EdgeDrawing::detectEllipses() modifies the caller's input image when Params::Sigma < 1.0

System information (version)

• OpenCV => 4.x head (reproduced on 4.14.0)
• Operating System / Platform => Ubuntu (Linux 5.15) 64 Bit
• Compiler => gcc 9.4

Detailed description

When EdgeDrawing::Params::Sigma < 1.0, detectEdges() sets smoothImage = srcImage; (modules/ximgproc/src/edge_drawing.cpp:411 on 4.x), so smoothImage aliases the caller's input Mat. detectEllipses() then runs GaussianBlur(srcImage, smoothImage, Size(), 0.50) (:2718) — an in-place blur that silently overwrites the user's input image (the PFmode path in detectEdges() at :430 does the same with sigma 0.4). This violates the InputArray read-only contract and also corrupts any later use of the source image.

The reference implementation ED_Lib always blurs into a separately allocated smoothImage; the aliasing shortcut is unique to the OpenCV port.

Steps to reproduce (C++)

#include <opencv2/imgproc.hpp>
#include <opencv2/ximgproc.hpp>
#include <cstdio>
using namespace cv;

int main() {
    Mat img(200, 200, CV_8UC1, Scalar(255));
    circle(img, Point(100, 100), 60, Scalar(0), 2);
    Mat backup = img.clone();

    Ptr<ximgproc::EdgeDrawing> ed = ximgproc::createEdgeDrawing();
    ed->params.Sigma = 0.0;                 // Sigma < 1.0
    ed->detectEdges(img);
    std::vector<Vec6d> ellipses;
    ed->detectEllipses(ellipses);

    printf("input pixels modified = %d (expected 0)\n",
           countNonZero(img != backup));   // prints 1916
    return 0;
}

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files