From 57f328a27c5f48e40c7a2ba3e8e9c3c222097fb7 Mon Sep 17 00:00:00 2001
From: Vincent Rabaud <vrabaud@google.com>
Date: Wed, 9 Sep 2026 16:58:58 +0200
Subject: [PATCH] Only optimize upon 4 distortion parameters when the input has
 dim==4

---
 modules/calib/src/calibration.cpp             | 17 +++++--
 modules/calib/test/test_cameracalibration.cpp | 47 +++++++++++++++++++
 2 files changed, 60 insertions(+), 4 deletions(-)

diff --git a/modules/calib/src/calibration.cpp b/modules/calib/src/calibration.cpp
index 034581ce6f..9e57a0af2d 100644
--- a/modules/calib/src/calibration.cpp
+++ b/modules/calib/src/calibration.cpp
@@ -2861,9 +2861,10 @@ static Mat prepareDistCoeffs(Mat& distCoeffs0, int rtype, int outputSize)
     int n = sz.area();
     if (n > 0)
         CV_Assert(sz.width == 1 || sz.height == 1);
+    CV_Assert(n == 0 || n == 4 || n == 5 || n == 8 || n == 12 || n == 14);
     CV_Assert((int)distCoeffs0.total() <= outputSize);
     Mat distCoeffs = Mat::zeros(sz.width == 1 ? Size(1, outputSize) : Size(outputSize, 1), rtype);
-    if( n ==  4 || n ==  5 || n ==  8 || n == 12 || n == 14 )
+    if( n > 0 )
     {
         distCoeffs0.convertTo(distCoeffs(Rect(Point(), sz)), rtype);
     }
@@ -2932,6 +2933,7 @@ double calibrateCameraRO(InputArrayOfArrays _objectPoints,
     Mat cameraMatrix = _cameraMatrix.getMat();
     cameraMatrix = prepareCameraMatrix(cameraMatrix, rtype, flags);
     Mat distCoeffs = _distCoeffs.getMat();
+    int dist_total = distCoeffs.total();
     distCoeffs =
         (flags & CALIB_THIN_PRISM_MODEL) &&
         !(flags & CALIB_TILTED_MODEL) ?
@@ -2940,7 +2942,10 @@ double calibrateCameraRO(InputArrayOfArrays _objectPoints,
     if( !(flags & CALIB_RATIONAL_MODEL) &&
     (!(flags & CALIB_THIN_PRISM_MODEL)) &&
     (!(flags & CALIB_TILTED_MODEL)))
-        distCoeffs = distCoeffs.rows == 1 ? distCoeffs.colRange(0, 5) : distCoeffs.rowRange(0, 5);
+    {
+        int out_size = (dist_total == 0) ? 5 : std::min(5, dist_total);
+        distCoeffs = distCoeffs.rows == 1 ? distCoeffs.colRange(0, out_size) : distCoeffs.rowRange(0, out_size);
+    }
 
     int nimages = int(_objectPoints.total());
     CV_Assert( nimages > 0 );
@@ -3115,6 +3120,8 @@ double stereoCalibrate( InputArrayOfArrays _objectPoints,
     Mat cameraMatrix2 = _cameraMatrix2.getMat();
     Mat distCoeffs1 = _distCoeffs1.getMat();
     Mat distCoeffs2 = _distCoeffs2.getMat();
+    int dist1_total = distCoeffs1.total();
+    int dist2_total = distCoeffs2.total();
     cameraMatrix1 = prepareCameraMatrix(cameraMatrix1, rtype, flags);
     cameraMatrix2 = prepareCameraMatrix(cameraMatrix2, rtype, flags);
     distCoeffs1 = prepareDistCoeffs(distCoeffs1, rtype, 14);
@@ -3124,8 +3131,10 @@ double stereoCalibrate( InputArrayOfArrays _objectPoints,
     (!(flags & CALIB_THIN_PRISM_MODEL)) &&
     (!(flags & CALIB_TILTED_MODEL)))
     {
-        distCoeffs1 = distCoeffs1.rows == 1 ? distCoeffs1.colRange(0, 5) : distCoeffs1.rowRange(0, 5);
-        distCoeffs2 = distCoeffs2.rows == 1 ? distCoeffs2.colRange(0, 5) : distCoeffs2.rowRange(0, 5);
+        int out_size1 = (dist1_total == 0) ? 5 : std::min(5, dist1_total);
+        int out_size2 = (dist2_total == 0) ? 5 : std::min(5, dist2_total);
+        distCoeffs1 = distCoeffs1.rows == 1 ? distCoeffs1.colRange(0, out_size1) : distCoeffs1.rowRange(0, out_size1);
+        distCoeffs2 = distCoeffs2.rows == 1 ? distCoeffs2.colRange(0, out_size2) : distCoeffs2.rowRange(0, out_size2);
     }
 
     if((flags & CALIB_USE_EXTRINSIC_GUESS) == 0)
diff --git a/modules/calib/test/test_cameracalibration.cpp b/modules/calib/test/test_cameracalibration.cpp
index 7fcc41f991..0093b40b63 100644
--- a/modules/calib/test/test_cameracalibration.cpp
+++ b/modules/calib/test/test_cameracalibration.cpp
@@ -1898,4 +1898,51 @@ TEST(Calib_StereoCalibrate, regression_22421)
     EXPECT_LE(terr, 0.0000001);
 }
 
+TEST(Calib_CalibrateCamera, size4DistortionCoeffs)
+{
+    std::vector<std::vector<cv::Point3f>> objectPoints(3);
+    std::vector<std::vector<cv::Point2f>> imagePoints(3);
+    cv::Size boardSize(9, 6);
+    for (int i = 0; i < 3; i++) {
+        for (int y = 0; y < boardSize.height; y++) {
+            for (int x = 0; x < boardSize.width; x++) {
+                objectPoints[i].push_back(cv::Point3f(x * 0.1f, y * 0.1f, 0.f));
+            }
+        }
+    }
+
+    cv::Matx33d cameraMatrix(800, 0, 320, 0, 800, 240, 0, 0, 1);
+    cv::Vec4d distCoeffs(0.1, -0.05, 0.001, -0.002);
+    for (int i = 0; i < 3; i++) {
+        cv::Vec3d rvec(0.1 * i, -0.2 * i, 0.05 * i);
+        cv::Vec3d tvec(-0.1 * i, 0.05 * i, 1.0 + 0.1 * i);
+        cv::projectPoints(objectPoints[i], rvec, tvec, cameraMatrix, distCoeffs, imagePoints[i]);
+    }
+
+    // 1. Test calibrateCamera
+    cv::Matx33d cameraMatrix_est = cameraMatrix;
+    cv::Mat1d distCoeffs_est = cv::Mat1d::zeros(1, 4);
+    std::vector<cv::Mat> rvecs, tvecs;
+    double rms = cv::calibrateCamera(objectPoints, imagePoints, cv::Size(640, 480),
+                                     cameraMatrix_est, distCoeffs_est, rvecs, tvecs,
+                                     cv::CALIB_USE_INTRINSIC_GUESS);
+
+    EXPECT_LT(rms, 1e-4);
+    EXPECT_LE(cv::norm(cv::Vec4d(distCoeffs_est), distCoeffs, NORM_INF), 1e-4);
+
+    // 2. Test stereoCalibrate
+    cv::Matx33d K1 = cameraMatrix;
+    cv::Matx33d K2 = cameraMatrix;
+    cv::Mat1d D1 = cv::Mat1d::zeros(1, 4);
+    cv::Mat1d D2 = cv::Mat1d::zeros(1, 4);
+    cv::Mat R, T, E, F;
+    double stereo_rms = cv::stereoCalibrate(objectPoints, imagePoints, imagePoints,
+                                            K1, D1, K2, D2, cv::Size(640, 480),
+                                            R, T, E, F, cv::CALIB_USE_INTRINSIC_GUESS);
+
+    EXPECT_LT(stereo_rms, 1e-4);
+    EXPECT_LE(cv::norm(cv::Vec4d(D1), distCoeffs, NORM_INF), 1e-4);
+    EXPECT_LE(cv::norm(cv::Vec4d(D2), distCoeffs, NORM_INF), 1e-4);
+}
+
 }} // namespace