Add attribute read/write supports to the HDF module.

The following data types for attributes are implemented:
 - int
 - double
 - cv::String
 - cv::InputArray

modules/hdf/include/opencv2/hdf/hdf5.hpp

@@ -103,13 +103,109 @@ public:
      */
     CV_WRAP virtual bool hlexists( const String& label ) const = 0;
 
-    /* @overload */
+    /**
+     * Check whether a given attribute exits or not in the root group.
+     *
+     * @param atlabel the attribute name to be checked.
+     * @return true if the attribute exists, false otherwise.
+     *
+     * @sa atdelete, atwrite, atread
+     */
+    CV_WRAP virtual bool atexists(const String& atlabel) const = 0;
+
+    /**
+     * Delete an attribute from the root group.
+     *
+     * @param atlabel the attribute to be deleted.
+     *
+     * @note CV_Error() is called if the given attribute does not exist. Use atexists()
+     * to check whether it exists or not beforehand.
+     *
+     * @sa atexists, atwrite, atread
+     */
+    CV_WRAP virtual void atdelete(const String& atlabel) = 0;
+
+    /**
+     * Write an attribute inside the root group.
+     *
+     * @param value attribute value.
+     * @param atlabel attribute name.
+     *
+     * The following example demonstrates how to write an attribute of type cv::String:
+     *
+     *  @snippet samples/read_write_attributes.cpp snippets_write_str
+     *
+     * @note CV_Error() is called if the given attribute already exists. Use atexists()
+     * to check whether it exists or not beforehand. And use atdelete() to delete
+     * it if it already exists.
+     *
+     * @sa atexists, atdelete, atread
+     */
+    CV_WRAP virtual void atwrite(const int value, const String& atlabel) = 0;
+
+    /**
+     * Read an attribute from the root group.
+     *
+     * @param value address where the attribute is read into
+     * @param atlabel attribute name
+     *
+     * The following example demonstrates how to read an attribute of type cv::String:
+     *
+     *  @snippet samples/read_write_attributes.cpp snippets_read_str
+     *
+     * @note The attribute MUST exist, otherwise CV_Error() is called. Use atexists()
+     * to check if it exists beforehand.
+     *
+     * @sa atexists, atdelete, atwrite
+     */
+    CV_WRAP virtual void atread(int* value, const String& atlabel) = 0;
+
+    /** @overload */
+    CV_WRAP virtual void atwrite(const double value, const String& atlabel) = 0;
+
+    /** @overload */
+    CV_WRAP virtual void atread(double* value, const String& atlabel) = 0;
+
+    /** @overload */
+    CV_WRAP virtual void atwrite(const String& value, const String& atlabel) = 0;
+
+    /** @overload */
+    CV_WRAP virtual void atread(String* value, const String& atlabel) = 0;
+
+    /**
+     * Write an attribute into the root group.
+     *
+     * @param value attribute value. Currently, only n-d continuous multi-channel arrays are supported.
+     * @param atlabel attribute name.
+     *
+     * @note CV_Error() is called if the given attribute already exists. Use atexists()
+     * to check whether it exists or not beforehand. And use atdelete() to delete
+     * it if it already exists.
+     *
+     * @sa atexists, atdelete, atread.
+     */
+    CV_WRAP virtual void atwrite(InputArray value, const String& atlabel) = 0;
+
+    /**
+     * Read an attribute from the root group.
+     *
+     * @param value attribute value. Currently, only n-d continuous multi-channel arrays are supported.
+     * @param atlabel attribute name.
+     *
+     * @note The attribute MUST exist, otherwise CV_Error() is called. Use atexists()
+     * to check if it exists beforehand.
+     *
+     * @sa atexists, atdelete, atwrite
+     */
+    CV_WRAP virtual void atread(OutputArray value, const String& atlabel) = 0;
+
+    /** @overload */
     CV_WRAP virtual void dscreate( const int rows, const int cols, const int type,
                  const String& dslabel ) const = 0;
-    /* @overload */
+    /** @overload */
     CV_WRAP virtual void dscreate( const int rows, const int cols, const int type,
                  const String& dslabel, const int compresslevel ) const = 0;
-    /* @overload */
+    /** @overload */
     CV_WRAP virtual void dscreate( const int rows, const int cols, const int type,
                  const String& dslabel, const int compresslevel, const vector<int>& dims_chunks ) const = 0;
     /** @brief Create and allocate storage for two dimensional single or multi channel dataset.

modules/hdf/samples/create_groups.cpp

-// This file is part of OpenCV project.
-// It is subject to the license terms in the LICENSE file found in the top-level directory
-// of this distribution and at http://opencv.org/license.html.
-
 /**
  * @file create_groups.cpp
  * @author Fangjun Kuang <csukuangfj dot at gmail dot com>

modules/hdf/samples/create_read_write_datasets.cpp

-// This file is part of OpenCV project.
-// It is subject to the license terms in the LICENSE file found in the top-level directory
-// of this distribution and at http://opencv.org/license.html.
-
 /**
- * @file create_read_write.cpp
+ * @file create_read_write_datasets.cpp
  * @author Fangjun Kuang <csukuangfj dot at gmail dot com>
  * @date December 2017
  *
@@ -13,14 +9,6 @@
  *
  */
 
-#ifdef __GNUC__
-#  pragma GCC diagnostic ignored "-Wmissing-declarations"
-#  if defined __clang__ || defined __APPLE__
-#    pragma GCC diagnostic ignored "-Wmissing-prototypes"
-#    pragma GCC diagnostic ignored "-Wextra"
-#  endif
-#endif
-
 //! [tutorial]
 #include <iostream>
 
@@ -29,7 +17,7 @@
 
 using namespace cv;
 
-void write_root_group_single_channel()
+static void write_root_group_single_channel()
 {
     String filename = "root_group_single_channel.h5";
     String dataset_name = "/single"; // Note that it is a child of the root group /
@@ -61,7 +49,7 @@ void write_root_group_single_channel()
     h5io->close();
 }
 
-void write_single_channel()
+static void write_single_channel()
 {
     String filename = "single_channel.h5";
     String parent_name = "/data";
@@ -98,7 +86,7 @@ void write_single_channel()
  * creating, reading and writing multiple-channel matrices
  * are the same with single channel matrices
  */
-void write_multiple_channels()
+static void write_multiple_channels()
 {
     String filename = "two_channels.h5";
     String parent_name = "/data";

modules/hdf/samples/read_write_attributes.cpp

+/**
+ * @file read_write_attributes.cpp
+ * @author Fangjun Kuang <csukuangfj dot at gmail dot com>
+ * @date December 2017
+ *
+ * @brief It demonstrates how to read and write attributes inside the
+ * root group.
+ *
+ * Currently, only the following datatypes can be used as attributes:
+ *  - cv::String
+ *  - int
+ *  - double
+ *  - cv::InputArray (n-d continuous multichannel arrays)
+ *
+ * Although HDF supports associating attributes with both datasets and groups,
+ * only support for the root group is implemented by OpenCV at present.
+ */
+
+//! [tutorial]
+#include <iostream>
+
+#include <opencv2/core.hpp>
+#include <opencv2/hdf.hpp>
+
+using namespace cv;
+
+static void read_write_attributes()
+{
+    String filename = "attributes.h5";
+
+    //! [tutorial_open_file]
+    Ptr<hdf::HDF5> h5io = hdf::open(filename);
+    //! [tutorial_open_file]
+
+    //! [tutorial_write_mat]
+    String attr_mat_name = "array attribute";
+    Mat attr_mat;
+    attr_mat = (cv::Mat_<float>(2, 3) << 0, 1, 2, 3, 4, 5, 6);
+    if (!h5io->atexists(attr_mat_name))
+        h5io->atwrite(attr_mat, attr_mat_name);
+    //! [tutorial_write_mat]
+
+    //! [snippets_write_str]
+    String attr_str_name = "string attribute";
+    String attr_str = "Hello HDF5 from OpenCV!";
+    if (!h5io->atexists(attr_str_name))
+        h5io->atwrite(attr_str, attr_str_name);
+    //! [snippets_write_str]
+
+    String attr_int_name = "int attribute";
+    int attr_int = 123456;
+    if (!h5io->atexists(attr_int_name))
+        h5io->atwrite(attr_int, attr_int_name);
+
+    String attr_double_name = "double attribute";
+    double attr_double = 45678.123;
+    if (!h5io->atexists(attr_double_name))
+        h5io->atwrite(attr_double, attr_double_name);
+
+    // read attributes
+    Mat expected_attr_mat;
+    int expected_attr_int;
+    double expected_attr_double;
+
+    //! [snippets_read_str]
+    String expected_attr_str;
+    h5io->atread(&expected_attr_str, attr_str_name);
+    //! [snippets_read_str]
+
+    //! [tutorial_read_mat]
+    h5io->atread(expected_attr_mat, attr_mat_name);
+    //! [tutorial_read_mat]
+    h5io->atread(&expected_attr_int, attr_int_name);
+    h5io->atread(&expected_attr_double, attr_double_name);
+
+    // check results
+    CV_Assert(norm(attr_mat - expected_attr_mat) < 1e-10);
+    CV_Assert(attr_str.compare(expected_attr_str) == 0);
+    CV_Assert(attr_int == expected_attr_int);
+    CV_Assert(fabs(attr_double - expected_attr_double) < 1e-10);
+
+    //! [tutorial_close_file]
+    h5io->close();
+    //! [tutorial_close_file]
+}
+
+int main()
+{
+    read_write_attributes();
+
+    return 0;
+}
+//! [tutorial]

modules/hdf/src/hdf5.cpp

@@ -61,6 +61,21 @@ public:
     // check if object / link exists
     virtual bool hlexists( const String& label ) const;
 
+    virtual bool atexists(const String& atlabel) const;
+    virtual void atdelete(const String& atlabel);
+
+    virtual void atwrite(const int value, const String& atlabel);
+    virtual void atread(int* value, const String& atlabel);
+
+    virtual void atwrite(const double value, const String& atlabel);
+    virtual void atread(double* value, const String& atlabel);
+
+    virtual void atwrite(const String& value, const String& atlabel);
+    virtual void atread(String* value, const String& atlabel);
+
+    virtual void atwrite(InputArray value, const String& atlabel);
+    virtual void atread(OutputArray value, const String& atlabel);
+
     /*
      * h5 group
      */
@@ -218,7 +233,7 @@ inline hid_t HDF5Impl::GetH5type( int cvType ) const
         h5Type = H5T_NATIVE_INT;
         break;
       default:
-        CV_Error( Error::StsInternal, "Unknown cvType." );
+        CV_Error_(Error::StsInternal, ("Unknown cvType: %d.", cvType));
     }
     return h5Type;
 }
@@ -242,7 +257,7 @@ inline int HDF5Impl::GetCVtype( hid_t h5Type ) const
     else if ( H5Tequal( h5Type, H5T_NATIVE_INT    ) )
       cvType = CV_32S;
     else
-      CV_Error( Error::StsInternal, "Unknown H5Type." );
+      CV_Error_(Error::StsInternal, ("Unknown H5Type: %d.", h5Type));
 
     return cvType;
 }
@@ -302,15 +317,229 @@ bool HDF5Impl::hlexists( const String& label ) const
     return exists;
 }
 
+bool HDF5Impl::atexists(const String& atlabel) const
+{
+    bool res = false;
+
+    // save old error handler
+    void *errdata;
+    H5E_auto2_t errfunc;
+    hid_t stackid = H5E_DEFAULT;
+    H5Eget_auto(stackid, &errfunc, &errdata);
+
+    // turn off error handling
+    H5Eset_auto(stackid, NULL, NULL);
+
+    hid_t attr = H5Aopen_name(m_h5_file_id, atlabel.c_str());
+    if (attr >= 0)
+    {
+        res = true;
+        H5Aclose(attr);
+    }
+
+    // restore previous error handler
+    H5Eset_auto(stackid, errfunc, errdata);
+
+    return res;
+}
+
+void HDF5Impl::atdelete(const String& atlabel)
+{
+    if (!atexists(atlabel))
+        CV_Error_(Error::StsInternal,("The attribute '%s' does not exist!", atlabel.c_str()));
+
+    H5Adelete(m_h5_file_id, atlabel.c_str());
+}
+
+void HDF5Impl::atwrite(const int value, const String& atlabel)
+{
+    if (atexists(atlabel))
+        CV_Error_(Error::StsInternal,("The attribute '%s' already exists!", atlabel.c_str()));
+
+    hid_t aid = H5Screate(H5S_SCALAR);;
+    hid_t attr = H5Acreate2(m_h5_file_id, atlabel.c_str(), H5T_NATIVE_INT, aid,
+                            H5P_DEFAULT, H5P_DEFAULT);
+    H5Awrite(attr, H5T_NATIVE_INT, &value);
+
+    H5Sclose(aid);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atread(int* value, const String& atlabel)
+{
+    if (!value)
+        CV_Error(Error::StsBadArg, "NULL pointer");
+
+    if (!atexists(atlabel))
+        CV_Error_(Error::StsInternal, ("Attribute '%s' does not exist!", atlabel.c_str()));
+
+    hid_t attr = H5Aopen(m_h5_file_id, atlabel.c_str(), H5P_DEFAULT);
+    H5Aread(attr, H5T_NATIVE_INT, value);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atwrite(const double value, const String& atlabel)
+{
+    if (atexists(atlabel))
+        CV_Error_(Error::StsInternal,("The attribute '%s' already exists!", atlabel.c_str()));
+
+    hid_t aid = H5Screate(H5S_SCALAR);;
+    hid_t attr = H5Acreate2(m_h5_file_id, atlabel.c_str(), H5T_NATIVE_DOUBLE, aid,
+                            H5P_DEFAULT, H5P_DEFAULT);
+    H5Awrite(attr, H5T_NATIVE_DOUBLE, &value);
+
+    H5Sclose(aid);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atread(double* value, const String& atlabel)
+{
+    if (!value)
+        CV_Error(Error::StsBadArg, "NULL pointer");
+
+    if (!atexists(atlabel))
+        CV_Error_(Error::StsInternal, ("Attribute '%s' does not exist!", atlabel.c_str()));
+
+    hid_t attr = H5Aopen(m_h5_file_id, atlabel.c_str(), H5P_DEFAULT);
+    H5Aread(attr, H5T_NATIVE_DOUBLE, value);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atwrite(const String& value, const String& atlabel)
+{
+    if (atexists(atlabel))
+        CV_Error_(Error::StsInternal,("The attribute '%s' already exists!", atlabel.c_str()));
+
+    hid_t aid = H5Screate(H5S_SCALAR);
+    hid_t atype = H5Tcopy(H5T_C_S1);
+    H5Tset_size(atype, value.size()+1);
+    H5Tset_strpad(atype, H5T_STR_NULLTERM);
+
+    hid_t attr = H5Acreate2(m_h5_file_id, atlabel.c_str(), atype, aid, H5P_DEFAULT, H5P_DEFAULT);
+    H5Awrite(attr, atype, value.c_str());
+
+    H5Sclose(aid);
+    H5Tclose(atype);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atread(String* value, const String& atlabel)
+{
+    if (!value)
+        CV_Error(Error::StsBadArg, "NULL pointer");
+
+    if (!atexists(atlabel))
+        CV_Error_(Error::StsInternal, ("Attribute '%s' does not exist!", atlabel.c_str()));
+
+    hid_t attr = H5Aopen(m_h5_file_id, atlabel.c_str(), H5P_DEFAULT);
+    hid_t atype = H5Aget_type(attr);
+    H5T_class_t type_class = H5Tget_class(atype);
+    if (type_class != H5T_STRING)
+    {
+        H5Tclose(atype);
+        H5Aclose(attr);
+        CV_Error_(Error::StsInternal, ("Attribute '%s' is not of string type!", atlabel.c_str()));
+    }
+    size_t size = H5Tget_size(atype);
+    *value = String(size, 0); // allocate space
+
+    hid_t atype_mem = H5Tget_native_type(atype, H5T_DIR_ASCEND);
+    H5Aread(attr, atype_mem, const_cast<char*>(value->c_str()));
+
+    H5Tclose(atype_mem);
+    H5Tclose(atype);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atwrite(InputArray value, const String& atlabel)
+{
+    if (atexists(atlabel))
+        CV_Error_(Error::StsInternal,("The attribute '%s' already exists!", atlabel.c_str()));
+
+    Mat value_ = value.getMat();
+
+    if (!value_.isContinuous())
+        CV_Error(Error::StsInternal, "Only continuous array are implemented. Current array is not continuous!");
+
+    int ndims = value_.dims;
+
+    vector<hsize_t> dim_vec(ndims);
+    for (int i = 0; i < ndims; i++)
+        dim_vec[i] = value_.size[i];
+
+    hid_t dtype = GetH5type(value_.type());
+    if (value_.channels() > 1)
+    {
+        hsize_t dims[1] = { (hsize_t)value_.channels()};
+        dtype = H5Tarray_create(dtype, 1, dims);
+    }
+
+    hid_t aid = H5Screate(H5S_SIMPLE);
+    H5Sset_extent_simple(aid, ndims, dim_vec.data(), NULL);
+
+    hid_t attr = H5Acreate2(m_h5_file_id, atlabel.c_str(), dtype,
+                            aid, H5P_DEFAULT, H5P_DEFAULT);
+
+    H5Awrite(attr, dtype, value_.data);
+
+    if (value_.channels() > 1)
+        H5Tclose(dtype);
+
+    H5Sclose(aid);
+    H5Aclose(attr);
+}
+
+void HDF5Impl::atread(OutputArray value, const String& atlabel)
+{
+    if (!atexists(atlabel))
+        CV_Error_(Error::StsInternal, ("Attribute '%s' does not exist!", atlabel.c_str()));
+
+    hid_t attr = H5Aopen(m_h5_file_id, atlabel.c_str(), H5P_DEFAULT);
+    hid_t atype  = H5Aget_type(attr);
+    hid_t aspace = H5Aget_space(attr);
+    int rank = H5Sget_simple_extent_ndims(aspace);
+
+    vector<hsize_t> dim_vec_(rank);
+    H5Sget_simple_extent_dims(aspace, dim_vec_.data(), NULL);
+    vector<int> dim_vec(dim_vec_.begin(), dim_vec_.end());
+
+    int nchannels = 1;
+    hid_t h5type;
+    if (H5Tget_class(atype) == H5T_ARRAY)
+    {
+        hsize_t dims;
+        H5Tget_array_dims(atype, &dims);
+        nchannels = (int) dims;
+
+        hid_t super_type = H5Tget_super(atype);
+        h5type = H5Tget_native_type(super_type, H5T_DIR_ASCEND);
+        H5Tclose(super_type);
+    }
+    else
+        h5type = H5Tget_native_type(atype, H5T_DIR_ASCEND);
+
+    int dtype = GetCVtype(h5type);
+
+    value.create(rank, dim_vec.data(), CV_MAKETYPE(dtype, nchannels));
+    H5Aread(attr, atype, value.getMat().data);
+
+    H5Sclose(aspace);
+    H5Tclose(atype);
+    H5Aclose(attr);
+}
+
 /*
  * h5 group
  */
 
 void HDF5Impl::grcreate( const String& grlabel )
 {
-  hid_t gid = H5Gcreate( m_h5_file_id, grlabel.c_str(),
-                H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
-  H5Gclose( gid );
+    if (hlexists(grlabel))
+        CV_Error_(Error::StsInternal, ("Requested group '%s' already exists.", grlabel.c_str()));
+
+    hid_t gid = H5Gcreate(m_h5_file_id, grlabel.c_str(),
+                          H5P_DEFAULT, H5P_DEFAULT, H5P_DEFAULT);
+    H5Gclose(gid);
 }
 
 /*
@@ -358,7 +587,7 @@ vector<int> HDF5Impl::dsgetsize( const String& dslabel, int dims_flag ) const
         SizeVect.resize( n_dims );
     }
     else
-      CV_Error( Error::StsInternal, "Unknown dimension flag." );
+      CV_Error_(Error::StsInternal, ("Unknown dimension flag: %d", dims_flag));
 
     // fill with size data
     for ( size_t d = 0; d < SizeVect.size(); d++ )
@@ -479,7 +708,7 @@ void HDF5Impl::dscreate( const int n_dims, const int* sizes, const int type,
     CV_Assert( compresslevel >= H5_NONE && compresslevel <= 9 );
 
     if ( hlexists( dslabel ) == true )
-      CV_Error( Error::StsInternal, "Requested dataset already exists." );
+      CV_Error_(Error::StsInternal, ("Requested dataset '%s' already exists.", dslabel.c_str()));
 
     int channs = CV_MAT_CN( type );
 
@@ -802,7 +1031,7 @@ void HDF5Impl::dsinsert( InputArray Array, const String& dslabel,
 
     // check dataset exists
     if ( hlexists( dslabel ) == false )
-      CV_Error( Error::StsInternal, "Dataset does not exist." );
+      CV_Error_(Error::StsInternal, ("Dataset '%s' does not exist.", dslabel.c_str()));
 
     Mat matrix = Array.getMat();
 
@@ -935,7 +1164,7 @@ void HDF5Impl::kpcreate( const int size, const String& kplabel,
     CV_Assert( compresslevel >= H5_NONE && compresslevel <= 9 );
 
     if ( hlexists( kplabel ) == true )
-      CV_Error( Error::StsInternal, "Requested dataset already exists." );
+      CV_Error_(Error::StsInternal, ("Requested dataset '%s' already exists.", kplabel.c_str()));
 
     hsize_t dchunk[1];
     hsize_t dsdims[1];
@@ -1058,7 +1287,7 @@ void HDF5Impl::kpinsert( const vector<KeyPoint> keypoints, const String& kplabel
 
     // check dataset exists
     if ( hlexists( kplabel ) == false )
-      CV_Error( Error::StsInternal, "Dataset does not exist." );
+      CV_Error_(Error::StsInternal, ("Dataset '%s' does not exist.", kplabel.c_str()));
 
     hsize_t dsddims[1];
     hsize_t doffset[1];

modules/hdf/test/test_hdf5.cpp

@@ -61,6 +61,9 @@ TEST_F(HDF5_Test, create_a_single_group)
     EXPECT_EQ(m_hdf_io->hlexists(group_name), true);
     EXPECT_EQ(m_hdf_io->hlexists("child"), false);
 
+    // It should fail since it creates a group with an existing name
+    EXPECT_ANY_THROW(m_hdf_io->grcreate(group_name));
+
     m_hdf_io->close();
 }
 
@@ -210,3 +213,146 @@ TEST_F(HDF5_Test, write_read_dataset_2)
 
     m_hdf_io->close();
 }
+
+TEST_F(HDF5_Test, test_attribute)
+{
+    reset();
+
+    String attr_name = "test attribute name";
+    int attr_value = 0x12345678;
+
+    m_hdf_io = hdf::open(m_filename);
+    EXPECT_EQ(m_hdf_io->atexists(attr_name), false);
+
+    m_hdf_io->atwrite(attr_value, attr_name);
+    EXPECT_ANY_THROW(m_hdf_io->atwrite(attr_value, attr_name)); // error! it already exists
+
+    EXPECT_EQ(m_hdf_io->atexists(attr_name), true);
+
+    int expected_attr_value;
+    m_hdf_io->atread(&expected_attr_value, attr_name);
+    EXPECT_EQ(attr_value, expected_attr_value);
+
+    m_hdf_io->atdelete(attr_name);
+    EXPECT_ANY_THROW(m_hdf_io->atdelete(attr_name)); // error! Delete non-existed attribute
+
+    EXPECT_EQ(m_hdf_io->atexists(attr_name), false);
+
+    m_hdf_io->close();
+}
+
+TEST_F(HDF5_Test, test_attribute_int)
+{
+    reset();
+
+    String attr_name = "test int";
+    int attr_value = 0x12345678;
+
+    m_hdf_io = hdf::open(m_filename);
+
+    m_hdf_io->atwrite(attr_value, attr_name);
+
+    int expected_attr_value;
+    m_hdf_io->atread(&expected_attr_value, attr_name);
+    EXPECT_EQ(attr_value, expected_attr_value);
+
+    m_hdf_io->close();
+}
+
+TEST_F(HDF5_Test, test_attribute_double)
+{
+    reset();
+
+    String attr_name = "test double";
+    double attr_value = 123.456789;
+
+    m_hdf_io = hdf::open(m_filename);
+
+    m_hdf_io->atwrite(attr_value, attr_name);
+
+    double expected_attr_value;
+    m_hdf_io->atread(&expected_attr_value, attr_name);
+    EXPECT_NEAR(attr_value, expected_attr_value, 1e-9);
+
+    m_hdf_io->close();
+}
+
+TEST_F(HDF5_Test, test_attribute_String)
+{
+    reset();
+
+    String attr_name = "test-String";
+    String attr_value = "----_______----Hello HDF5----_______----\n";
+
+    m_hdf_io = hdf::open(m_filename);
+
+    m_hdf_io->atwrite(attr_value, attr_name);
+
+    String expected_attr_value;
+    m_hdf_io->atread(&expected_attr_value, attr_name);
+    EXPECT_EQ(attr_value.compare(expected_attr_value), 0);
+
+    m_hdf_io->close();
+}
+
+TEST_F(HDF5_Test, test_attribute_InutArray_OutputArray_2d)
+{
+    reset();
+
+    String attr_name = "test-InputArray-OutputArray-2d";
+    cv::Mat attr_value;
+
+    std::vector<int> depth_vec;
+    depth_vec.push_back(CV_8U); depth_vec.push_back(CV_8S);
+    depth_vec.push_back(CV_16U); depth_vec.push_back(CV_16S);
+    depth_vec.push_back(CV_32S); depth_vec.push_back(CV_32F);
+    depth_vec.push_back(CV_64F);
+
+    std::vector<int> channel_vec;
+    channel_vec.push_back(1); channel_vec.push_back(2);
+    channel_vec.push_back(3); channel_vec.push_back(4);
+    channel_vec.push_back(5); channel_vec.push_back(6);
+    channel_vec.push_back(7); channel_vec.push_back(8);
+    channel_vec.push_back(9); channel_vec.push_back(10);
+
+    std::vector<std::vector<int> > dim_vec;
+    std::vector<int> dim_2d;
+    dim_2d.push_back(2); dim_2d.push_back(3);
+    dim_vec.push_back(dim_2d);
+
+    std::vector<int> dim_3d;
+    dim_3d.push_back(2);
+    dim_3d.push_back(3);
+    dim_3d.push_back(4);
+    dim_vec.push_back(dim_3d);
+
+    std::vector<int> dim_4d;
+    dim_4d.push_back(2); dim_4d.push_back(3);
+    dim_4d.push_back(4); dim_4d.push_back(5);
+    dim_vec.push_back(dim_4d);
+
+    Mat expected_attr_value;
+
+    m_hdf_io = hdf::open(m_filename);
+    for (size_t i = 0; i < depth_vec.size(); i++)
+    for (size_t j = 0; j < channel_vec.size(); j++)
+    for (size_t k = 0; k < dim_vec.size(); k++)
+    {
+        if (m_hdf_io->atexists(attr_name))
+            m_hdf_io->atdelete(attr_name);
+
+        attr_value.create(dim_vec[k], CV_MAKETYPE(depth_vec[i], channel_vec[j]));
+        randu(attr_value, 0, 255);
+
+        m_hdf_io->atwrite(attr_value, attr_name);
+        m_hdf_io->atread(expected_attr_value, attr_name);
+
+        double diff = norm(attr_value - expected_attr_value);
+        EXPECT_NEAR(diff, 0, 1e-6);
+
+        EXPECT_EQ(attr_value.size(), expected_attr_value.size());
+        EXPECT_EQ(attr_value.type(), expected_attr_value.type());
+    }
+
+    m_hdf_io->close();
+}

modules/hdf/tutorials/create_groups/how_to_create_groups.markdown

@@ -3,13 +3,13 @@ Creating Groups {#tutorial_hdf_create_groups}
 
 Goal
 ----
+
 This tutorial will show you:
  - How to create a HDF5 file?
  - How to create a group?
  - How to check whether a given group exists or not?
  - How to create a subgroup?
 
-
 Source Code
 ----
 
@@ -35,7 +35,7 @@ Next, we create the group `Group1`
 @snippet samples/create_groups.cpp tutorial_create_group
 
 Note that we have to check whether `/Group1` exists or not using
-the function `hlexists` before creating it. You can not create
+the function cv::hdf::HDF5::hlexists() before creating it. You can not create
 a group with an existing name. Otherwise, an error will occur.
 
 Then, we create the subgroup named `Subgroup1`. In order to

modules/hdf/tutorials/create_read_write_dataset/create_read_write_dataset.markdown

@@ -3,12 +3,13 @@ Creating, Writing and Reading Datasets {#tutorial_hdf_create_read_write_datasets
 
 Goal
 ----
+
 This tutorial shows you:
  - How to create a dataset?
  - How to write a `cv::Mat` to a dataset?
  - How to read a `cv::Mat` from a dataset?
 
-@note Currently, it supports only reading and writing `cv::Mat` and the matrix should be continuous
+@note Currently, it supports only reading and writing cv::Mat and the matrix should be continuous
 in memory. Supports for other data types have not been implemented yet.
 
 Source Code
@@ -36,7 +37,7 @@ the dataset name is `/single`, which is inside the root group, we can use
 @snippet samples/create_read_write_datasets.cpp tutorial_write_root_single_channel
 
 to write the data directly to the dataset without the need of creating
-it beforehand. Because it is created inside `HDF5::dswrite()`
+it beforehand. Because it is created inside cv::hdf::HDF5::dswrite()
 automatically.
 
 @warning This applies only to datasets that reside inside the root group.
@@ -59,7 +60,7 @@ Results
 ----
 
 Figure 1 shows the result visualized using the tool HDFView for the file
-`root_group_sinle_channel`. The results
+`root_group_single_channel`. The results
 of matrices for datasets that are not the direct children of the root group
 are given in Figure 2 and Figure 3, respectively.
 

modules/hdf/tutorials/read_write_attributes/read_write_attributes.markdown

+Reading and Writing Attributes{#tutorial_hdf_read_write_attributes}
+===============================
+
+Goal
+----
+This tutorial shows you:
+ - How to write attributes?
+ - How to read attributes?
+
+@note Although attributes can be associated with groups and datasets, only attributes
+with the root group are implemented in OpenCV. Supported attribute types are
+`int`, `double`, `cv::String` and `cv::InputArray` (only for continuous arrays).
+
+Source Code
+----
+
+The following code demonstrates reading and writing attributes
+inside the root group with data types `cv::Mat`, `cv::String`, `int`
+and `double`.
+
+You can download the code from [here][1] or find it in the file
+`modules/hdf/samples/read_write_attributes.cpp` of the opencv_contrib source code library.
+
+@snippet samples/read_write_attributes.cpp tutorial
+
+Explanation
+----
+
+The first step is to open the HDF5 file:
+
+@snippet samples/read_write_attributes.cpp tutorial_open_file
+
+Then we use cv::hdf::HDF5::atwrite() to write attributes by specifying its value and name:
+
+@snippet samples/read_write_attributes.cpp tutorial_write_mat
+
+@warning Before writing an attribute, we have to make sure that
+the attribute does not exist using cv::hdf::HDF5::atexists().
+
+To read an attribute, we use cv::hdf::HDF5::atread() by specifying the attribute name
+
+@snippet samples/read_write_attributes.cpp tutorial_read_mat
+
+In the end, we have to close the HDF file
+
+@snippet samples/read_write_attributes.cpp tutorial_close_file
+
+Results
+----
+
+Figure 1 and Figure 2 give the results visualized using the tool HDFView.
+
+![Figure 1: Attributes of the root group](pics/attributes-file.png)
+
+![Figure 2: Detailed attribute information](pics/attributes-details.png)
+
+[1]: https://github.com/opencv/opencv_contrib/tree/master/modules/hdf/samples/read_write_attributes.cpp

modules/hdf/tutorials/table_of_content_hdf.markdown

@@ -2,9 +2,9 @@ The Hierarchical Data Format (hdf) I/O {#tutorial_table_of_content_hdf}
 =====================================
 
 Here you will know how to read and write a HDF5 file using OpenCV.
-Currently, only `cv::Mat` is supported.
+Specifically, it shows you how to read/write groups, datasets and attributes.
 
-Note that the HDF5 library has to be installed in your system
+@note The HDF5 library has to be installed in your system
 to use this module.
 
 -   @subpage tutorial_hdf_create_groups
@@ -22,3 +22,11 @@ to use this module.
     *Author:* Fangjun Kuang
 
     You will learn how to create, read and write datasets.
+
+-   @subpage tutorial_hdf_read_write_attributes
+
+    *Compatibility:* \> OpenCV 3.4
+
+    *Author:* Fangjun Kuang
+
+    You will learn how to read and write attributes.