adjustingFocus" options:NSKeyValueObservingOptionNew context:MyAdjustingFocusObservationContext];
        [device addObserver:self forKeyPath:@"adjustingWhiteBalance" options:NSKeyValueObservingOptionNew context:MyAdjustingWhiteBalanceObservationContext];
    }
}

-(void) actuallyCapture
{
    if ([session isRunning] == NO)
        return;

    connection = [output connectionWithMediaType:AVMediaTypeVideo];
    [output captureStillImageAsynchronouslyFromConnection:connection completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) {
        // save file etc ...
    }];
}

The idea is to check if camera device is adjusting focus, exposure or white balance. If not call actuallyCapture right away. If it is adjusting - add observers and call actuallyCapture from the observeValueForKeyPath.

The problem is that the addObserver calls are never called because the device returns all adjustingX==NO - but still, the captured photo is dark.

What might be the reason? Am I waiting for white balance and exposure adjustments properly?

It's hard to debug for me because I only own those devices that work fine.

Answer 1

I managed to solve this issue myself. Here's how I did that:

Set observers for adjustingExposure, adjustingFocus and adjustingWhiteBalance:

[self.device addObserver:self forKeyPath:@"adjustingExposure" options:NSKeyValueObservingOptionNew context:MyAdjustingExposureObservationContext];
[self.device addObserver:self forKeyPath:@"adjustingFocus" options:NSKeyValueObservingOptionNew context:MyAdjustingFocusObservationContext];
[self.device addObserver:self forKeyPath:@"adjustingWhiteBalance" options:NSKeyValueObservingOptionNew context:MyAdjustingWhiteBalanceObservationContext];

To capture photo initialise a AVCaptureSession but set a 1s delay timer and actually capture after it fires:

-(void) shootPhoto
{
    dispatch_async(self.sessionQueue, ^{
        if([self setupCamera]) {
            self.sessionInitialized = YES;
            [self.session startRunning];

            self.isWaitingToCaptureImage = YES;
            dispatch_async(dispatch_get_main_queue(), ^{
                self.captureDelayTimer = [NSTimer scheduledTimerWithTimeInterval:1.0
                                                                          target:self
                                                                        selector:@selector(actuallyCapture)
                                                                        userInfo:nil
                                                                         repeats:NO];
            });
        }
    });
}

In the observeValueForKeyPath:ofObject:change:context check if all three adjustments are already done and if they are - cancel the timer set above and shoot the photo:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if(!self.sessionInitialized || !self.isWaitingToCaptureImage) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        return;
    }

    if (context != MyAdjustingExposureObservationContext && context != MyAdjustingFocusObservationContext && context != MyAdjustingWhiteBalanceObservationContext) {
        [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
        return;
    } else {
        if (self.device.adjustingExposure || self.device.adjustingFocus || self.device.adjustingWhiteBalance) {
            NSLog(@"not ready to capture yet");
            return;
        } else {
            NSLog(@"ready to capture");
            if (self.captureDelayTimer && self.captureDelayTimer.isValid) {
                [self.captureDelayTimer invalidate];
                self.captureDelayTimer = nil;
            }
            [self actuallyCaptureDispatch];
        }
    }
}