Using VNRecognizeTextRequest on image drawn in UIImageView returns empty results

Question

I have an UIImageView in which I draw handwritten text, using UIGraphicsBeginImageContext to create the bitmap image.

I pass this image to an OCR func:

  func ocrText(onImage: UIImage?) {    
    let request = VNRecognizeTextRequest { request, error in
                guard let observations = request.results as? [VNRecognizedTextObservation] else {
                    fatalError("Received invalid observations") }
          print("observations", observations.count)  // count is 0
          for observation in observations {
             if observation.topCandidates(1).isEmpty {   
               continue
             }
           }
        }       // end of request
     request.recognitionLanguages = ["fr"]

     let requests = [request]

     DispatchQueue.global(qos: .userInitiated).async {
         let ocrGroup = DispatchGroup()
         guard let img = onImage?.cgImage else { return }  
         crGroup.enter()
         let handler = VNImageRequestHandler(cgImage: img, options: [:])
         try? handler.perform(requests)
         ocrGroup.leave()
         crGroup.wait() 
       }
    }

Problem is that observations is an empty array.

But, If I save UIImage to the photo album:

    UIImageWriteToSavedPhotosAlbum(image, nil, nil, nil)

and read back image from the album with imagePicker and pass this image to ocrText, it works.

So it seems there is a format change to the image (or metadata?) when saved to album and that VNRecognizer needs those data.

Is there a way to change directly the original bitmap image format, without going through the storage on photo album ?

Or am I missing something in the use of VNRecognizeTextRequest ?

Answer 1

I finally found a way to get it. I save the image to a file as jpeg and read the file back. This didn't work with png, but works with jpeg.