From c3fe92d813ca17a523559ed09ab3bd19b0b6d4b3 Mon Sep 17 00:00:00 2001
From: Gursimar Singh <gursimar@bigvisionllc.com>
Date: Wed, 21 May 2025 11:10:11 +0530
Subject: [PATCH] Merge pull request #27270 from
 gursimarsingh:bug_fix_unstable_crf

Bug fix unstable crf #27270

### 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.
- [x] The feature is well documented and sample code can be built with the project CMake

The PR resolves the issue for triangle Weights used by debevec algorithm being non zero at extremes.
It resolves #24966

The fix needs ground truth data to be changed in order to pass existing tests. PR to opencv_extra: https://github.com/opencv/opencv_extra/pull/1253
---
 modules/photo/src/hdr_common.cpp | 10 ++++---
 modules/photo/test/test_hdr.cpp  | 49 ++++++++++++++++++++++++++++----
 2 files changed, 50 insertions(+), 9 deletions(-)

diff --git a/modules/photo/src/hdr_common.cpp b/modules/photo/src/hdr_common.cpp
index 983efe3792..5e4bea9d85 100644
--- a/modules/photo/src/hdr_common.cpp
+++ b/modules/photo/src/hdr_common.cpp
@@ -63,10 +63,12 @@ Mat triangleWeights()
 {
     // hat function
     Mat w(LDR_SIZE, 1, CV_32F);
-    int half = LDR_SIZE / 2;
-    for(int i = 0; i < LDR_SIZE; i++) {
-        w.at<float>(i) = i < half ? i + 1.0f : LDR_SIZE - i;
-    }
+    int half   = LDR_SIZE / 2;
+    int maxVal = LDR_SIZE - 1;
+    for (int i = 0; i < LDR_SIZE; i++)
+        w.at<float>(i) = (i < half)
+            ? static_cast<float>(i)
+            : static_cast<float>(maxVal - i);
     return w;
 }
 
diff --git a/modules/photo/test/test_hdr.cpp b/modules/photo/test/test_hdr.cpp
index a26e83e49e..264a7d7257 100644
--- a/modules/photo/test/test_hdr.cpp
+++ b/modules/photo/test/test_hdr.cpp
@@ -187,11 +187,9 @@ TEST(Photo_MergeDebevec, regression)
     Mat result, expected;
     loadImage(test_path + "merge/debevec.hdr", expected);
     merge->process(images, result, times, response);
-
     Ptr<Tonemap> map = createTonemap();
     map->process(result, result);
     map->process(expected, expected);
-
     checkEqual(expected, result, 1e-2f, "Debevec");
 }
 
@@ -221,16 +219,15 @@ TEST(Photo_CalibrateDebevec, regression)
     loadExposureSeq(test_path + "exposures/", images, times);
     loadResponseCSV(test_path + "calibrate/debevec.csv", expected);
     Ptr<CalibrateDebevec> calibrate = createCalibrateDebevec();
-
     calibrate->process(images, response, times);
     Mat diff = abs(response - expected);
     diff = diff.mul(1.0f / response);
     double max;
     minMaxLoc(diff, NULL, &max);
 #if defined(__arm__) || defined(__aarch64__)
-    ASSERT_LT(max, 0.2);
+    ASSERT_LT(max, 0.25);
 #else
-    ASSERT_LT(max, 0.1);
+    ASSERT_LT(max, 0.15);
 #endif
 }
 
@@ -266,4 +263,46 @@ TEST(Photo_CalibrateRobertson, bug_18180)
     EXPECT_EQ(0.0, cv::norm(response, response_no_nans, NORM_L2));
 }
 
+TEST(Photo_CalibrateDebevec, bug_24966)
+{
+    string test_path = string(cvtest::TS::ptr()->get_data_path()) + "hdr/";
+    vector<Mat> all_images;
+    vector<float> all_times;
+    loadExposureSeq(test_path + "exposures/", all_images, all_times);
+    // Use a balanced subset of exposures
+    vector<int> selected_indices = {1,2,3,4,5};
+    vector<Mat> images;
+    vector<float> times;
+    for (int idx : selected_indices) {
+        images.push_back(all_images[idx]);
+        times.push_back(all_times[idx]);
+    }
+    // Run CRF estimation for different sample points
+    vector<int> sample_points = {200,300,400};
+    vector<Mat> responses;
+    for (int samples : sample_points) {
+        Ptr<CalibrateDebevec> calibrate = createCalibrateDebevec(samples);
+        Mat response;
+        calibrate->process(images, response, times);
+        Mat roi = response.rowRange(15, 240); //Checking CRF only in the middle of the image
+        responses.push_back(roi);
+    }
+
+    // Compare consecutive pairs of CRFs
+    for (size_t i = 0; i < responses.size()-1; ++i) {
+        Mat diff = abs(responses[i] - responses[i+1]);
+        double max_diff;
+        minMaxLoc(diff, nullptr, &max_diff);
+        cout << "max_diff = " << max_diff << endl;
+        #if defined(__aarch64__) && defined(__APPLE__)
+            ASSERT_LT(max_diff, 10) << "CRF instability detected between samples="
+                << sample_points[i] << " and " << sample_points[i+1]
+                << " (max diff = " << max_diff << ")";
+        #else
+            ASSERT_LT(max_diff, 5) << "CRF instability detected between samples="
+                << sample_points[i] << " and " << sample_points[i+1]
+                << " (max diff = " << max_diff << ")";
+        #endif
+    }
+}
 }} // namespace