1.目标

在这篇文章中，我们将学习

• 使用OpenCV的cv::minAreaRect函数
• 使用OpenCV的cv::fitEllipse函数

2.主要函数讲解

2.1 cv::minAreaRect

C++:
RotatedRect cv::minAreaRect(InputArray points)
Python:
cv.minAreaRect(points[, ])-> retval
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参数
points:2D点集，存储在std::vector<>或Mat中
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查找包含输入2D点集的最小面积的旋转矩形。
该函数计算并返回指定点集的最小面积边界矩形(可能经过旋转)。

2.2 cv::fitEllipse

C++
RotatedRect cv::fitEllipse(InputArray points	)
Python:
cv.fitEllipse(points[, ]) ->	retval
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拟合一组二维点的椭圆。
该函数计算最适合(在最小二乘意义上)一组2D点的椭圆。它返回内接椭圆的旋转矩形。

3.代码案例

3.1 Python代码

import cv2 as cv
import numpy as np
import argparse
import random as rng

rng.seed(12345)


def thresh_callback(val):
    global src_gray
    threshold = val
    src_gray = cv.GaussianBlur(src_gray, (3, 3), 0.1)
    canny_output = cv.Canny(src_gray, threshold, threshold * 2)

    contours, _ = cv.findContours(canny_output, cv.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

    # Find the rotated rectangles and ellipses for each contour
    minRect = [None] * len(contours)
    minEllipse = [None] * len(contours)
    for i, c in enumerate(contours):
        minRect[i] = cv.minAreaRect(c)
        if c.shape[0] > 5:
            minEllipse[i] = cv.fitEllipse(c)
    # Draw contours + rotated rects + ellipses

    drawing = np.zeros((canny_output.shape[0], canny_output.shape[1], 3), dtype=np.uint8)

    for i, c in enumerate(contours):
        color = (rng.randint(0, 256), rng.randint(0, 256), rng.randint(0, 256))
        # contour
        cv.drawContours(drawing, contours, i, color)
        # ellipse
        if c.shape[0] > 5:
            cv.ellipse(drawing, minEllipse[i], color, 2)
        # rotated rectangle
        box = cv.boxPoints(minEllipse[i])
        # box = cv.boxPoints(minRect[i])
        box = np.intp(box)  # np.intp: Integer used for indexing (same as C ssize_t; normally either int32 or int64)
        # box = np.int0(box)  # normally either int32 or int64)
        cv.drawContours(drawing, [box], 0, color)


    cv.imshow('Contours', drawing)


parser = argparse.ArgumentParser(
    description='Code for Creating Bounding rotated boxes and ellipses for contours tutorial.')
parser.add_argument('--input', help='Path to input image.', default='balloon1.png')
args = parser.parse_args()
src = cv.imread(cv.samples.findFile(args.input))
if src is None:
    print('Could not open or find the image:', args.input)
    exit(0)
# Convert image to gray and blur it
src_gray = cv.cvtColor(src, cv.COLOR_BGR2GRAY)
src_gray = cv.blur(src_gray, (3, 3))
source_window = 'Source'
cv.namedWindow(source_window)
cv.imshow(source_window, src)
max_thresh = 255
thresh = 100  # initial threshold
cv.createTrackbar('Canny Thresh:', source_window, thresh, max_thresh, thresh_callback)
thresh_callback(thresh)
cv.waitKey()
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3.2 C++代码

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include 
using namespace cv;
using namespace std;
Mat src_gray;
int thresh = 100;
RNG rng(12345);
void thresh_callback(int, void* );
int main( int argc, char** argv )
{
    CommandLineParser parser( argc, argv, "{@input | stuff.jpg | input image}" );
    Mat src = imread( samples::findFile( parser.get<String>( "@input" ) ) );
    if( src.empty() )
    {
        cout << "Could not open or find the image!\n" << endl;
        cout << "Usage: " << argv[0] << " " << endl;
        return -1;
    }
    cvtColor( src, src_gray, COLOR_BGR2GRAY );
    blur( src_gray, src_gray, Size(3,3) );
    const char* source_window = "Source";
    namedWindow( source_window );
    imshow( source_window, src );
    const int max_thresh = 255;
    createTrackbar( "Canny thresh:", source_window, &thresh, max_thresh, thresh_callback );
    thresh_callback( 0, 0 );
    waitKey();
    return 0;
}
void thresh_callback(int, void* )
{
    Mat canny_output;
    Canny( src_gray, canny_output, thresh, thresh*2 );
    vector<vector<Point> > contours;
    findContours( canny_output, contours, RETR_TREE, CHAIN_APPROX_SIMPLE, Point(0, 0) );
    vector<RotatedRect> minRect( contours.size() );
    vector<RotatedRect> minEllipse( contours.size() );
    for( size_t i = 0; i < contours.size(); i++ )
    {
        minRect[i] = minAreaRect( contours[i] );
        if( contours[i].size() > 5 )
        {
            minEllipse[i] = fitEllipse( contours[i] );
        }
    }
    Mat drawing = Mat::zeros( canny_output.size(), CV_8UC3 );
    for( size_t i = 0; i< contours.size(); i++ )
    {
        Scalar color = Scalar( rng.uniform(0, 256), rng.uniform(0,256), rng.uniform(0,256) );
        // contour
        drawContours( drawing, contours, (int)i, color );
        // ellipse
        ellipse( drawing, minEllipse[i], color, 2 );
        // rotated rectangle
        Point2f rect_points[4];
        minRect[i].points( rect_points );
        for ( int j = 0; j < 4; j++ )
        {
            line( drawing, rect_points[j], rect_points[(j+1)%4], color );
        }
    }
    imshow( "Contours", drawing );
}
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参考目录

https://docs.opencv.org/5.x/de/d62/tutorial_bounding_rotated_ellipses.html