fix#9570: implement mat ptr for generic types

The original template based mat ptr for indexing is not implemented,
add the similar implementation as uchar type, but cast to
user-defined type from the uchar pointer.

modules/core/include/opencv2/core/mat.inl.hpp

@@ -1058,6 +1058,34 @@ const uchar* Mat::ptr(const int* idx) const
     return p;
 }
 
+template<typename _Tp> inline
+_Tp* Mat::ptr(const int* idx)
+{
+    int i, d = dims;
+    uchar* p = data;
+    CV_DbgAssert( d >= 1 && p );
+    for( i = 0; i < d; i++ )
+    {
+        CV_DbgAssert( (unsigned)idx[i] < (unsigned)size.p[i] );
+        p += idx[i] * step.p[i];
+    }
+    return (_Tp*)p;
+}
+
+template<typename _Tp> inline
+const _Tp* Mat::ptr(const int* idx) const
+{
+    int i, d = dims;
+    uchar* p = data;
+    CV_DbgAssert( d >= 1 && p );
+    for( i = 0; i < d; i++ )
+    {
+        CV_DbgAssert( (unsigned)idx[i] < (unsigned)size.p[i] );
+        p += idx[i] * step.p[i];
+    }
+    return (const _Tp*)p;
+}
+
 template<typename _Tp> inline
 _Tp& Mat::at(int i0, int i1)
 {

modules/core/test/test_mat.cpp

@@ -1804,4 +1804,24 @@ TEST(Mat_, from_initializer_list)
     ASSERT_DOUBLE_EQ(norm(A, B, NORM_INF), 0.);
 }
 
+
+TEST(Mat, template_based_ptr)
+{
+    Mat mat = (Mat_<float>(2, 2) << 11.0f, 22.0f, 33.0f, 44.0f);
+    int idx[2] = {1, 0};
+    ASSERT_FLOAT_EQ(33.0f, *(mat.ptr<float>(idx)));
+    idx[0] = 1;
+    idx[1] = 1;
+    ASSERT_FLOAT_EQ(44.0f, *(mat.ptr<float>(idx)));
+}
+
+TEST(Mat_, template_based_ptr)
+{
+    int dim[4] = {2, 2, 1, 2};
+    Mat_<float> mat = (Mat_<float>(4, dim) << 11.0f, 22.0f, 33.0f, 44.0f,
+                                              55.0f, 66.0f, 77.0f, 88.0f);
+    int idx[4] = {1, 0, 0, 1};
+    ASSERT_FLOAT_EQ(66.0f, *(mat.ptr<float>(idx)));
+}
+
 #endif