Merge pull request #26836 from chacha21:thresholding_compute_threshold_only

Add cv::THRESH_DRYRUN flag to get adaptive threshold values without thresholding #26836

A first proposal for #26777

Adds a `cv::THRESH_DRYRUN` flag to let cv::threshold() compute the threshold (useful for OTSU/TRIANGLE), but without actually running the thresholding. This flags is a proposal instead of a new function cv::computeThreshold()

- [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/imgproc/test/test_thresh.cpp

@@ -502,6 +502,25 @@ BIGDATA_TEST(Imgproc_Threshold, huge)
     ASSERT_EQ((uint64)nz, n / 2);
 }
 
+TEST(Imgproc_Threshold, threshold_dryrun)
+{
+    Size sz(16, 16);
+    Mat input_original(sz, CV_8U, Scalar::all(2));
+    Mat input = input_original.clone();
+    std::vector<int> threshTypes = {THRESH_BINARY, THRESH_BINARY_INV, THRESH_TRUNC, THRESH_TOZERO, THRESH_TOZERO_INV};
+    std::vector<int> threshFlags = {0, THRESH_OTSU, THRESH_TRIANGLE};
+    for(int threshType : threshTypes)
+    {
+        for(int threshFlag : threshFlags)
+        {
+            const int _threshType = threshType | threshFlag | THRESH_DRYRUN;
+            cv::threshold(input, input, 2.0, 0.0, _threshType);
+            EXPECT_MAT_NEAR(input, input_original, 0);
+        }
+    }
+}
+
+
 TEST(Imgproc_Threshold, regression_THRESH_TOZERO_IPP_16085)
 {
     Size sz(16, 16);