From a2a3f5e86c82044c2612d3dc638b505295c20f0f Mon Sep 17 00:00:00 2001
From: Maxim Smolskiy <mithridatus@mail.ru>
Date: Mon, 20 Jan 2025 14:25:40 +0300
Subject: [PATCH] Merge pull request #26773 from
 MaximSmolskiy:improve-robustness-for-ellipse-fitting

Improve robustness for ellipse fitting #26773

### Pull Request Readiness Checklist

Related to #26694

Current noise addition is not very good because for example it turns degenerate case of one horizontal line into degenerate case of two parallel horizontal lines

Improving noise addition leads to improved robustness of algorithms

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
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake
---
 modules/imgproc/src/shapedescr.cpp              |  9 +++++----
 modules/imgproc/test/test_fitellipse.cpp        | 11 +++++++++++
 modules/imgproc/test/test_fitellipse_ams.cpp    | 11 +++++++++++
 modules/imgproc/test/test_fitellipse_direct.cpp | 11 +++++++++++
 4 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/modules/imgproc/src/shapedescr.cpp b/modules/imgproc/src/shapedescr.cpp
index 007bf9ac62..ccb006b90c 100644
--- a/modules/imgproc/src/shapedescr.cpp
+++ b/modules/imgproc/src/shapedescr.cpp
@@ -340,9 +340,10 @@ double cv::contourArea( InputArray _contour, bool oriented )
 namespace cv
 {
 
-static inline Point2f getOfs(int i, float eps)
+static inline Point2f getOfs(float eps)
 {
-    return Point2f(((i & 1)*2 - 1)*eps, ((i & 2) - 1)*eps);
+    RNG& rng = theRNG();
+    return Point2f(rng.uniform(-eps, eps), rng.uniform(-eps, eps));
 }
 
 static RotatedRect fitEllipseNoDirect( InputArray _points )
@@ -419,7 +420,7 @@ static RotatedRect fitEllipseNoDirect( InputArray _points )
         float eps = (float)(s/(n*2)*1e-3);
         for( i = 0; i < n; i++ )
         {
-            Point2f p = ptsf_copy[i] + getOfs(i, eps);
+            const Point2f p = ptsf_copy[i] + getOfs(eps);
             ptsf_copy[i] = p;
         }
 
@@ -744,7 +745,7 @@ cv::RotatedRect cv::fitEllipseDirect( InputArray _points )
         for( i = 0; i < n; i++ )
         {
             Point2f p = is_float ? ptsf[i] : Point2f((float)ptsi[i].x, (float)ptsi[i].y);
-            Point2f delta = getOfs(i, eps);
+            const Point2f delta = getOfs(eps);
             double px = (p.x + delta.x - c.x)*scale, py = (p.y + delta.y - c.y)*scale;
 
             A.at<double>(i,0) = px*px;
diff --git a/modules/imgproc/test/test_fitellipse.cpp b/modules/imgproc/test/test_fitellipse.cpp
index 3e6d0478ca..169751acab 100644
--- a/modules/imgproc/test/test_fitellipse.cpp
+++ b/modules/imgproc/test/test_fitellipse.cpp
@@ -102,4 +102,15 @@ TEST(Imgproc_FitEllipse_JavaCase, accuracy) {
     EXPECT_NEAR(e.size.height, sqrt(2.)*2, 0.4);
 }
 
+TEST(Imgproc_FitEllipse_HorizontalLine, accuracy) {
+    vector<Point2f> pts({{-300, 100}, {-200, 100}, {-100, 100}, {0, 100}, {100, 100}, {200, 100}, {300, 100}});
+    const RotatedRect el = fitEllipse(pts);
+
+    EXPECT_NEAR(el.center.x, -100, 100);
+    EXPECT_NEAR(el.center.y, 100, 1);
+    EXPECT_NEAR(el.size.width, 1, 1);
+    EXPECT_GE(el.size.height, 150);
+    EXPECT_NEAR(el.angle, 90, 0.1);
+}
+
 }} // namespace
diff --git a/modules/imgproc/test/test_fitellipse_ams.cpp b/modules/imgproc/test/test_fitellipse_ams.cpp
index 92e7f2e7a2..5c672bec89 100644
--- a/modules/imgproc/test/test_fitellipse_ams.cpp
+++ b/modules/imgproc/test/test_fitellipse_ams.cpp
@@ -337,4 +337,15 @@ TEST(Imgproc_FitEllipseAMS_Issue_7, accuracy) {
     EXPECT_TRUE(checkEllipse(ellipseAMSTest, ellipseAMSTrue, tol));
 }
 
+TEST(Imgproc_FitEllipseAMS_HorizontalLine, accuracy) {
+    vector<Point2f> pts({{-300, 100}, {-200, 100}, {-100, 100}, {0, 100}, {100, 100}, {200, 100}, {300, 100}});
+    const RotatedRect el = fitEllipseAMS(pts);
+
+    EXPECT_NEAR(el.center.x, -100, 100);
+    EXPECT_NEAR(el.center.y, 100, 1);
+    EXPECT_NEAR(el.size.width, 1, 1);
+    EXPECT_GE(el.size.height, 150);
+    EXPECT_NEAR(el.angle, 90, 0.1);
+}
+
 }} // namespace
diff --git a/modules/imgproc/test/test_fitellipse_direct.cpp b/modules/imgproc/test/test_fitellipse_direct.cpp
index 4ae8c4dc76..e41c52764d 100644
--- a/modules/imgproc/test/test_fitellipse_direct.cpp
+++ b/modules/imgproc/test/test_fitellipse_direct.cpp
@@ -337,4 +337,15 @@ TEST(Imgproc_FitEllipseDirect_Issue_7, accuracy) {
     EXPECT_TRUE(checkEllipse(ellipseDirectTest, ellipseDirectTrue, tol));
 }
 
+TEST(Imgproc_FitEllipseDirect_HorizontalLine, accuracy) {
+    vector<Point2f> pts({{-300, 100}, {-200, 100}, {-100, 100}, {0, 100}, {100, 100}, {200, 100}, {300, 100}});
+    const RotatedRect el = fitEllipseDirect(pts);
+
+    EXPECT_NEAR(el.center.x, 0, 100);
+    EXPECT_NEAR(el.center.y, 100, 1);
+    EXPECT_NEAR(el.size.width, 2, 2);
+    EXPECT_NEAR(el.size.height, 600, 100);
+    EXPECT_NEAR(el.angle, 90, 0.1);
+}
+
 }} // namespace