OpenCV Android (java) character detection and font recognition

Question

I'm working on android app, which determines which font is used on a text image. So I need to extract every character from image and don't know how to do it precisely. Furthermore, when I'm trying to process an image I have one result...but my classmate has different (for example, more or less noise). The problem with character detection is that:

1) it detects also some noise blobs on image and shows it in rectangles (I thought about detectMultiScale... but I have doubts about it, maybe there are easiest ways to detect characters)

2) it detects several contours of one character (for example inner and outer radius of letter "o")

And question for the future: I'm going to create a DB with images (for now just 3 fonts) of different letters of fonts and compare them with an image of letters from photo. Maybe someone could recommend a better way to do it.
So this is part of code with image processing(I'm still playing with values of blur, threshold and Canny... but there was no really positive result):

Imgproc.cvtColor(sImage, grayImage, Imgproc.COLOR_BGR2GRAY); //градации серого
Imgproc.GaussianBlur(grayImage,blurImage,new Size(5, 5),0); //размытие
        Imgproc.adaptiveThreshold(blurImage, thresImage, 255, Imgproc.ADAPTIVE_THRESH_MEAN_C, Imgproc.THRESH_BINARY, 101, 39);
        List<MatOfPoint> contours = new ArrayList<MatOfPoint>();
        Mat hierarchy = new Mat();

        Imgproc.Canny(thresImage, binImage, 30, 10, 3, true); //контур
        Imgproc.findContours(binImage, contours, hierarchy, Imgproc.RETR_TREE, Imgproc.CHAIN_APPROX_SIMPLE, new Point(0, 0));

        hierarchy.release();
        Imgproc.drawContours(binImage, contours, -1, new Scalar(255, 255, 255));//, 2, 8, hierarchy, 0, new Point());


        MatOfPoint2f approxCurve = new MatOfPoint2f();

        //For each contour found
        for (int i = 0; i < contours.size(); i++) {
            //Convert contours(i) from MatOfPoint to MatOfPoint2f
            MatOfPoint2f contour2f = new MatOfPoint2f(contours.get(i).toArray());
            //Processing on mMOP2f1 which is in type MatOfPoint2f
            double approxDistance = Imgproc.arcLength(contour2f, true) * 0.02;
            Imgproc.approxPolyDP(contour2f, approxCurve, approxDistance, true);

            //Convert back to MatOfPoint
            MatOfPoint points = new MatOfPoint(approxCurve.toArray());


            // Get bounding rect of contour
            Rect rect = Imgproc.boundingRect(points);

            // draw enclosing rectangle (all same color, but you could use variable i to make them unique)
            Imgproc.rectangle(binImage, new Point(rect.x, rect.y), new Point(rect.x + rect.width, rect.y + rect.height), new Scalar(255, 255, 255), 5);

        }

And screen (not actually with processing values from code, just one with better results):

Original:

(unfortunately, I can't add more than 2 links to show more examples)
There were situations, when picture from this screen looked pretty good, but another pictures looked like with shapeless blobs.

Answer 1

Your code is fine, you just need to make a minor tweaks to get it work properly.

• Firstly, the image size is very large, you can safely reduce it to 20% of current size without suffering a major loss in accuracy. Due to larger image size all the functions would perform slower.

• You dont need to perform adaptive threshold before Canny, canny works perfectly on gray-scale images as well, You need to adjust the params as:

  Canny(img, threshold1=170, threshold2=250)
  

  which yields an image as:

• [Optional] If you want to de-noise the image then you can try with morphological operations like erode and dilate.

• Now you are ready to find the contours. The mistake in your code was using Imgproc.RETR_TREE flag you need to use Imgproc.RETR_EXTERNAL flag to get only the outer contours and not the nested inner contours.

  At this step you may have some unwanted small contours, which can be filtered as:

  // ** Below code if for reference purposes only, consult OpenCV docs for proper API methods
  int character_area_lower_thresh = 10;
  for (Contour c:contours) {
       if (Imgproc.contourArea(c) > character_area_lower_thresh) {
           // Desired contour, do what ever you want to do
           Rect r = Imgproc.boundingRect(c);
       }
  }