Merge pull request #29878 from sridhar-git05:fix-broadcast-zero-dimension

core: handle zero-sized broadcast dimensions #29878

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- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
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- [x] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
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### Changes

- Added test coverage for the zero-sized dimension case in `cv::broadcast()`.
- The test exercises the `false` branch of `_flatten_for_broadcast()`.
- Fixed division by zero when the broadcast destination has zero elements.

### Test

- `opencv_test_core.exe --gtest_filter=BroadcastTo.*`

All `BroadcastTo` tests pass.

Fixes #28910

modules/core/src/matrix_transform.cpp

@@ -1335,6 +1335,8 @@ void broadcast(InputArray _src, InputArray _shape, OutputArray _dst) {
             #undef OPENCV_CORE_BROADCAST_LOOP
         }
     } else {
+        if (dst.total() == 0)
+            return;
         // initial copy (src to dst)
         std::vector<size_t> step_src{src.step.p, src.step.p + dims_src};
         if (step_src.size() < static_cast<size_t>(dims_shape)) {

modules/core/test/test_arithm.cpp

@@ -2628,6 +2628,23 @@ TEST(BroadcastTo, basic) {
         broadcast(_src, shape, dst);
         fn_verify(ref, dst);
     }
+
+    {
+        std::vector<int> shape{1, 0};
+        std::vector<int> data;
+        Mat zero_src(static_cast<int>(shape.size()), shape.data(), CV_32FC1, data.data());
+
+        std::vector<int> target_shape{3, 0};
+        Mat dst;
+
+        broadcast(zero_src, target_shape, dst);
+
+        EXPECT_EQ(dst.dims, 2);
+        EXPECT_EQ(dst.size[0], 3);
+        EXPECT_EQ(dst.size[1], 0);
+        EXPECT_EQ(dst.total(), 0u);
+    }
+
 }
 
 TEST(Core_minMaxIdx, regression_9207_2)