Need to modify image before displaying on AVCaptureVideoPreviewLayer

I'm using AVCaptureSession to capture video from my iPhone 3G and I need to capture the image and change it before displaying o开发者_JS百科n my AVCaptureVideoPreviewLayer. In my current implementation, I am simply implementing the captureOutput: method and displaying the UIImage in a UIImageView, but that does not work for some reason.

Any ideas? Subclass AVCaptureSession somehow?

Thanks!

Check out this https://developer.apple.com/library/ios/#samplecode/SquareCam/Introduction/Intro.html I think it does this. It draws face boxes onto faces.

- (void)drawFaceBoxesForFeatures:(NSArray *)features forVideoBox:(CGRect)clap orientation:(UIDeviceOrientation)orientation// videoMirrored:(const BOOL)ISVIDEOMIRRORED
{
    NSArray *sublayers = [NSArray arrayWithArray:[previewLayer sublayers]];
    NSInteger sublayersCount = [sublayers count], currentSublayer = 0;
    NSInteger featuresCount = [features count], currentFeature = 0;

    [CATransaction begin];
    [CATransaction setValue:(id)kCFBooleanTrue forKey:kCATransactionDisableActions];

    // hide all the face layers
    for ( CALayer *layer in sublayers ) {
        if ( [[layer name] isEqualToString:@"FaceLayer"] )
            [layer setHidden:YES];
    }   

    if ( featuresCount == 0 || !detectFaces ) {
        [CATransaction commit];
        return; // early bail.
    }

    CGSize parentFrameSize = [previewView frame].size;
    NSString *gravity = [previewLayer videoGravity];
    //BOOL isMirrored = isVideoMirrored;
#warning Deprecated
    const BOOL isMirrored = [previewLayer isMirrored]; // deprecated
    CGRect previewBox = [SquareCamViewController videoPreviewBoxForGravity:gravity 
                                                               frameSize:parentFrameSize 
                                                            apertureSize:clap.size];

    for ( CIFaceFeature *ff in features ) {
        // find the correct position for the square layer within the previewLayer
        // the feature box originates in the bottom left of the video frame.
        // (Bottom right if mirroring is turned on)
        CGRect faceRect = [ff bounds];

        // flip preview width and height
        CGFloat temp = faceRect.size.width;
        faceRect.size.width = faceRect.size.height;
        faceRect.size.height = temp;
        temp = faceRect.origin.x;
        faceRect.origin.x = faceRect.origin.y;
        faceRect.origin.y = temp;
        // scale coordinates so they fit in the preview box, which may be scaled
        CGFloat widthScaleBy = previewBox.size.width / clap.size.height;
        CGFloat heightScaleBy = previewBox.size.height / clap.size.width;
        faceRect.size.width *= widthScaleBy;
        faceRect.size.height *= heightScaleBy;
        faceRect.origin.x *= widthScaleBy;
        faceRect.origin.y *= heightScaleBy;

        if ( isMirrored )
            faceRect = CGRectOffset(faceRect, previewBox.origin.x + previewBox.size.width - faceRect.size.width - (faceRect.origin.x * 2), previewBox.origin.y);
        else
            faceRect = CGRectOffset(faceRect, previewBox.origin.x, previewBox.origin.y);

        CALayer *featureLayer = nil;

        // re-use an existing layer if possible
        while ( !featureLayer && (currentSublayer < sublayersCount) ) {
            CALayer *currentLayer = [sublayers objectAtIndex:currentSublayer++];
            if ( [[currentLayer name] isEqualToString:@"FaceLayer"] ) {
                featureLayer = currentLayer;
                [currentLayer setHidden:NO];
            }
        }

        // create a new one if necessary
        if ( !featureLayer ) {
            featureLayer = [CALayer new];
            [featureLayer setContents:(id)[square CGImage]];
            [featureLayer setName:@"FaceLayer"];
            [previewLayer addSublayer:featureLayer];
            [featureLayer release];
        }
        [featureLayer setFrame:faceRect];

        switch (orientation) {
            case UIDeviceOrientationPortrait:
                [featureLayer setAffineTransform:CGAffineTransformMakeRotation(DegreesToRadians(0.))];
                break;
            case UIDeviceOrientationPortraitUpsideDown:
                [featureLayer setAffineTransform:CGAffineTransformMakeRotation(DegreesToRadians(180.))];
                break;
            case UIDeviceOrientationLandscapeLeft:
                [featureLayer setAffineTransform:CGAffineTransformMakeRotation(DegreesToRadians(90.))];
                break;
            case UIDeviceOrientationLandscapeRight:
                [featureLayer setAffineTransform:CGAffineTransformMakeRotation(DegreesToRadians(-90.))];
                break;
            case UIDeviceOrientationFaceUp:
            case UIDeviceOrientationFaceDown:
            default:
                break; // leave the layer in its last known orientation
        }
        currentFeature++;
    }

    [CATransaction commit];
}