OpenCV C++ core and Python cv2 binding: Mat ownership and deallocation across the language boundary

The integration goal is to define clear ownership and release timing for cv::Mat objects that cross from OpenCV C++ core into the Python cv2 binding.

The C++ core uses reference-counted cv::Mat with shallow copy semantics. The Python binding wraps a Mat in a capsule with a custom deallocator that decrements the C++ refcount when the Python object is finalized. Ownership is transferred on the C++ to Python boundary for functions returning Mat, and the Python wrapper holds the sole reference unless additional references are kept. UMat and GPU-backed Mats add a device memory layer whose lifetime is tied to both the C++ refcount and the Python wrapper lifetime. Behavior is version-sensitive between OpenCV 3.x and 4.x binding generation and varies with build options for OpenCL and CUDA.

OpenCV does not define a deterministic release contract for Mats created in C++ extensions and used from Python; explicit release is documented as available but not required. This leaves expectations about immediate native free versus GC-deferred free ambiguous across bindings.

What is the documented ownership transfer rule for Mats returned from C++ to Python and when is native memory guaranteed to be freed? Is explicit release required for deterministic deallocation of Mats originating in C++ extensions? How does UMat device buffer lifetime relate to Python wrapper finalization and C++ refcount?