Java OpenCV 无法正确检测轮廓

之所以整个图像被检测为一个轮廓，是因为findContours发现了白色轮廓(而不是黑色轮廓)，而阈值之后的整个图像是一个大的白色轮廓.

使用Imgproc.THRESH_BINARY_INV将黑白反转:

Imgproc.threshold(grayImage, thresholdImage, 242, 255, Imgproc.THRESH_BINARY_INV);

其他问题:

• 我们可以忽略被认为是噪声的小轮廓:

   double contour_area = Imgproc.contourArea(contour);
   if (contour_area < min_contour_area) {
       continue;
   }
  

• 在warpAffine之前添加白色填充--这是必需的，因为warpAffine可能会添加黑色边距(例如，考虑旋转45度):

   Mat rect = new Mat(image, boundingRect);
   rect = rect.clone(); //Create a clone of the cropped rectangle (required because copyMakeBorder refers to the large image).
   Mat rectWithBorder = new Mat();
   Core.copyMakeBorder(rect, rectWithBorder, emptySpace*2, emptySpace*2, emptySpace*2, emptySpace*2, Core.BORDER_CONSTANT, new Scalar(255, 255, 255));
  

• 围绕裁剪的矩形的中心旋转(而不是围绕rotatedRect.center):

   Point rect_center = new Point((rectWithBorder.cols()-1.0)/2, (rectWithBorder.rows()-1.0)/2);
   Mat rotationMatrix = Imgproc.getRotationMatrix2D(rect_center, angle, 1.0);  
   Mat rotatedRectMat = new Mat();
   Imgproc.warpAffine(rectWithBorder, rotatedRectMat, rotationMatrix, rectWithBorder.size());
  

• 删除warpAffine之后的多余页边距:

   Rect boundingRect2 = new Rect(emptySpace, emptySpace, rotatedRectMat.cols()-emptySpace*2, rotatedRectMat.rows()-emptySpace*2);
   Mat rectWithBorder2 = new Mat(rotatedRectMat, boundingRect2);
  

代码示例:

package myproject;

import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.Scalar;
import org.opencv.core.Rect;
import org.opencv.core.Point;
import org.opencv.core.MatOfPoint;
import org.opencv.core.MatOfPoint2f;
import org.opencv.core.RotatedRect;
import org.opencv.imgproc.Imgproc;
import org.opencv.imgcodecs.Imgcodecs;
import java.util.List;
import java.util.ArrayList;

class Sample {
    

static { System.loadLibrary(Core.NATIVE_LIBRARY_NAME); }


  
public static void main(String[] args) {
    // Load the image file
    // temp
    //File file = ResourceUtils.getFile("classpath:input_image.png");
    final String image_file_name = "input_image.png";
    
    // temp change to inputstream
    Mat image = Imgcodecs.imread(image_file_name);

    // Convert the image to grayscale
    Mat grayImage = new Mat();
    Imgproc.cvtColor(image, grayImage, Imgproc.COLOR_BGR2GRAY);

    // Apply a threshold to convert the image to black and white
    Mat thresholdImage = new Mat();
    //Imgproc.threshold(grayImage, thresholdImage, 242, 255, Imgproc.THRESH_BINARY);
    Imgproc.threshold(grayImage, thresholdImage, 242, 255, Imgproc.THRESH_BINARY_INV);
    Imgcodecs.imwrite("thresh.png", thresholdImage);

    // Find contours in the thresholded image
    List<MatOfPoint> contours = new ArrayList<>();
    Mat hierarchy = new Mat();
    Imgproc.findContours(thresholdImage, contours, hierarchy, Imgproc.RETR_EXTERNAL, Imgproc.CHAIN_APPROX_SIMPLE);

    // Set the amount of empty space to add around the cropped image
    int emptySpace = 20;
    
    double min_contour_area = 10000.0;  //Area threshold - ignore small contours with area less than 10000 (assume noise)

    // Loop over the contours
    for (int i = 0; i < contours.size(); i++) {
        MatOfPoint contour = contours.get(i);
        
        double contour_area = Imgproc.contourArea(contour);
        
        if (contour_area < min_contour_area) {
            continue;   //Ignore small contours with area less than min_contour_area (assume noise)
        }

        // Find the bounding box of the contour
        Rect boundingRect = Imgproc.boundingRect(contour);

        // Crop the rectangle from the original image using the bounding box coordinates
        Mat rect = new Mat(image, boundingRect);
        rect = rect.clone(); //Create a clone of the cropped rectangle (required because copyMakeBorder refers to the large image). 
        
        //Imgcodecs.imwrite("rect.png", rect); //Save rect for testing

        // Straighten the rectangle
        RotatedRect rotatedRect = Imgproc.minAreaRect(new MatOfPoint2f(contour.toArray()));
        double angle = rotatedRect.angle;
        if (rotatedRect.size.width > rotatedRect.size.height) {
            angle += 90;
        }
               
        // Add empty space around the rect - add larger padding before rotation because warpAffine may add black margins
        Mat rectWithBorder = new Mat();
        Core.copyMakeBorder(rect, rectWithBorder, emptySpace*2, emptySpace*2, emptySpace*2, emptySpace*2, Core.BORDER_CONSTANT, new Scalar(255, 255, 255));
        
        //Imgcodecs.imwrite("rectWithBorder.png", rectWithBorder); //Save rectWithBorder for testing
        
        //Mat rotationMatrix = Imgproc.getRotationMatrix2D(rotatedRect.center, angle, 1.0);  
        Point rect_center = new Point((rectWithBorder.cols()-1.0)/2, (rectWithBorder.rows()-1.0)/2);
        Mat rotationMatrix = Imgproc.getRotationMatrix2D(rect_center, angle, 1.0);  //Rotate around the center of rect.
        Mat rotatedRectMat = new Mat();
        Imgproc.warpAffine(rectWithBorder, rotatedRectMat, rotationMatrix, rectWithBorder.size());
        
        //Imgcodecs.imwrite("rotatedRectMat.png", rotatedRectMat); //Save rotatedRectMat for testing
        
        //Crop the center of rotatedRectMat (remove the extra emptySpace padding from each side).
        Rect boundingRect2 = new Rect(emptySpace, emptySpace, rotatedRectMat.cols()-emptySpace*2, rotatedRectMat.rows()-emptySpace*2);
        Mat rectWithBorder2 = new Mat(rotatedRectMat, boundingRect2);


        // Save the rectangle with border as a separate image file
        Imgcodecs.imwrite(String.format("output_image_%d.png", i), rectWithBorder2);
    }
}

}

Result: