Support custom options in AVCodecContext (av_dict_set) in FFMPEG backend

[peters]

I'm looking for guidance in regards to submitting a PR that will allow you to specify raw options (i.e preset) using av_dict_set. This applies to VideoCapture and VideoWriter The use case here being that you could support encoding videos targeting a specific device (i.e Apple iPad). Or tune encoding performance for a given codec i.e. VP9

Specify backend options by instantiating a new VideoWriter (!?)

VideoWriter::VideoWriter(const String& filename, int fourcc, double fps, Size frameSize, bool isColor=true, std::map<std::string, std::string> backendOptions)

[alalek]

We prefer do not use STL classes in exported functions/methods (ABI is different in many cases, for example Debug/Release MSVS builds, moreover FFMPEG wrapper is build via MINGW tools, so it will not work). (upd 2019/12/06: not actual anymore, but bindings and wrapper API should be adopted)

What is about of using environment variable and parse passed value via av_dict_parse_string call in cap_ffmpeg_impl.hpp?

Suggested names:

• OPENCV_VIDEOWRITER_FFMPEG_OPTIONS OPENCV_FFMPEG_WRITER_OPTIONS
• OPENCV_VIDEOCAPTURE_FFMPEG_OPTIONS OPENCV_FFMPEG_CAPTURE_OPTIONS (currently we have forced hack with "rtsp_transport=tcp")

Suggested format:

param1=value1,param2=value2

BTW, Could you add link or example with useful parameters and possible values (to select appropriate keys/values separators)?

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Related patch for VideoCapture + OPENCV_FFMPEG_CAPTURE_OPTIONS: #9292

[grzegorzk]

Hi @alalek , I browse the repository and can see OPENCV_FFMPEG_CAPTURE_OPTIONS env in there, but not OPENCV_FFMPEG_WRITER_OPTIONS - is the former applied to both VideoReader and VideoWriter?