iOS 5 - AVCaptureDevice setting focus point and focus mode freezes the live camera picture

问题

I'm using the following method to set point of focus since iOS 4:

- (void) focusAtPoint:(CGPoint)point
{
    AVCaptureDevice *device = [[self captureInput] device];

    NSError *error;

     if ([device isFocusModeSupported:AVCaptureFocusModeAutoFocus] &&
         [device isFocusPointOfInterestSupported])
     {
         if ([device lockForConfiguration:&error]) {
             [device setFocusPointOfInterest:point];
             [device setFocusMode:AVCaptureFocusModeAutoFocus];
             [device unlockForConfiguration];
         } else {
             NSLog(@"Error: %@", error);
         }
     }
}

On iOS 4 devices this works without any problems. But on iOS 5 the live camera feed freezes and after some seconds gets completely black. There is no exception or error thrown.

The error won't occur if I comment out either setFocusPointOfInterest or setFocusMode. So the combination of them both will lead to this behavior.

回答1:

The point you've given the setFocusPointOfInterest: function is incorrect. It's the reason why it's crashing.

Add this method to your program and use the value returned by this function

    - (CGPoint)convertToPointOfInterestFromViewCoordinates:(CGPoint)viewCoordinates 
{
    CGPoint pointOfInterest = CGPointMake(.5f, .5f);
    CGSize frameSize = [[self videoPreviewView] frame].size;

    AVCaptureVideoPreviewLayer *videoPreviewLayer = [self prevLayer];

    if ([[self prevLayer] isMirrored]) {
        viewCoordinates.x = frameSize.width - viewCoordinates.x;
    }    

    if ( [[videoPreviewLayer videoGravity] isEqualToString:AVLayerVideoGravityResize] ) {
        pointOfInterest = CGPointMake(viewCoordinates.y / frameSize.height, 1.f - (viewCoordinates.x / frameSize.width));
    } else {
        CGRect cleanAperture;
        for (AVCaptureInputPort *port in [[[[self captureSession] inputs] lastObject] ports]) {
            if ([port mediaType] == AVMediaTypeVideo) {
                cleanAperture = CMVideoFormatDescriptionGetCleanAperture([port formatDescription], YES);
                CGSize apertureSize = cleanAperture.size;
                CGPoint point = viewCoordinates;

                CGFloat apertureRatio = apertureSize.height / apertureSize.width;
                CGFloat viewRatio = frameSize.width / frameSize.height;
                CGFloat xc = .5f;
                CGFloat yc = .5f;

                if ( [[videoPreviewLayer videoGravity] isEqualToString:AVLayerVideoGravityResizeAspect] ) {
                    if (viewRatio > apertureRatio) {
                        CGFloat y2 = frameSize.height;
                        CGFloat x2 = frameSize.height * apertureRatio;
                        CGFloat x1 = frameSize.width;
                        CGFloat blackBar = (x1 - x2) / 2;
                        if (point.x >= blackBar && point.x <= blackBar + x2) {
                            xc = point.y / y2;
                            yc = 1.f - ((point.x - blackBar) / x2);
                        }
                    } else {
                        CGFloat y2 = frameSize.width / apertureRatio;
                        CGFloat y1 = frameSize.height;
                        CGFloat x2 = frameSize.width;
                        CGFloat blackBar = (y1 - y2) / 2;
                        if (point.y >= blackBar && point.y <= blackBar + y2) {
                            xc = ((point.y - blackBar) / y2);
                            yc = 1.f - (point.x / x2);
                        }
                    }
                } else if ([[videoPreviewLayer videoGravity] isEqualToString:AVLayerVideoGravityResizeAspectFill]) {
                    if (viewRatio > apertureRatio) {
                        CGFloat y2 = apertureSize.width * (frameSize.width / apertureSize.height);
                        xc = (point.y + ((y2 - frameSize.height) / 2.f)) / y2;
                        yc = (frameSize.width - point.x) / frameSize.width;
                    } else {
                        CGFloat x2 = apertureSize.height * (frameSize.height / apertureSize.width);
                        yc = 1.f - ((point.x + ((x2 - frameSize.width) / 2)) / x2);
                        xc = point.y / frameSize.height;
                    }

                }

                pointOfInterest = CGPointMake(xc, yc);
                break;
            }
        }
    }

    return pointOfInterest;
}

回答2:

I want to give some additional info to @Louis 's answer.

According to Apple's documents (Please pay attention to the bold part):

In addition, a device may support a focus point of interest. You test for support using focusPointOfInterestSupported. If it’s supported, you set the focal point using focusPointOfInterest. You pass a CGPoint where {0,0} represents the top left of the picture area, and {1,1} represents the bottom right in landscape mode with the home button on the right—this applies even if the device is in portrait mode.

We should involve the orientation when calculate FocusPointOfInterest.

来源：https://stackoverflow.com/questions/7919485/ios-5-avcapturedevice-setting-focus-point-and-focus-mode-freezes-the-live-came