Vision, VNDetectTextRectanglesRequest - can't

问题:

I want to use VNDetectTextRectanglesRequest from a Vision framework to detect regions in an image containing only one character, number '9', with the white background. I'm using following code to do this:

 private func performTextDetection() {
    let textRequest = VNDetectTextRectanglesRequest(completionHandler: self.detectTextHandler)
    textRequest.reportCharacterBoxes = true
    textRequest.preferBackgroundProcessing = false

    let handler = VNImageRequestHandler(cgImage: loadedImage.cgImage!, options: [:])

    DispatchQueue.global(qos: .userInteractive).async {
        do {
            try handler.perform([textRequest])
        } catch {
            print ("Error")
        }
    }
}

func detectTextHandler(request: VNRequest, error: Error?) {
    guard let observations = request.results, !observations.isEmpty else {
        fatalError("no results")
    }

    print("there is result")
}

Number of observations results I get is 0, however if I provide an image with text '123' on black background, '123' is detected as a region with text. The described problem also occurs for 2 digit numbers, '22' on white background also doesn't get detected.

Why does a Vision API detect only 3 digits+ numbers on white background in my case?