Text recognition from a live video stream using ML kit (with CMSampleBuffer)

Question

I'm trying to modify the on-device text recognition example provided by Google here to make it work with a live camera feed.

When holding the camera over text (that works with the image example) my console produces the following in a stream before ultimately running out of memory:

2018-05-16 10:48:22.129901+1200 TextRecognition[32138:5593533] An empty result returned from from GMVDetector for VisionTextDetector.

This is my video capture method:

func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {

        if let textDetector = self.textDetector {

            let visionImage = VisionImage(buffer: sampleBuffer)
            let metadata = VisionImageMetadata()
            metadata.orientation = .rightTop
            visionImage.metadata = metadata

            textDetector.detect(in: visionImage) { (features, error) in
                guard error == nil, let features = features, !features.isEmpty else {
                    // Error. You should also check the console for error messages.
                    // ...
                    return
                }

                // Recognized and extracted text
                print("Detected text has: \(features.count) blocks")
                // ...
            }

        }

    }

Is this the right way to do it?

Answer 1

ML Kit has long migrated out of Firebase and became a standalone SDK (migration guide).

The Quick Start sample app in Swift showing how to do text recognition from a live video stream using ML Kit (with CMSampleBuffer) is now available here:

https://github.com/googlesamples/mlkit/tree/master/ios/quickstarts/textrecognition/TextRecognitionExample

The live feed is implemented in the CameraViewController.swift:

https://github.com/googlesamples/mlkit/blob/master/ios/quickstarts/textrecognition/TextRecognitionExample/CameraViewController.swift

Answer 2

ML Kit is still in the process of adding sample code for CMSampleBuffer usage to Firebase Quick Start.

In the meantime, below code works for CMSampleBuffer.

Set up AV Capture (use kCVPixelFormatType_32BGRA for kCVPixelBufferPixelFormatTypeKey):

@property(nonatomic, strong) AVCaptureSession *session;
@property(nonatomic, strong) AVCaptureVideoDataOutput *videoDataOutput;

- (void)setupVideoProcessing {
  self.videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
  NSDictionary *rgbOutputSettings = @{
      (__bridge NSString*)kCVPixelBufferPixelFormatTypeKey :  @(kCVPixelFormatType_32BGRA)
  };
  [self.videoDataOutput setVideoSettings:rgbOutputSettings];

  if (![self.session canAddOutput:self.videoDataOutput]) {
    [self cleanupVideoProcessing];
    NSLog(@"Failed to setup video output");
    return;
  }
  [self.videoDataOutput setAlwaysDiscardsLateVideoFrames:YES];
  [self.videoDataOutput setSampleBufferDelegate:self queue:self.videoDataOutputQueue];
  [self.session addOutput:self.videoDataOutput];
}

Consume the CMSampleBuffer and run detection:

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

  CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
  size_t imageWidth = CVPixelBufferGetWidth(imageBuffer);
  size_t imageHeight = CVPixelBufferGetHeight(imageBuffer);

  AVCaptureDevicePosition devicePosition = self.isUsingFrontCamera ? AVCaptureDevicePositionFront : AVCaptureDevicePositionBack;

  // Calculate the image orientation.
  UIDeviceOrientation deviceOrientation = [[UIDevice currentDevice] orientation];
  ImageOrientation orientation =
      [ImageUtility imageOrientationFromOrientation:deviceOrientation
                        withCaptureDevicePosition:devicePosition
                         defaultDeviceOrientation:[self deviceOrientationFromInterfaceOrientation]];
  // Invoke text detection.
  FIRVisionImage *image = [[FIRVisionImage alloc] initWithBuffer:sampleBuffer];
  FIRVisionImageMetadata *metadata = [[FIRVisionImageMetadata alloc] init];
  metadata.orientation = orientation;
  image.metadata = metadata;

  FIRVisionTextDetectionCallback callback =
      ^(NSArray<id<FIRVisionText>> *_Nullable features, NSError *_Nullable error) {
     ...
  };

 [self.textDetector detectInImage:image completion:callback];
}

The helper function of ImageUtility used above to determine the orientation:

+ (FIRVisionDetectorImageOrientation)imageOrientationFromOrientation:(UIDeviceOrientation)deviceOrientation
                             withCaptureDevicePosition:(AVCaptureDevicePosition)position
                              defaultDeviceOrientation:(UIDeviceOrientation)defaultOrientation {
  if (deviceOrientation == UIDeviceOrientationFaceDown ||
      deviceOrientation == UIDeviceOrientationFaceUp ||
      deviceOrientation == UIDeviceOrientationUnknown) {
    deviceOrientation = defaultOrientation;
  }
  FIRVisionDetectorImageOrientation orientation = FIRVisionDetectorImageOrientationTopLeft;
  switch (deviceOrientation) {
    case UIDeviceOrientationPortrait:
      if (position == AVCaptureDevicePositionFront) {
        orientation = FIRVisionDetectorImageOrientationLeftTop;
      } else {
        orientation = FIRVisionDetectorImageOrientationRightTop;
      }
      break;
    case UIDeviceOrientationLandscapeLeft:
      if (position == AVCaptureDevicePositionFront) {
        orientation = FIRVisionDetectorImageOrientationBottomLeft;
      } else {
        orientation = FIRVisionDetectorImageOrientationTopLeft;
      }
      break;
    case UIDeviceOrientationPortraitUpsideDown:
      if (position == AVCaptureDevicePositionFront) {
        orientation = FIRVisionDetectorImageOrientationRightBottom;
      } else {
        orientation = FIRVisionDetectorImageOrientationLeftBottom;
      }
      break;
    case UIDeviceOrientationLandscapeRight:
      if (position == AVCaptureDevicePositionFront) {
        orientation = FIRVisionDetectorImageOrientationTopRight;
      } else {
        orientation = FIRVisionDetectorImageOrientationBottomRight;
      }
      break;
    default:
      orientation = FIRVisionDetectorImageOrientationTopLeft;
      break;
  }

  return orientation;
}