From 64bbaa9f41be3e96a883cdbc11635eb84f18140c Mon Sep 17 00:00:00 2001
From: Alexander Smorkalov <2536374+asmorkalov@users.noreply.github.com>
Date: Mon, 10 Nov 2025 08:00:17 +0300
Subject: [PATCH] Merge pull request #27972 from asmorkalov:as/dst_mat_type

Force empty output type where it's defined in API #27972

The PR replaces:
- https://github.com/opencv/opencv/pull/27936
- https://github.com/opencv/opencv/pull/21059

Empty matrix has undefined type, so user code should not relay on the output type, if it's empty. The PR introduces some exceptions:
- copyTo documentation defines, that the method re-create output buffer and set it's type.
- convertTo has output type as parameter and output type is defined and expected.
---
 modules/core/src/arithm.cpp           | 12 ++++++++++++
 modules/core/src/convert.dispatch.cpp |  2 ++
 modules/core/src/copy.cpp             |  1 +
 modules/core/src/umatrix.cpp          |  1 +
 modules/core/test/test_mat.cpp        |  9 +++++++++
 modules/core/test/test_umat.cpp       |  8 ++++++++
 6 files changed, 33 insertions(+)

diff --git a/modules/core/src/arithm.cpp b/modules/core/src/arithm.cpp
index 5c1033be81..ccb104cebe 100644
--- a/modules/core/src/arithm.cpp
+++ b/modules/core/src/arithm.cpp
@@ -1113,6 +1113,10 @@ void cv::add( InputArray src1, InputArray src2, OutputArray dst,
     if (src1.empty() && src2.empty())
     {
         dst.release();
+        if (dtype >= 0)
+        {
+            dst.create(0, 0, dtype);
+        }
         return;
     }
 
@@ -1140,6 +1144,10 @@ void cv::subtract( InputArray _src1, InputArray _src2, OutputArray _dst,
     if (_src1.empty() && _src2.empty())
     {
         _dst.release();
+        if (dtype >= 0)
+        {
+            _dst.create(0, 0, dtype);
+        }
         return;
     }
 
@@ -1345,6 +1353,10 @@ void cv::addWeighted( InputArray src1, double alpha, InputArray src2,
     if (src1.empty() && src2.empty())
     {
         dst.release();
+        if (dtype >= 0)
+        {
+            dst.create(0, 0, dtype);
+        }
         return;
     }
 
diff --git a/modules/core/src/convert.dispatch.cpp b/modules/core/src/convert.dispatch.cpp
index a33708a557..0df033af6a 100644
--- a/modules/core/src/convert.dispatch.cpp
+++ b/modules/core/src/convert.dispatch.cpp
@@ -252,6 +252,7 @@ void Mat::convertTo(OutputArray dst, int type_, double alpha, double beta) const
     if (empty())
     {
         dst.release();
+        dst.create(size(), type_ >= 0 ? type_ : type());
         return;
     }
 
@@ -318,6 +319,7 @@ void UMat::convertTo(OutputArray dst, int type_, double alpha, double beta) cons
     if (empty())
     {
         dst.release();
+        dst.create(size(), type_ >= 0 ? type_ : type());
         return;
     }
 
diff --git a/modules/core/src/copy.cpp b/modules/core/src/copy.cpp
index 68d6b938e7..43f477aeae 100644
--- a/modules/core/src/copy.cpp
+++ b/modules/core/src/copy.cpp
@@ -447,6 +447,7 @@ void Mat::copyTo( OutputArray _dst ) const
     if( empty() )
     {
         _dst.release();
+        _dst.create(size(), type());
         return;
     }
 
diff --git a/modules/core/src/umatrix.cpp b/modules/core/src/umatrix.cpp
index 65d50458dc..ceb4c064ec 100644
--- a/modules/core/src/umatrix.cpp
+++ b/modules/core/src/umatrix.cpp
@@ -1153,6 +1153,7 @@ void UMat::copyTo(OutputArray _dst) const
     if( empty() )
     {
         _dst.release();
+        _dst.create(size(), type());
         return;
     }
 
diff --git a/modules/core/test/test_mat.cpp b/modules/core/test/test_mat.cpp
index ae8d5ddfda..9ba47755b4 100644
--- a/modules/core/test/test_mat.cpp
+++ b/modules/core/test/test_mat.cpp
@@ -1385,6 +1385,15 @@ TEST(Core_Mat, push_back)
     }
 }
 
+TEST(Core_Mat, copyToConvertTo_Empty)
+{
+    cv::Mat A(0, 0, CV_16SC2), B, C;
+    A.copyTo(B);
+    ASSERT_EQ(A.type(), B.type());
+    A.convertTo(C, CV_32SC2);
+    ASSERT_EQ(C.type(), CV_32SC2);
+}
+
 TEST(Core_Mat, copyNx1ToVector)
 {
     cv::Mat_<uchar> src(5, 1);
diff --git a/modules/core/test/test_umat.cpp b/modules/core/test/test_umat.cpp
index 34e6d2be75..a1fc434dc6 100644
--- a/modules/core/test/test_umat.cpp
+++ b/modules/core/test/test_umat.cpp
@@ -1199,6 +1199,14 @@ TEST(UMat, async_cleanup_without_call_chain_warning)
     }
 }
 
+TEST(UMat, copyToConvertTo_Empty)
+{
+    cv::UMat A(0, 0, CV_16SC2), B, C;
+    A.copyTo(B);
+    ASSERT_EQ(A.type(), B.type());
+    A.convertTo(C, CV_32SC2);
+    ASSERT_EQ(C.type(), CV_32SC2);
+}
 
 ///////////// oclCleanupCallback threadsafe check (#5062) /////////////////////