compatibility with opencv4

[wkcn]

Description

To compatible with multiple versions of OpenCV, define the old OpenCV macros to support OpenCV4.

In OpenCV4, opencv2/imgproc/imgproc_c.h and opencv2/videoio/videoio_c.h define these macros, however I couldn't include them since it would bring redefinition problems.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Add a new file src/io/opencv_compatibility.h to define the OpenCV macros.
• Modify Makefile to find the path of opencv4

Comments

There is a similar PR #13559, Thank @daleydeng

[anirudhacharya]

@mxnet-label-bot add [pr-awaiting-review]

[szha]

Seems like this time the CI is reporting an actual problem. @wkcn could you take a look?

[wkcn]

@szha Yes. OpenCV2 does not have libopencv_imgcodecs.so, but OpenCV3/4 need it.

I would like to add the variable OPENCV_DEPS_PATH, then check whether $(OPENCV_DEPS_PATH)/lib/libopencv_imgcodecs.(a|so) exists.

[wkcn]

I do not know what OpenCV shared library we did not link. It raises undefined reference in unit-cpu CI.

[szha]

@wkcn it might be that the system has an opencv2 installation while the build is statically linking opencv3.

[wkcn]

@szha I see. I used wrong linking order. I will fix it. Thank you!

[szha]

Thanks for the compatibility fix and the improvement 💯

[dbsxdbsx]

As far as I know, when I tried to build mxnet on win10 with cuda 10.1. It failed with compling error.

incomplete type is not allowed	mxnet	c:\opencv\build\include\opencv2\core\cvstd_wrapper.hpp	45	

[wkcn]

@dbsxdbsx Could you please open an issue and provide more error message?
I only fix the compatibility on Linux in this PR.

[dbsxdbsx]

@wkcn, thanks. I posted it #14385.

[reminisce]

This can be problematic when USE_OPENCV_INC_PATH is not defined in config.mk, because it's an empty string at the time. In that situation, no correct paths to headers and libs of opencv are actually added to CFLAGS and LDFLAGS.

[szha]

@reminisce good catch. @wkcn how about we add a default value for empty value here to help with cases where people already have old config.mk without the new variables?

[wkcn]

@reminisce @szha Hi! I have fixed it in the PR #14424