From e0b489e91774dacd5ed73a7120db291ea49441a2 Mon Sep 17 00:00:00 2001
From: Adrian Kretz <me@akretz.com>
Date: Tue, 27 Feb 2024 06:10:21 +0100
Subject: [PATCH] Merge pull request #25050 from akretz:fix_issue_24330

Handle degenerate cases in RQDecomp3x3 #25050

The point of the Givens rotations here is to iteratively set the lower left matrix entries to zero. If an element is zero already, we don't need to do anything. This resolves #24330.

### 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
- [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.
- [ ] The feature is well documented and sample code can be built with the project CMake
---
 modules/calib3d/src/calibration.cpp           | 12 ++++++------
 .../test/test_decompose_projection.cpp        | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/modules/calib3d/src/calibration.cpp b/modules/calib3d/src/calibration.cpp
index c03e3a1c34..d488e4569e 100644
--- a/modules/calib3d/src/calibration.cpp
+++ b/modules/calib3d/src/calibration.cpp
@@ -3188,8 +3188,8 @@ cvRQDecomp3x3( const CvMat *matrixM, CvMat *matrixR, CvMat *matrixQ,
     Qx = ( 0  c  s ), c = m33/sqrt(m32^2 + m33^2), s = m32/sqrt(m32^2 + m33^2)
          ( 0 -s  c )
     */
-    s = matM[2][1];
-    c = matM[2][2];
+    s = std::abs(matM[2][1]) > DBL_EPSILON ? matM[2][1] : 0;
+    c = std::abs(matM[2][1]) > DBL_EPSILON ? matM[2][2] : 1;
     z = 1./std::sqrt(c * c + s * s + DBL_EPSILON);
     c *= z;
     s *= z;
@@ -3207,8 +3207,8 @@ cvRQDecomp3x3( const CvMat *matrixM, CvMat *matrixR, CvMat *matrixQ,
     Qy = ( 0  1  0 ), c = m33/sqrt(m31^2 + m33^2), s = -m31/sqrt(m31^2 + m33^2)
          ( s  0  c )
     */
-    s = -matR[2][0];
-    c = matR[2][2];
+    s = std::abs(matR[2][0]) > DBL_EPSILON ? -matR[2][0] : 0;
+    c = std::abs(matR[2][0]) > DBL_EPSILON ? matR[2][2] : 1;
     z = 1./std::sqrt(c * c + s * s + DBL_EPSILON);
     c *= z;
     s *= z;
@@ -3227,8 +3227,8 @@ cvRQDecomp3x3( const CvMat *matrixM, CvMat *matrixR, CvMat *matrixQ,
          ( 0  0  1 )
     */
 
-    s = matM[1][0];
-    c = matM[1][1];
+    s = std::abs(matM[1][0]) > DBL_EPSILON ? matM[1][0] : 0;
+    c = std::abs(matM[1][0]) > DBL_EPSILON ? matM[1][1] : 1;
     z = 1./std::sqrt(c * c + s * s + DBL_EPSILON);
     c *= z;
     s *= z;
diff --git a/modules/calib3d/test/test_decompose_projection.cpp b/modules/calib3d/test/test_decompose_projection.cpp
index e43aa65b69..0e3a420d69 100644
--- a/modules/calib3d/test/test_decompose_projection.cpp
+++ b/modules/calib3d/test/test_decompose_projection.cpp
@@ -141,4 +141,23 @@ TEST(Calib3d_DecomposeProjectionMatrix, accuracy)
     test.safe_run();
 }
 
+TEST(Calib3d_DecomposeProjectionMatrix, degenerate_cases)
+{
+    for (int i = 0; i < 3; i++)
+    {
+        for (int j = 0; j < 2; j++)
+        {
+            cv::Matx34d P;
+            P(0, i) = 1;
+            P(1, (i + j + 1) % 3) = 1;
+            P(2, (i + 2 * j + 2) % 3) = 1;
+
+            cv::Matx33d K, R;
+            cv::Vec4d t;
+            decomposeProjectionMatrix(P, K, R, t);
+            EXPECT_LT(cv::norm(K * R, P.get_minor<3, 3>(0, 0), cv::NORM_INF), 1e-6);
+        }
+    }
+}
+
 }} // namespace