Merge pull request opencv#26446 from dkurt:file_storage_empty_and_1d_mat

* Change style of empty and 1d Mat in FileStorage

* Remove misleading 1d Mat test

Author: dkurt

modules/core/src/persistence_types.cpp

@@ -12,7 +12,7 @@ void write( FileStorage& fs, const String& name, const Mat& m )
 {
     char dt[22];
 
-    if( m.dims <= 2 )
+    if( m.dims == 2 || m.empty() )
     {
         fs.startWriteStruct(name, FileNode::MAP, String("opencv-matrix"));
         fs << "rows" << m.rows;
@@ -151,6 +151,11 @@ void read(const FileNode& node, Mat& m, const Mat& default_mat)
     CV_Assert(!data_node.empty());
 
     size_t nelems = data_node.size();
+    if (nelems == 0)
+    {
+        m = Mat();
+        return;
+    }
     CV_Assert(nelems == m.total()*m.channels());
 
     data_node.readRaw(dt, (uchar*)m.ptr(), m.total()*m.elemSize());

modules/core/test/test_io.cpp

@@ -2018,7 +2018,25 @@ T fsWriteRead(const T& expectedValue, const char* ext)
     return value;
 }
 
+void testExactMat(const Mat& src, const char* ext)
+{
+    bool srcIsEmpty = src.empty();
+    Mat dst = fsWriteRead(src, ext);
+    EXPECT_EQ(dst.empty(), srcIsEmpty);
+    EXPECT_EQ(src.dims, dst.dims);
+    EXPECT_EQ(src.size, dst.size);
+    if (!srcIsEmpty)
+    {
+        EXPECT_EQ(0.0, cv::norm(src, dst, NORM_INF));
+    }
+}
+
 typedef testing::TestWithParam<const char*> FileStorage_exact_type;
+TEST_P(FileStorage_exact_type, empty_mat)
+{
+    testExactMat(Mat(), GetParam());
+}
+
 TEST_P(FileStorage_exact_type, long_int)
 {
     for (const int64_t expected : std::vector<int64_t>{INT64_MAX, INT64_MIN, -1, 1, 0})