Merge pull request #27270 from gursimarsingh:bug_fix_unstable_crf

Bug fix unstable crf #27270

### Pull Request Readiness Checklist

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- [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

@@ -63,10 +63,12 @@ Mat triangleWeights()
 {
     // hat function
     Mat w(LDR_SIZE, 1, CV_32F);
-    int half = LDR_SIZE / 2;
+    int half   = LDR_SIZE / 2;
-    for(int i = 0; i < LDR_SIZE; i++) {
+    int maxVal = LDR_SIZE - 1;
-        w.at<float>(i) = i < half ? i + 1.0f : LDR_SIZE - i;
+    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;
 }
 

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