Support OpenCV 5

[csparker247]

Homebrew's unversioned opencv formula is now 5.0.0, and other distros will follow. RT currently requires major version 4 and cannot configure against 5.

It is not only the version pin

The obvious change is the two places that pin the major version:

File	Line
cmake/FindDependencies.cmake:6	find_package(OpenCV 4 QUIET REQUIRED)
cmake/Config.cmake.in:17	find_dependency(OpenCV 4)

The second one matters as much as the first — it is the installed package config, so it constrains downstream consumers of rt::core, not just our own build.

But relaxing the version alone will not link. OpenCV 5 reorganised the modules RT uses, comparing modules/ on the upstream 4.x and 5.x branches:

4.x:  calib3d   core dnn features2d flann gapi         highgui imgcodecs imgproc ... stitching
5.x:  calib     core dnn features   flann geometry ptcloud stereo highgui imgcodecs imgproc ... stitching

calib3d is split across calib / geometry / stereo, and features2d is renamed features. So these link targets do not exist in OpenCV 5:

• core/CMakeLists.txt:66 — opencv_calib3d
• core/CMakeLists.txt:67 — opencv_features2d

modules/calib/CMakeLists.txt on 5.x declares ocv_define_module(calib ...), i.e. the target is opencv_calib, and a code search for opencv_calib3d across 5.x *.cmake returns nothing — there are no legacy target aliases.

The headers are fine

opencv2/calib3d.hpp and opencv2/features2d.hpp still ship on 5.x as compatibility shims. The whole of modules/calib/include/opencv2/calib3d.hpp is:

#ifndef OPENCV_CALIB3D_HPP
#define OPENCV_CALIB3D_HPP
#include "opencv2/geometry.hpp"
#include "opencv2/stereo.hpp"
#include "opencv2/calib.hpp"
#endif

So core/src/LandmarkDetector.cpp:7-8 can stay as-is. Only the CMake target names need to change.

Surface area is small

RT touches very little of the affected modules — one file:

• core/src/LandmarkDetector.cpp:57 — cv::SIFT::create(), cv::DescriptorMatcher::create(FLANNBASED), cv::KeyPoint (features2d → features)
• core/src/LandmarkDetector.cpp:185 — cv::estimateAffinePartial2D (on 5.x this is declared in modules/geometry/include/opencv2/geometry/3d.hpp, so the target is likely opencv_geometry rather than opencv_calib — worth confirming)

Everything else is core, imgproc, imgcodecs, which are unchanged.

Also worth cleaning up while here

core/CMakeLists.txt:70 links opencv_stitching, but nothing in the tree includes opencv2/stitching* or references cv::Stitcher / cv::detail:: stitching symbols. It appears to be a stale dependency and can probably be dropped. (The module does still exist on 5.x, so it is not a blocker either way.)

Suggested shape

Supporting 4 and 5 concurrently is probably worth it rather than a hard cutover, since distros will straddle the versions for a while:

find_package(OpenCV 4 QUIET)
if(NOT OpenCV_FOUND)
  find_package(OpenCV 5 QUIET REQUIRED)
endif()

if(OpenCV_VERSION_MAJOR GREATER_EQUAL 5)
  set(RT_OPENCV_GEOM opencv_calib opencv_geometry opencv_features)
else()
  set(RT_OPENCV_GEOM opencv_calib3d opencv_features2d)
endif()

Related

Once this lands, the macOS CI workaround added in #23 can be reverted: the Brewfile can go back to the unversioned opencv, and the -DOpenCV_DIR=$(brew --prefix opencv@4)/lib/cmake/opencv4 line in .github/workflows/ci.yml (needed because versioned Homebrew formulae are keg-only) can be dropped.