Until some recent changes were made, OpenCV selected the default DNN backend either as DNN_BACKEND_INFERENCE_ENGINE, if available, or DNN_BACKEND_OPENCV, otherwise. Recently, this was removed because OpenCV supports multiple plugins/backends, as discussed in #24085.

This patch has two aims: First, fix the documentation since there is no extended default backend logic anymore. Next, it introduces a CMake variable OPENCV_DNN_BACKEND_DEFAULT that can be used to define a user-specific default backend. If this is not set, it defaults to DNN_BACKEND_OPENCV. Still, compiling with OpenVINO/Inference Engine support allows to default to it by adding the CMake argument -D "OPENCV_DNN_BACKEND_DEFAULT=DNN_BACKEND_INFERENCE_ENGINE". Selecting a different default backend is similarly possible, only define OPENCV_DNN_BACKEND_DEFAULT accordingly (e.g. for CUDA). In any case, defining a configuration parameter (e.g. as an environment variable) overrules the compiled-in default such that no existing logic is changed.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• 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
• The PR is proposed to the proper branch
• 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.
• The feature is well documented and sample code can be built with the project CMake