From fe160f3eed3ec0344baff4bfb6a0771d01b5882d Mon Sep 17 00:00:00 2001
From: Anshu <anshubaka2004@gmail.com>
Date: Wed, 4 Mar 2026 17:46:07 +0530
Subject: [PATCH] Merge pull request #28548 from
 0AnshuAditya0:fix-minEnclosingCircle-welzl-28546
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imgproc: fix minEnclosingCircle O(n^3) worst case by adding Welzl shuffle #28548

Welzl's algorithm requires random permutation of input points to
achieve expected O(n) time. Without shuffling, sorted inputs such
as those produced by findContours() trigger O(n^3) worst case.

Fix: copy input to std::vector<PT> and apply cv::randShuffle()
before processing. Uses OpenCV's RNG so cv::setRNGSeed() ensures
reproducible behavior.

Original benchmark (5088 contour points, Release, AVX2):
findContours output: 3.93 ms → 0.033 ms (119x speedup)
Random points: 0.051 ms → 0.049 ms (no regression)

Perf test results (this PR, Release, AVX2):
| Input | N | Time |
|-------|---|------|
| Sequential circle points | 10000 | 0.03 ms |
| Sequential circle points | 5000 | 0.01 ms |
| Random points (CV_32F) | 100000 | 1.87 ms |
| Random points (CV_32S) | 100000 | 2.81 ms |

Fixes #28546

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake
---
 modules/imgproc/perf/perf_contours.cpp | 35 ++++++++++++++++++++++++++
 modules/imgproc/src/shapedescr.cpp     | 25 +++++++++++++++++-
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/modules/imgproc/perf/perf_contours.cpp b/modules/imgproc/perf/perf_contours.cpp
index bc8b530016..8fabdb334b 100644
--- a/modules/imgproc/perf/perf_contours.cpp
+++ b/modules/imgproc/perf/perf_contours.cpp
@@ -106,4 +106,39 @@ PERF_TEST_P(TestBoundingRect, BoundingRect,
     SANITY_CHECK_NOTHING();
 }
 
+typedef TestBaseWithParam< tuple<MatDepth, int> > TestMinEnclosingCircle;
+PERF_TEST_P(TestMinEnclosingCircle, minEnclosingCircle,
+    Combine(
+        testing::Values(CV_32S, CV_32F),
+        Values(400, 1000, 10000, 100000)
+    ))
+{
+    int ptType = get<0>(GetParam());
+    int n = get<1>(GetParam());
+    Mat pts(n, 2, ptType);
+    declare.in(pts, WARMUP_RNG);
+
+    Point2f center;
+    float radius;
+    TEST_CYCLE() minEnclosingCircle(pts, center, radius);
+    SANITY_CHECK_NOTHING();
+}
+
+typedef TestBaseWithParam<int> TestMinEnclosingCircleWorstCase;
+PERF_TEST_P(TestMinEnclosingCircleWorstCase, minEnclosingCircle_sequential,
+    Values(400, 1000, 5000, 10000))
+{
+    int n = GetParam();
+    vector<Point2f> contour;
+    for(int i = 0; i < n; ++i) {
+        float angle = (float)(i * 2 * CV_PI / n);
+        contour.push_back(Point2f(cos(angle) * 100, sin(angle) * 100));
+    }
+
+    Point2f center;
+    float radius;
+    TEST_CYCLE() minEnclosingCircle(contour, center, radius);
+    SANITY_CHECK_NOTHING();
+}
+
 } } // namespace
diff --git a/modules/imgproc/src/shapedescr.cpp b/modules/imgproc/src/shapedescr.cpp
index a713cfeed4..6ddd855e32 100644
--- a/modules/imgproc/src/shapedescr.cpp
+++ b/modules/imgproc/src/shapedescr.cpp
@@ -130,8 +130,31 @@ void findSecondPoint(const PT *pts, int i, Point2f &center, float &radius)
 
 
 template<typename PT>
-static void findMinEnclosingCircle(const PT *pts, int count, Point2f &center, float &radius)
+static void findMinEnclosingCircle(const PT *pts_in, int count, Point2f &center, float &radius)
 {
+    // Welzl's algorithm requires random permutation for expected O(n) time.
+    // Without shuffling, sorted inputs trigger O(n^3) worst case.
+    cv::AutoBuffer<PT, 1024> pts_buf(count);
+    std::copy(pts_in, pts_in + count, pts_buf.data());
+    PT* pts = pts_buf.data();
+    if (count > 10)
+    {
+        Cv32suf x0, y0, xn, yn;
+        x0.f = (float)pts[0].x;
+        y0.f = (float)pts[0].y;
+        xn.f = (float)pts[count-1].x;
+        yn.f = (float)pts[count-1].y;
+
+        uint32_t seed = (uint32_t)count ^ x0.u ^ (y0.u << 8) ^ (xn.u << 16) ^ (yn.u << 24);
+        cv::RNG rng(seed);
+
+        for (int i = 1; i < count; ++i)
+        {
+            int j = rng.uniform(0, i + 1);
+            std::swap(pts[i], pts[j]);
+        }
+    }
+
     center.x = (float)(pts[0].x + pts[1].x) / 2.0f;
     center.y = (float)(pts[0].y + pts[1].y) / 2.0f;
     float dx = (float)(pts[0].x - pts[1].x);