Merge pull request #29252 from JonasPerolini:pr-aruco-bit-threshold-in-refine

Use validBitIdThreshold for Aruco refineDetectedMarkers #29252

The goal of this PR is to solve the issue raised by @vrabaud in https://github.com/opencv/opencv/pull/28289 (comment: https://github.com/opencv/opencv/pull/28289#discussion_r3355812646). 

**Issue:** 

`refineDetectedMarkers()` converted the extracted cell ratios with `convertTo(CV_8UC1)` (an implicit 0.5 threshold) before computing the code distance, ignoring `detectorParams.validBitIdThreshold`. 

**Solution:** 

Make the refine path consistent with the main detection path `Dictionary::identify`.

**Changes:**

- Add a `Dictionary::getDistanceToId()` overload that takes the float cell pixel ratio matrix and `validBitIdThreshold` (similar to how it's done for the `identify()` overload.
- Move the per cell distance computation into a private `getDistanceToIdImpl` helper used by both `identify()` and the new overload of `getDistanceToId()` to avoid repetitions.
- `refineDetectedMarkers()` now calls the new overload.

**Tests:**

- `CV_ArucoRefine.validBitIdThreshold`: a marker with one degraded cell is recovered at threshold 0.7 but not at 0.49.
- `CV_ArucoDictionary.getDistanceToIdCellPixelRatio`: unit-tests both `getDistanceToId` overloads.

All passed

modules/objdetect/misc/python/test/test_objdetect_aruco.py

@@ -144,6 +144,32 @@ class aruco_objdetect_test(NewOpenCVTests):
 
         self.assertEqual(dist, 0)
 
+    def test_getDistanceToId_cell_pixel_ratio(self):
+        aruco_dict = cv.aruco.getPredefinedDictionary(cv.aruco.DICT_4X4_50)
+        idx = 7
+        valid_bit_id_threshold = 0.49
+        bit_marker = np.array([[0, 1, 0, 1], [0, 1, 1, 1], [1, 1, 0, 0], [0, 1, 0, 0]], dtype=np.uint8)
+        ratio_marker = bit_marker.astype(np.float32)
+
+        # Same marker as test_getDistanceToId, but passed as float cell ratios.
+        dist = aruco_dict.getDistanceToId(ratio_marker, idx, True, valid_bit_id_threshold)
+        self.assertEqual(dist, 0)
+
+        # A small drift stays within the threshold.
+        accepted_ratio = ratio_marker.copy()
+        accepted_ratio[0, 0] = 0.4
+        dist = aruco_dict.getDistanceToId(accepted_ratio, idx, True, valid_bit_id_threshold)
+        self.assertEqual(dist, 0)
+
+        # A full flip crosses the threshold and counts as one bad cell.
+        erroneous_ratio = ratio_marker.copy()
+        erroneous_ratio[0, 0] = 1.0 - erroneous_ratio[0, 0]
+        dist = aruco_dict.getDistanceToId(onlyCellPixelRatio=erroneous_ratio,
+                                          id=idx,
+                                          allRotations=True,
+                                          validBitIdThreshold=valid_bit_id_threshold)
+        self.assertEqual(dist, 1)
+
     def test_aruco_detector(self):
         aruco_params = cv.aruco.DetectorParameters()
         aruco_dict = cv.aruco.getPredefinedDictionary(cv.aruco.DICT_4X4_250)