From d750d43aa2fd6c8e5dde5861c19afe612d57911b Mon Sep 17 00:00:00 2001
From: Dmitry Kurtaev <dmitry.kurtaev@gmail.com>
Date: Wed, 11 Jun 2025 17:37:18 +0300
Subject: [PATCH] Merge pull request #27432 from
 dkurt:d.kurtaev/ipp_distTransform

Correct IPP distanceTransform results with single thread #27432

### Pull Request Readiness Checklist

resolves #24082

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/distransform.cpp          | 16 +++++++++
 .../imgproc/test/test_distancetransform.cpp   | 33 +++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/modules/imgproc/src/distransform.cpp b/modules/imgproc/src/distransform.cpp
index 6a7026c8c8..66be0e8ea5 100644
--- a/modules/imgproc/src/distransform.cpp
+++ b/modules/imgproc/src/distransform.cpp
@@ -796,6 +796,22 @@ void cv::distanceTransform( InputArray _src, OutputArray _dst, OutputArray _labe
                     ippFree( pBuffer );
                     if (status>=0)
                     {
+                        // https://github.com/opencv/opencv/issues/24082
+                        // There is probably a rounding issue that leads to non-deterministic behavior
+                        // between runs on positions closer to zeros by x-axis in straight direction.
+                        // As a workaround, we detect the distances that expected to be exact
+                        // number of pixels and round manually.
+                        static const float correctionDiff = 1.0f / (1 << 11);
+                        for (int i = 0; i < dst.rows; ++i)
+                        {
+                            float* row = dst.ptr<float>(i);
+                            for (int j = 0; j < dst.cols; ++j)
+                            {
+                                float rounded = static_cast<float>(cvRound(row[j]));
+                                if (fabs(row[j] - rounded) <= correctionDiff)
+                                    row[j] = rounded;
+                            }
+                        }
                         CV_IMPL_ADD(CV_IMPL_IPP);
                         return;
                     }
diff --git a/modules/imgproc/test/test_distancetransform.cpp b/modules/imgproc/test/test_distancetransform.cpp
index bf272cd224..9692655fab 100644
--- a/modules/imgproc/test/test_distancetransform.cpp
+++ b/modules/imgproc/test/test_distancetransform.cpp
@@ -416,4 +416,37 @@ TEST(Imgproc_DistanceTransform, precise_long_dist)
     EXPECT_EQ(cv::norm(expected, dist, NORM_INF), 0);
 }
 
+TEST(Imgproc_DistanceTransform, ipp_deterministic_corner)
+{
+    setNumThreads(1);
+
+    Mat src(1, 4096, CV_8U, Scalar(255)), dist;
+    src.at<uint8_t>(0, 0) = 0;
+    distanceTransform(src, dist, DIST_L2, DIST_MASK_PRECISE);
+    for (int i = 0; i < src.cols; ++i)
+    {
+        float expected = static_cast<float>(i);
+        ASSERT_EQ(expected, dist.at<float>(0, i)) << cv::format("diff: %e", expected - dist.at<float>(0, i));
+    }
+}
+
+TEST(Imgproc_DistanceTransform, ipp_deterministic)
+{
+    setNumThreads(1);
+    RNG& rng = TS::ptr()->get_rng();
+    Mat src(1, 800, CV_8U, Scalar(255)), dist;
+    int p1 = cvtest::randInt(rng) % src.cols;
+    int p2 = cvtest::randInt(rng) % src.cols;
+    int p3 = cvtest::randInt(rng) % src.cols;
+    src.at<uint8_t>(0, p1) = 0;
+    src.at<uint8_t>(0, p2) = 0;
+    src.at<uint8_t>(0, p3) = 0;
+    distanceTransform(src, dist, DIST_L2, DIST_MASK_PRECISE);
+    for (int i = 0; i < src.cols; ++i)
+    {
+        float expected = static_cast<float>(min(min(abs(i - p1), abs(i - p2)), abs(i - p3)));
+        ASSERT_EQ(expected, dist.at<float>(0, i)) << cv::format("diff: %e", expected - dist.at<float>(0, i));
+    }
+}
+
 }} // namespace