From 6c3b195a57ae6605486d8d2b7cf767a536a960e4 Mon Sep 17 00:00:00 2001
From: Benjamin Knecht <knechtb@google.com>
Date: Mon, 24 Feb 2025 17:33:09 +0100
Subject: [PATCH] Make sure serialization with single dict preserves old
 behavior

---
 .../objdetect/src/aruco/aruco_detector.cpp    | 17 +++++---
 .../objdetect/test/test_arucodetection.cpp    | 42 +++++++++++++++++++
 2 files changed, 53 insertions(+), 6 deletions(-)

diff --git a/modules/objdetect/src/aruco/aruco_detector.cpp b/modules/objdetect/src/aruco/aruco_detector.cpp
index 3fabaf94c7..7f062b01cd 100644
--- a/modules/objdetect/src/aruco/aruco_detector.cpp
+++ b/modules/objdetect/src/aruco/aruco_detector.cpp
@@ -1407,13 +1407,18 @@ void ArucoDetector::refineDetectedMarkers(InputArray _image, const Board& _board
 }
 
 void ArucoDetector::write(FileStorage &fs) const {
-    fs << "dictionaries" << "[";
-    for (auto& dictionary : arucoDetectorImpl->dictionaries) {
-        fs << "{";
-        dictionary.writeDictionary(fs);
-        fs << "}";
+    // preserve old format for single dictionary case
+    if (1 == arucoDetectorImpl->dictionaries.size()) {
+        arucoDetectorImpl->dictionaries[0].writeDictionary(fs);
+    } else {
+        fs << "dictionaries" << "[";
+        for (auto& dictionary : arucoDetectorImpl->dictionaries) {
+            fs << "{";
+            dictionary.writeDictionary(fs);
+            fs << "}";
+        }
+        fs << "]";
     }
-    fs << "]";
     arucoDetectorImpl->detectorParams.writeDetectorParameters(fs);
     arucoDetectorImpl->refineParams.writeRefineParameters(fs);
 }
diff --git a/modules/objdetect/test/test_arucodetection.cpp b/modules/objdetect/test/test_arucodetection.cpp
index c3138312a4..00cce307a0 100644
--- a/modules/objdetect/test/test_arucodetection.cpp
+++ b/modules/objdetect/test/test_arucodetection.cpp
@@ -6,6 +6,15 @@
 #include "opencv2/objdetect/aruco_detector.hpp"
 #include "opencv2/calib3d.hpp"
 
+namespace cv::aruco {
+    bool operator==(const Dictionary& d1, const Dictionary& d2);
+    bool operator==(const Dictionary& d1, const Dictionary& d2) {
+        return d1.markerSize == d2.markerSize
+            && std::equal(d1.bytesList.begin<uchar>(), d1.bytesList.end<uchar>(), d2.bytesList.begin<uchar>())
+            && d1.maxCorrectionBits == d2.maxCorrectionBits;
+    };
+}
+
 namespace opencv_test { namespace {
 
 /**
@@ -675,6 +684,9 @@ TEST(CV_ArucoMultiDict, noDict)
     EXPECT_THROW({
         detector.removeDictionary(0);
     }, Exception);
+    EXPECT_THROW({
+        detector.setDictionaries({});
+    }, Exception);
 }
 
 
@@ -717,6 +729,36 @@ TEST(CV_ArucoMultiDict, multiMarkerDetection)
 }
 
 
+TEST(CV_ArucoMultiDict, serialization)
+{
+    const std::string fileName("test_aruco_serialization.json");
+    aruco::ArucoDetector detector;
+    {
+        FileStorage fs(fileName, FileStorage::Mode::WRITE);
+        detector.write(fs);
+        fs.release();
+        FileStorage test_fs(fileName, FileStorage::Mode::READ);
+        aruco::ArucoDetector test_detector;
+        test_detector.read(test_fs.root());
+        // compare default constructor result
+        EXPECT_EQ(aruco::getPredefinedDictionary(aruco::DICT_4X4_50), test_detector.getDictionary());
+    }
+    detector.addDictionary(aruco::getPredefinedDictionary(aruco::DICT_5X5_100));
+    {
+        FileStorage fs(fileName, FileStorage::Mode::WRITE);
+        detector.write(fs);
+        fs.release();
+        FileStorage test_fs(fileName, FileStorage::Mode::READ);
+        aruco::ArucoDetector test_detector;
+        test_detector.read(test_fs.root());
+        // check for one additional dictionary
+        ASSERT_EQ(2ul, test_detector.getDictionaries().size());
+        EXPECT_EQ(aruco::getPredefinedDictionary(aruco::DICT_4X4_50), test_detector.getDictionary());
+        EXPECT_EQ(aruco::getPredefinedDictionary(aruco::DICT_5X5_100), test_detector.getDictionary(1));
+    }
+}
+
+
 struct ArucoThreading: public testing::TestWithParam<aruco::CornerRefineMethod>
 {
     struct NumThreadsSetter {