On Tue, May 17, 2016 at 7:22 PM, Ronald S. Bultje <rsbul...@gmail.com> wrote: > 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).
Maybe the ports to those other languages should devise an interface appropriate for that language, and we should keep a clean C interface? - Hendrik _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
