Hi, On Tue, May 17, 2016 at 12:34 PM, Hendrik Leppkes <h.lepp...@gmail.com> wrote:
> On Tue, May 17, 2016 at 6:26 PM, Ronald S. Bultje <rsbul...@gmail.com> > wrote: > > Hi, > > > > On Tue, May 17, 2016 at 11:18 AM, Michael Niedermayer < > > mich...@niedermayer.cc> wrote: > > > >> AVCodecContext contains all the fields that AVCodecParameters has. > >> Examples are width, height, bitrate, profile, sample_rate, channels > >> > >> In AVCodecContext these fields are accessible through AVOptions > >> in AVCodecParameters these fields are not accessible through AVOptions > >> and your code will crash mysteriously if you try to access them that > >> way > >> > >> This is a inconsistency that was not there before > > > > > > So, at the risk of going semi-offtopic, let me take a different angle at > > this problem. > > > > AVCodecParameter is supposed to be an intermediate layer that removes the > > need for lavf to access lavc structs like AVCodecContext, right? > > > > So, I worked (a long long time ago) on this software called GStreamer, > and > > specifically, one of the plugins I worked on early on was gst-ffmpeg. I > > loved working on it because it practically added so many codecs to the > > player which were previously unsupported. Yay! Now, the API back then was > > horrible. You had an AVCodecContext (no AVOptions) and there were > > approximately 100 million fields that could be set. Most were > undocumented, > > and it was totally unclear which needed to be set. For example, to play a > > WMA file, you might have to set block_align, but for other audio codecs, > > you do not [1]. > > > > For video, sometimes you would have to set extradata (e.g. for SVQ3), but > > other times, you would not. Huffyuv required bits_per_sample, but h264 > did > > not. > > > > So how do you write generic user-facing code where the demuxer and > decoder > > actually are separate entities and merely interact over a serialized > > communication channel? You can't serialize AVCodecContext. I mean, you > > could, but that's a terrible idea. And nothing documented which fields > were > > required to be set and which weren't. > > > > I was hoping that AVCodecParameters would solve this. Ideally, it would > be > > a dictionary of fields that are to be shared between demuxer and decoder > > (or codec and muxer). But I think it has sort of strayed off of that > intent > > and is just a flat minimized AVCodecContext as it existed in 2003. > > > > Can we fix this? I think AVOptions would be ideally suited to fix this. > > AVClass maybe helps also. But if introspection suggests I can set > > block_align on the h264 decoder, or width on an audio decoder, or who > knows > > what. Then I don't think AVOptions or AVClass in AVCodecParameters helps > at > > all. It just creates a minimized AVCodecContext from 2003 that will > slowly > > become the current AVCodecContext, which is not all that useful for > people > > that don't use ffmpeg.exe... > > > > > > This is C, I don't want to have to run introspection on an object to > find out what I can set. > A clean and doumented struct is a far more effective way to get this > information across, which is also why I don't want AVOptions for such > a struct, if its not needed to set private options, it would just > double the need for docs (both in doxy and in the AVOption > definition), and to figure out which fields actually exist, you still > have to read the struct or a comment in the header file - so why not > use the struct in the first place? > > Why use something "generic" with a dict or some weirdness, which loses > the strong typing and self-documentation of such a struct? Sounds like > you want to be working in another language, honestly. :) > AVOption will never "know" which fields a particular codec will need, > so that point is not going to get solved either way. It's not for us (ffmpeg.c developers), it's for people using the ffmpeg libraries on other language platforms (e.g. python). Ronald _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
