Image preprocessing of water meter with OpenCv Java, OCR with Tesseract

Question

I'm trying to develop simple application (OpenCv, Tesseract and Java) where i need to get numbers from a photo of water meter. I am newbie to OpenCV and i am stuck on detection of numbers in rectangles.

So i want to achieve "00295" value as result.

Here is a example of water meter But i am not able to achieve this result.

Steps:

Apply Gray filter
GaussianBlur filter 3x3
Sobel filter Threshold
And doing OCR with number characters allowed only

But in result i get bunch of random numbers from other labels. Can you please give some suggestions and show way how to detect this 5 rectangles and get digits from them ? Thanks in advance.

Here is code:


    private static final int
            CV_THRESH_OTSU = 8;
    public static void main(String[] args) {

        System.loadLibrary(Core.NATIVE_LIBRARY_NAME);

        Mat img = new Mat();
        Mat imgGray = new Mat();
        Mat imgGaussianBlur = new Mat();
        Mat imgSobel = new Mat();
        Mat imgThreshold = new Mat();

        //Path to picture
        String inputFilePath = "D:/OCR/test.jpg";
        img = Imgcodecs.imread(inputFilePath);
        Imgcodecs.imwrite("preprocess/1_True_Image.png", img);

        Imgproc.cvtColor(img, imgGray, Imgproc.COLOR_BGR2GRAY);
        Imgcodecs.imwrite("preprocess/2_imgGray.png", imgGray);

        Imgproc.GaussianBlur(imgGray,imgGaussianBlur, new Size(3, 3),0);
        Imgcodecs.imwrite("preprocess/3_imgGaussianBlur.png", imgGray);

        Imgproc.Sobel(imgGaussianBlur, imgSobel, -1, 1, 0);
        Imgcodecs.imwrite("preprocess/4_imgSobel.png", imgSobel);

        Imgproc.threshold(imgSobel, imgThreshold, 0, 255,  CV_THRESH_OTSU);
        Imgcodecs.imwrite("preprocess/5_imgThreshold.png", imgThreshold);

        File imageFile = new File("preprocess/5_imgThreshold.png");
        Tesseract tesseract = new Tesseract();
        //tessdata directory
        tesseract.setDatapath("tessdata");
        tesseract.setTessVariable("tessedit_char_whitelist", "0123456789");
        try {
            String result = tesseract.doOCR(imageFile);
            System.out.println(result);
        } catch (TesseractException e) {
            System.err.println(e.getMessage());
        }
    }
}

score 0 · Answer 1 · answered Mar 25 '20 at 02:40

If the position of the water filter won't change from image to image, could you just manually crop the image to your desired size? Also, after you blur the image, try using an adaptive threshold followed by canny edge detection. As a result, your image will only have the hard edges present. Then you could find contours on the image and filter through those contours till they fit the desired size that you want.

Image preprocessing of water meter with OpenCv Java, OCR with Tesseract

1 Answers1