COMP: Migrate OpenCV Video Bridge from legacy C API to C++ API

[hjmjohnson]

Summary

Migrates Modules/Video/BridgeOpenCV from the OpenCV legacy C API to the
modern C++ API. The C API (IplImage*, CvCapture*, CvVideoWriter*,
cvCaptureFromFile, cvQueryFrame, CV_CAP_PROP_*, etc.) was deprecated in
OpenCV 2.x, moved to optional headers in OpenCV 3.x, and removed entirely in
OpenCV 4.0, causing build failures on any system with OpenCV 4+.

Changes per commit

1. Fix OpenCVBasicTypeBridge template deduction (OpenCV 3/4)

• The cv::Matx<T, VRows, VColumns> specialization used unsigned int template
  parameters but cv::Matx uses int. Cast (unsigned int)VRows in the partial
  specialization to allow the compiler to deduce both sides correctly.

2. Migrate OpenCVVideoIO to C++ API

• Remove four custom deleter structs (IplImageDeleter, CvCaptureDeleter, etc.)
  and all std::unique_ptr wrappers around C API types.
• Replace member types with RAII C++ value types: cv::Mat, cv::VideoCapture,
  cv::VideoWriter — these manage their own lifetime with no custom cleanup needed.
• Rewrite all methods using the C++ API: cv::VideoCapture::isOpened(),
  operator>> for frame reads, cv::VideoWriter::write(), cv::cvtColor(),
  cv::VideoWriter::fourcc().
• Replace memcpy with std::copy_n for type-safe buffer copy.
• Remove conditional OpenCV 2.4/3.x C header includes; use only
  opencv2/videoio.hpp and opencv2/imgproc.hpp.

3. Migrate OpenCVVideoCapture and test files to C++ API

• Replace IplImage* frame handling in retrieve() with cv::Mat non-owning
  constructor wrapping the ITK frame buffer directly (no copy needed).
• Replace CV_FOURCC(...) macro with cv::VideoWriter::fourcc(...) (×2 in
  constructors).
• Replace all CV_CAP_PROP_* macros with cv::CAP_PROP_* scoped enums in
  set()/get() methods.
• In test files: replace CvCapture*/cvCaptureFromCAM/cvReleaseCapture with
  cv::VideoCapture; replace all remaining CV_CAP_PROP_*/CV_FOURCC macro
  usage; remove legacy C headers (videoio_c.h, imgproc_c.h, highgui.h).

Backward Compatibility

All C++ API replacements used have been available since OpenCV 2.0 or earlier.
This PR does not raise the minimum required OpenCV version.

Legacy C API (removed in OpenCV 4)	Modern C++ Replacement	Available Since
IplImage* / cvCreateImageHeader	cv::Mat(rows, cols, type, ptr)	OpenCV 1.x (C++ wrappers)
CvCapture* / cvCaptureFromFile	cv::VideoCapture(filename)	OpenCV 2.0
CvCapture* / cvCaptureFromCAM	cv::VideoCapture(index)	OpenCV 2.0
cvQueryFrame / cvGetNextImage	cv::VideoCapture::operator>>	OpenCV 2.0
cvGetCaptureProperty	cv::VideoCapture::get(propId)	OpenCV 2.0
cvSetCaptureProperty	cv::VideoCapture::set(propId, val)	OpenCV 2.0
CvVideoWriter* / cvCreateVideoWriter	cv::VideoWriter(file, fourcc, fps, size)	OpenCV 2.0
cvWriteFrame	cv::VideoWriter::write(frame)	OpenCV 2.0
cvReleaseCapture / cvReleaseVideoWriter	RAII (destructor / .release())	OpenCV 2.0
cvCvtColor / CV_BGR2RGB	cv::cvtColor / cv::COLOR_BGR2RGB	OpenCV 2.0
CV_FOURCC('M','P','4','2') macro	cv::VideoWriter::fourcc('M','P','4','2')	OpenCV 2.1
CV_CAP_PROP_POS_FRAMES macro	cv::CAP_PROP_POS_FRAMES scoped enum	OpenCV 3.0
CV_CAP_PROP_FPS macro	cv::CAP_PROP_FPS scoped enum	OpenCV 3.0
CV_CAP_PROP_FOURCC macro	cv::CAP_PROP_FOURCC scoped enum	OpenCV 3.0
CV_CAP_PROP_FRAME_WIDTH/HEIGHT macros	cv::CAP_PROP_FRAME_WIDTH/HEIGHT enums	OpenCV 3.0
CV_CAP_PROP_POS_AVI_RATIO macro	cv::CAP_PROP_POS_AVI_RATIO scoped enum	OpenCV 3.0
CV_CAP_PROP_POS_MSEC macro	cv::CAP_PROP_POS_MSEC scoped enum	OpenCV 3.0

Note: The cv::CAP_PROP_* scoped enums were introduced in OpenCV 3.0.
The CV_CAP_PROP_* macro aliases for them still exist in OpenCV 3.x (via
videoio_c.h) but were removed in OpenCV 4.0. Since ITK already requires
OpenCV 3+ for other reasons, using cv::CAP_PROP_* directly is safe.

Test plan

• Verify CI builds pass on Linux, macOS, Windows with OpenCV 4.x
• Verify all OpenCV Video Bridge tests pass (ctest -R OpenCV)

🤖 Generated with Claude Code

[N-Dekker]

Please don't use C-style cast! Is an explicit conversion really necessary? (If so, you may consider unsigned{VRows} and unsigned{VColumns}, which are 100% type safe.)

[hjmjohnson]

Good catch — replaced both C-style casts with static_cast<unsigned int>(VRows) and static_cast<unsigned int>(VColumns). The explicit conversion is necessary here: the template parameter VRows is deduced as int (to match cv::Matx), but itk::Matrix requires unsigned int, so a cast in the specialization pattern is required for deduction to succeed. Squashed into the same commit.

[N-Dekker]

Thanks, but are you sure that the explicit cast is necessary? Do you get a warning, otherwise?

itk::Matrix is declared as follows:

template <typename T, unsigned int VRows = 3, unsigned int VColumns = 3> 
class ITK_TEMPLATE_EXPORT Matrix 

ITK/Modules/Core/Common/include/itkMatrix.h

Lines 51 to 52 in 2b7e6d4

	template <typename T, unsigned int VRows = 3, unsigned int VColumns = 3>
	class ITK_TEMPLATE_EXPORT Matrix

I think it should be perfectly fine to pass a signed integer as argument for VRows or VColumns, for example itk::Matrix<float, 2, 2>. (2 is a signed int.)

[hjmjohnson]

I did get a failure that this fixed. It is hard to test this code in all scenarios, so I figured that being explicit is a "won't hurt" condition.

[N-Dekker]

Well, there is a guideline: Avoid casts

So I would only add static_cast when it is really necessary 🤷 That's why I suggested to just remove the cast. Or otherwise use modern T{ x } conversion.

[hjmjohnson]

@dzenanz This looks like perhaps it is something that supports Kitware projects. Do you know a good candidate to review this PR?

[greptile-apps]

Greptile Summary

This PR migrates Modules/Video/BridgeOpenCV from the OpenCV C API (removed in OpenCV 4.0) to the modern C++ API, fixing build failures on any system with OpenCV 4+. All seven changed files are updated consistently: raw pointer members (IplImage*, CvCapture*, CvVideoWriter*) are replaced with RAII value types (cv::Mat, cv::VideoCapture, cv::VideoWriter), all deprecated macros (CV_FOURCC, CV_CAP_PROP_*) are replaced with their scoped-enum equivalents, and version-guarded legacy header blocks are collapsed to clean modern includes. The template deduction fix in itkOpenCVBasicTypeBridge.h is a correct solution to the unsigned int vs int mismatch between itk::Matrix and cv::Matx. The migration is thorough and the RAII ownership model is applied correctly throughout.

Key findings:

• OpenWriter() in itkOpenCVVideoIO.cxx sets m_WriterOpen = true unconditionally without checking m_Writer.isOpened() — if the writer fails to open silently, all subsequent Write() calls will no-op, discarding frames with no diagnostic output.
• The new isContinuous() guard added to the cross-validation memcmp in itkOpenCVVideoIOTest.cxx silently bypasses the pixel comparison instead of failing the test when the guard is false.
• ReadImageInformation() correctly delegates file-existence/openability checking to CanReadFile() before re-opening, so no missing isOpened() guard is needed on the file path.

Confidence Score: 5/5

• Safe to merge — both remaining findings are P2 quality/robustness suggestions with no impact on the primary read path.
• All findings are P2: the OpenWriter check is a pre-existing gap that the new API makes easy to close, and the isContinuous() guard affects only test coverage quality. Neither blocks correct behavior on the read path or represents a regression introduced by this PR. The migration itself is correct and complete.
• itkOpenCVVideoIO.cxx (OpenWriter check) and itkOpenCVVideoIOTest.cxx (isContinuous guard)

Important Files Changed

Filename	Overview
Modules/Video/BridgeOpenCV/include/itkOpenCVBasicTypeBridge.h	Template parameters changed from unsigned int to int (matching cv::Matx) with static_cast<unsigned int> for the itk::Matrix partial specialization; correctly resolves deduction mismatch between OpenCV and ITK template conventions.
Modules/Video/BridgeOpenCV/include/itkOpenCVVideoCapture.hxx	Removes all legacy IplImage/C-header includes; replaces CV_FOURCC/CV_CAP_PROP_* macros with C++ equivalents; retrieve() now wraps the ITK buffer in a non-owning cv::Mat (correct rows/cols ordering) and uses cv::cvtColor directly — clean migration with correct dimension mapping.
Modules/Video/BridgeOpenCV/include/itkOpenCVVideoIO.h	Cleans up version-guarded legacy includes; replaces IplImage*/CvCapture*/CvVideoWriter* raw pointer members with RAII cv::Mat/cv::VideoCapture/cv::VideoWriter value members; also removes the duplicate private: label.
Modules/Video/BridgeOpenCV/src/itkOpenCVVideoIO.cxx	Full C→C++ API migration: removes manual cvRelease* calls, adds explicit RGBA read path, replaces memcpy with std::copy_n; OpenWriter() sets m_WriterOpen = true unconditionally without checking m_Writer.isOpened() — silent data loss if writer fails to open.
Modules/Video/BridgeOpenCV/test/itkOpenCVVideoIOTest.cxx	Migrates cross-validation capture from CvCapture*/cvQueryFrame to cv::VideoCapture/operator>>; adds isContinuous() guard around memcmp that silently skips comparison when false rather than failing the test.
Modules/Video/BridgeOpenCV/test/itkOpenCVVideoCaptureTest.cxx	Drops legacy videoio_c.h/imgproc_c.h conditional includes; replaces all CV_CAP_PROP_*/CV_FOURCC macros with cv::CAP_PROP_*/cv::VideoWriter::fourcc — straightforward, correct migration.
Modules/Video/BridgeOpenCV/test/itkOpenCVVideoIOFactoryTest.cxx	Replaces CvCapture*/cvCaptureFromCAM/cvReleaseCapture with cv::VideoCapture/isOpened()/release() — clean, correct migration.

Sequence Diagram

sequenceDiagram
    participant Caller
    participant OpenCVVideoIO
    participant cv_VideoCapture as cv::VideoCapture
    participant cv_VideoWriter as cv::VideoWriter

    Note over OpenCVVideoIO: Read path (C++ API)
    Caller->>OpenCVVideoIO: ReadImageInformation()
    OpenCVVideoIO->>cv_VideoCapture: open(filename) / open(cameraIndex)
    cv_VideoCapture-->>OpenCVVideoIO: isOpened()
    OpenCVVideoIO->>cv_VideoCapture: operator>> (tempFrame)
    OpenCVVideoIO->>cv_VideoCapture: get(CAP_PROP_FRAME_COUNT/FPS/WIDTH/HEIGHT)
    cv_VideoCapture-->>OpenCVVideoIO: metadata

    Caller->>OpenCVVideoIO: Read(buffer)
    OpenCVVideoIO->>cv_VideoCapture: operator>> (tempIm)
    OpenCVVideoIO->>OpenCVVideoIO: cvtColor BGR→RGB (or BGRA→RGBA)
    OpenCVVideoIO->>OpenCVVideoIO: std::copy_n(m_CVImage.data → buffer)

    Note over OpenCVVideoIO: Write path (C++ API)
    Caller->>OpenCVVideoIO: SetWriterParameters(fps, dim, fourCC, nChannels)
    OpenCVVideoIO->>OpenCVVideoIO: VideoWriter::fourcc(...)
    Caller->>OpenCVVideoIO: Write(buffer)
    OpenCVVideoIO->>cv_VideoWriter: open(filename, fourcc, fps, size)
    OpenCVVideoIO->>OpenCVVideoIO: cvtColor (GRAY/RGB → BGR)
    OpenCVVideoIO->>cv_VideoWriter: write(m_CVImage)

    Note over OpenCVVideoIO: Cleanup (RAII)
    Caller->>OpenCVVideoIO: FinishReadingOrWriting()
    OpenCVVideoIO->>cv_VideoCapture: release()
    OpenCVVideoIO->>cv_VideoWriter: release()

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_{Reviews (2): Last reviewed commit: "COMP: Replace legacy OpenCV C API in Vid..." | Re-trigger Greptile}

[dzenanz]

Looks good. I have some program which uses Video\BridgeOpenCV\include\itkOpenCVBasicTypeBridge.h, and it keeps working after these changes.