Remove dependency on OpenCV for capture and encoding

[dzhoshkun]

GIFT-Grab uses OpenCV for:

• Xvid encoding / decoding
• Capturing BGRA frames from Epiphan DVI2PCIe Duo and network streams

However OpenCV depends on FFmpeg, presumably for these tasks. FFmpeg is already used in GIFT-Grab as well. And OpenCV is quite a heavy-weight dependency in this regard. So it might be worth removing it in favour of using FFmpeg for all that OpenCV is needed for in GIFT-Grab.

With this, the OpenCV dependency will become optional, and for the time being confined to #10

[tvercaut]

That makes sense. One thing we discussed a long time ago though was to be able to decorate/adapt a VideoFrame (https://github.com/gift-surg/GIFT-Grab/blob/master/src/api/videoframe.h) to present it as an OpenCV image if OpenCV is specified as a dependency. This would ease integrating GIFT-Grab in processing pipelines.

[dzhoshkun]

• depends on Set up GitLab CI/CD for GitHub #7

[dzhoshkun]

A more thorough search, which:

• should be revisited (for repo items to be updated) before closing this issue,

has revealed that OpenCV is also used for capturing BGRA frames from Epiphan frame grabbers.

• so BGRA captures from Epiphan should be supported by an appropriate alternative.

For instance Epiphan SDK provides the V2U_GRABFRAME_FORMAT_ARGB32 flag (i.e. BGRA in reverse order). I reckon libVLC will provide an equivalent as well. However BGRA does not seem to be available. Maybe:

• check how this is implemented in OpenCV (as OpenCV uses video4linux under the hood, AFAIK).

[dzhoshkun]

@tvercaut I'm going to address OpenCV compatibility in a separate issue #10

[dzhoshkun]

This thread might be useful for supporting BGRA capture with libVLC.

[dzhoshkun]

A video-4-linux example https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/v4l2grab.c.html potentially useful for #9 (comment)

[dzhoshkun]

There's OpenCV color conversion involved in the current implementation, so we could as well use the Epiphan SDK's V2U_GRABFRAME_FORMAT_ARGB32 (#9 (comment)) and do the conversion manually!

[dzhoshkun]

OpenCV seems to use loops for color conversion

[dzhoshkun]

• run CI up to above commit with an Epiphan device!
• more restructuring is needed e.g. in CMake configuration

[dzhoshkun]

FFmpeg seems to nail it:

[0] % ./tests/rgbswap/argb_to_bgra_profiler     
Profiling ARGB => BGRA conversion for 1920 x 1080 image
Loop (success) took: 11333 usec
memcpy took: 3739 usec
Function (success) took: 35876 usec
FFmpeg (success) took: 10 usec

[tvercaut]

As per #10, OpenCV compatibility is an important feature for many (but not all) users. Should we rename this issue as "Make OpenCV dependency optional" or "Remove mandatory core dependency on OpenCV"?

[dzhoshkun]

@tvercaut Agreed, I've updated the title and the description to make this issue more specific.