Merge pull request #29632 from ArneshBanerjee/fix-29568-extractchannel-inplace

core: restore in-place support for extractChannel (#29568)

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))

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.