fillPolygon API usage

C++
1

Hello All,

I am fairly new to OpenCV library.

I observed a behavior of the fillPoly opencv function.

When I draw to polygons with the exact same co-ordinates twice, the fillPoly function clears the filled polygon.

I would like to know whether it is an expected beahvior of the fillPoly function

Below is the sample snippet of my code

void CreateTestImage(){

    cv::Mat image_buffer = cv::Mat::zeros( 200, 200, CV_8UC3 );
    std::vector< std::vector< cv::Point > > polygons;
    std::vector< cv::Point > poly1 {    cv::Point(155,155),
                                                              cv::Point(155,195),  
                                                              cv::Point(195,195),
                                                              cv::Point(195,155),
                                                              cv::Point(155,155),
                                    };

    polygons.push_back(poly1);
    // on addition of this line the polygon is non-filled.
   // when I comment this line, I get a filled polygon
    polygons.push_back(poly1);  

    auto ncolor = cv::Scalar(0,255,255);
    fillPoly( image_buffer,polygons,ncolor,cv::LINE_8 );
    
    std::string imageFile("test-1.png");
    cv::imwrite(imageFile,image_buffer);

}

Thanks in advance

2

that’s intentional.

the idea is that one can draw “holes” inside of other polygons, and also arbitrary overlapping polygons.

import numpy as np
import cv2 as cv

canvas = np.zeros((50, 50), np.float32)

poly1 = np.array([[10,20], [40,20], [40,30], [10,30]]).reshape((-1, 1, 2))
poly2 = np.array([[20,10], [30,10], [30,40], [20,40]]).reshape((-1, 1, 2))

cv.fillPoly(canvas, [poly1, poly2], 1.0, cv.LINE_8)

cv.namedWindow("canvas", cv.WINDOW_NORMAL)
cv.imshow("canvas", canvas)
cv.waitKey(-1)
cv.destroyAllWindows()

poly1
poly1502×532 1.83 KB
poly1 and poly2
poly1 and poly2502×532 1.88 KB
poly2
poly2502×532 1.79 KB

3

Thanks for immediate response.

The example shows there is hole created when we merge horizontal bar and vertical bar.
Is there a way to merge both without creating holes and keeping the polygon color as white using OpenCV. So the resulting polygon is filled “+”.

4

draw them individually.

either draw one on top of another (same picture), or draw both into separate pictures and then merge using arithmetic or binary operations.