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/include/opencv2/imgproc.hpp

@@ -329,7 +329,8 @@ enum ThresholdTypes {
     THRESH_TOZERO_INV = 4, //!< \f[\texttt{dst} (x,y) =  \fork{0}{if \(\texttt{src}(x,y) > \texttt{thresh}\)}{\texttt{src}(x,y)}{otherwise}\f]
     THRESH_MASK       = 7,
     THRESH_OTSU       = 8, //!< flag, use Otsu algorithm to choose the optimal threshold value
-    THRESH_TRIANGLE   = 16 //!< flag, use Triangle algorithm to choose the optimal threshold value
+    THRESH_TRIANGLE   = 16, //!< flag, use Triangle algorithm to choose the optimal threshold value
+    THRESH_DRYRUN     = 128 //!< flag, compute threshold only (useful for OTSU/TRIANGLE) but does not actually run thresholding
 };
 
 //! adaptive threshold algorithm

modules/imgproc/include/opencv2/imgproc/types_c.h

@@ -610,9 +610,11 @@ enum
     CV_THRESH_MASK        =7,
     CV_THRESH_OTSU        =8, /**< use Otsu algorithm to choose the optimal threshold value;
                                  combine the flag with one of the above CV_THRESH_* values */
-    CV_THRESH_TRIANGLE    =16  /**< use Triangle algorithm to choose the optimal threshold value;
+    CV_THRESH_TRIANGLE    =16,  /**< use Triangle algorithm to choose the optimal threshold value;
                                  combine the flag with one of the above CV_THRESH_* values, but not
                                  with CV_THRESH_OTSU */
+    CV_THRESH_DRYRUN      =128 /**< compute threshold only (useful for OTSU/TRIANGLE) but does not
+                                  actually run thresholding */
 };
 
 /** Adaptive threshold methods */