Make options changeable after codec or (de)muxer has been opened

[Tjoppen]

Overview

Currently, as best I can tell, it is only possible to set options once: when a codec or (de)muxer is opened. There are cases where one might want to change some option afterward. My use case is changing -crf in the libx264 encoder during encoding based on user preference and changing network conditions.

Existing FFmpeg API

The av_opt_set*() in libavutil/opt.h. av_opt_get*() are probably also prudent to implement.

Doxy link: https://ffmpeg.org/doxygen/trunk/group__avoptions.html

Expected PyAV API

The av_opt functions are very generic, so the easiest route would probably be to guarantee that all relevant classes have self.ptr exposed that is guaranteed to be passable to the av_opt functions. This could be hidden from the user so that they only have to pass a CodecContext, OutputContainer or InputContainer.

Example based on aiortc's h264.py:

from av.avutil import av_opt_set_int, av_opt_get_int

class H264Encoder(Encoder):
    # [...]
    def set_crf(self, crf: int)
        av_opt_set_int(self.codec, "crf", crf) # search_flags could be optional, default to 0

    def get_crf(self) -> int:
        return av_opt_get_int(self.codec, "crf")

Errors returned by av_opt_get_int() could be raised as Exceptions, which simplifies the get functions like above.

It would also be useful to expose AV_OPT_SEARCH_* and optionally allow setting them on the set and get calls.

[Tjoppen]

Here's a little proof-of-concept that works well enough for my purposes. In context.pyx:

cdef class CodecContext(object):
    # Default search_flags to AV_OPT_SEARCH_CHILDREN
    def opt_set_int(self, name: str, value: int, search_flags: int = 1):
        ret = lib.av_opt_set_int(self.ptr, name, value, search_flags)
        if ret != 0:
            raise RuntimeError(f'av_opt_set_int() = {ret}')

    def opt_get_int(self, name: str, search_flags: int = 1):
        cdef int64_t val
        ret = lib.av_opt_get_int(self.ptr, name, search_flags, &val)
        if ret != 0:
            raise RuntimeError(f'av_opt_get_int() = {ret}')
        return val

And of course in avutil.pxd right after av_opt_set_int():

    cdef int av_opt_get_int(
        void *obj,
        char *name,
        int search_flags,
        int64_t *out_val
    )

It'd be better if all classes inherited some base class from which void* ptr can be gotten, perhaps via an abstract property or something.

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