java - 检测图像中的矩形会产生不需要的结果(opencv，java)

Question

我有这张图片，我已经对其进行了阈值处理以将其转换为二进制图像和黑白图像。见下图:

我想用一个字母提取每个盒子，这是通过以下代码完成的:

 List<MatOfPoint> contours = new ArrayList<MatOfPoint>();  
  Imgproc.findContours(destination3, contours, new Mat(), Imgproc.RETR_LIST,Imgproc.CHAIN_APPROX_SIMPLE);

         MatOfPoint2f approxCurve = new MatOfPoint2f();
         int x = 1;
         //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);
             if(rect.height > 50 && rect.height < 100) {
              // draw enclosing rectangle (all same color, but you could use variable i to make them unique)
             //Core.rectangle(destination, new Point(rect.x,rect.y), new Point(rect.x+rect.width,rect.y+rect.height), new Scalar(255, 0, 255), 3); 
             Rect roi = new Rect(rect.x, rect.y, rect.width, rect.height);
             Mat cropped = new Mat(destination3, roi);
             Highgui.imwrite("letter"+x+".jpg", cropped);
             x++;
             }
         }

但是提取出来的字母是全黑的，看起来就像是填充了白色的字母一样，如下图所示。我该如何解决？我的代码有什么问题？

Answer 1

来自 findContours() 的文档: (强调我的)

Note: Source image is modified by this function. Also, the function does not take into account 1-pixel border of the image (it's filled with 0's and used for neighbor analysis in the algorithm), therefore the contours touching the image border will be clipped.

您看到的奇怪图像是 findContours() 对 destination3 进行修改的结果。通过调用 clone() 将数据的深拷贝传递给 findContours()，您应该能够获得正确的结果。您调用 findContours() 的行将如下所示:

Imgproc.findContours(destination3.clone(), contours, new Mat(), Imgproc.RETR_LIST,Imgproc.CHAIN_APPROX_SIMPLE);
                             //   ^^ Vive la difference!