[opencv] Add opencv@3

[purcell]

OpenCV was recently bumped to 4.x in #35521, but a lot of non-Homebrew-packaged code does not yet compile against 4.x, which was only released just over a month ago.

Therefore, since there is an opencv@2 formula, it also seems prudent to provide opencv@3. This PR does that, based on the most recent version of opencv.rb for OpenCV 3.x.

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[SMillerDev]

The tests fail on Jenkins, could you check those?

[purcell]

Thanks. I managed to reproduce this locally, and have pushed a fix: 79c7aef

I determined this by noting that the formula installed its Python modules into directories with the following names:

/usr/local/Cellar/opencv@3/3.4.5/lib/python2.7/ (6 files)
/usr/local/Cellar/opencv@3/3.4.5/lib/python3.7/ (6 files)

[fxcoudert]

For what it's worth: the 3 formulas in Homebrew core that depend on OpenCV could be adapted to OpenCV 4 by a simple patch, which was already available upstream in most cases. The breaking changes appear limited.

The criteria for new versions formulas in Homebrew core are here https://docs.brew.sh/Versions. They include:

Upstream should have a release branch for the versioned formulae version and still make security updates for that version

So: does the OpenCV project maintain the 2.x branch, and plans to make future releases?

[fxcoudert]

No bottle block, please.

[purcell]

No bottle block, please.

Removed in e1f9843

[purcell]

So: does the OpenCV project maintain the 3.x branch, and plans to make future releases?

Yes, according to this:

Branch 3.4 will be switched to maintanence mode: only bugfixes and light features will be accepted. BTW, release 3.4.4 is ready too!

[purcell]

For what it's worth: the 3 formulas in Homebrew core that depend on OpenCV could be adapted to OpenCV 4 by a simple patch, which was already available upstream in most cases. The breaking changes appear limited.

Agreed, but people also use Homebrew as a source of native libraries for non-Homebrew work, so bridging changes like this can be helpful. In my case, I'm using language bindings for OpenCV that are not yet compatible with OpenCV 4. With any luck, I'll get them updated, but it's still only a matter of weeks since the OpenCV 4 release. Obviously after a few months it would be best to drop these versioned formulae where possible: there may well be a case for dropping opencv@2 immediately.

[fxcoudert]

@purcell our “problem” is: it's hard to drop a versioned formula once it's introduced, since people rely on it. While it's easier to just tell people affected negatively by the version migration to just pin the version on their local system for now, until their other software becomes compatible.

To give an example, last month opencv had 21,064 installs, and opencv@2 had 1,641. While the later is clearly smaller, it's not exactly negligible.

[purcell]

Yep, understandably difficult to strike the right balance. You folks seem to do a wonderful job, though, so thanks!

[neggert]

FWIW, there's at least one place where the 4.x release does not maintain feature parity with 3.x. Quoting from the changelog:

For now base64 support is not complete (only loading base64-encoded XML and YAML is supported, encoding is not supported at all)

I'm not going to be able to migrate my code until they fix this, so I'd really appreciate having opencv@3 available.

[fxcoudert]

@BrewTestBot test this please

[fxcoudert]

That all sounds reasonable, let's go for it!

[fxcoudert]

Thanks @purcell for your contribution to Homebrew!

[purcell]

Thanks @purcell for your contribution to Homebrew!

Cheers @fxcoudert, your help and work on homebrew is appreciated.