From ba5b40b1a21ba083fae8270ef6b58eaf4821a7b1 Mon Sep 17 00:00:00 2001
From: Arnesh Banerjee <linkrinku13@gmail.com>
Date: Thu, 30 Jul 2026 03:09:43 +0530
Subject: [PATCH] core: restore in-place support for extractChannel (#29568)

extractChannel() stopped supporting in-place operation (dst aliasing
src) after commit 416bf3253 (PR #23473), which replaced

    Mat src = _src.getMat();
    _dst.create(src.dims, &src.size[0], depth);

with a single up-front

    _dst.createSameSize(_src, depth);
    ...
    Mat src = _src.getMat();

When _src and _dst reference the same array, the up-front reallocation
reshapes the shared buffer to a single channel before the multi-channel
source header is fetched. mixChannels() then sees a 1-channel source and
throws for any coi >= 1.

Move createSameSize() back to after the source Mat/UMat is obtained, so
the source header keeps the original multi-channel data alive across the
destination reallocation. This preserves the 0D/1D handling introduced
by createSameSize while restoring the pre-existing in-place contract.

Adds regression test Core_Mat.extractChannel_inplace_29568.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 modules/core/src/channels.cpp  |  6 +++++-
 modules/core/test/test_mat.cpp | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/modules/core/src/channels.cpp b/modules/core/src/channels.cpp
index bc27918a0f..53f673f7f7 100644
--- a/modules/core/src/channels.cpp
+++ b/modules/core/src/channels.cpp
@@ -428,11 +428,14 @@ void cv::extractChannel(InputArray _src, OutputArray _dst, int coi)
     CV_Assert( 0 <= coi && coi < cn );
     int ch[] = { coi, 0 };
 
-    _dst.createSameSize(_src, depth);
 #ifdef HAVE_OPENCL
     if (ocl::isOpenCLActivated() && _src.dims() <= 2 && _dst.isUMat())
     {
         UMat src = _src.getUMat();
+        // NB: fetch the source before (re)allocating the destination so that
+        // in-place operation (_src and _dst referencing the same array) keeps
+        // the multi-channel source alive while dst is reshaped to 1 channel.
+        _dst.createSameSize(_src, depth);
         UMat dst = _dst.getUMat();
         mixChannels(std::vector<UMat>(1, src), std::vector<UMat>(1, dst), ch, 1);
         return;
@@ -440,6 +443,7 @@ void cv::extractChannel(InputArray _src, OutputArray _dst, int coi)
 #endif
 
     Mat src = _src.getMat();
+    _dst.createSameSize(_src, depth);
     Mat dst = _dst.getMat();
 
     CV_IPP_RUN_FAST(ipp_extractChannel(src, dst, coi))
diff --git a/modules/core/test/test_mat.cpp b/modules/core/test/test_mat.cpp
index 85aba95097..5400111957 100644
--- a/modules/core/test/test_mat.cpp
+++ b/modules/core/test/test_mat.cpp
@@ -1381,6 +1381,17 @@ TEST(Core_Mat, regression_9507)
     EXPECT_EQ(25u, m2.total());
 }
 
+TEST(Core_Mat, extractChannel_inplace_29568)
+{
+    // https://github.com/opencv/opencv/issues/29568
+    // extractChannel() must support in-place operation (dst aliasing src).
+    Mat mat(3, 3, CV_8UC3, Scalar(1, 2, 3));
+    ASSERT_NO_THROW(cv::extractChannel(mat, mat, 1));
+    EXPECT_EQ(1, mat.channels());
+    EXPECT_EQ(Size(3, 3), mat.size());
+    EXPECT_EQ(0, cv::norm(mat, Mat(3, 3, CV_8UC1, Scalar(2)), NORM_INF));
+}
+
 TEST(Core_Mat, empty)
 {
     // Should not crash.