On 20/02/17 10:44, Michael Niedermayer wrote: > On Mon, Feb 20, 2017 at 10:23:27AM +0000, Mark Thompson wrote: >> On 20/02/17 02:35, Michael Niedermayer wrote: >>> On Sun, Feb 19, 2017 at 09:29:33PM +0000, Mark Thompson wrote: >>>> On 19/02/17 21:04, Ronald S. Bultje wrote: >>>>> Hi, >>>>> >>>>> On Sun, Feb 19, 2017 at 12:23 PM, Mark Thompson <s...@jkqxz.net> wrote: >>>>> >>>>>> diff --git a/libavcodec/webp.c b/libavcodec/webp.c >>>>>> >>>>> [..] >>>>> >>>>>> + avctx->get_format = webp_get_format; >>>>> >>>>> >>>>> Docs say: >>>>> "decoding: Set by user, if not set the native format will be chosen." >>>>> So I don't think decoders are supposed to set this. >>>> >>>> The webp decoder does not support get_format. I suppose the user could >>>> technically store something there and then read it back, so save and >>>> restore the value across the relevant calls? >>> >>> This is quite ugly, why do you want to do that ? >>> >>> get_format is set by the user >>> the get_format API requires the function to choose one of the caller >>> provided formats and it can choose any. >>> I dont know what your function does but its different from the API. >>> It smells very much like a hack ... >>> >>> The one situation in which you can set get_format from libavcodec is >>> when there is a seperate codec context you created, that is that you >>> are its user. >>> >>> can you explain why this code messes with avctx->get_format ? >>> and for example doesnt change the code that calls get_format so that >>> it passes a correct pixel format list which then by any get_format() >>> results in a correct format ? >>> or am i missing something ? >> > >> The current WebP decoder calls the VP8 decoder /using the same >> AVCodecContext/. > > so they are kind of the same decoder > > >> Previously the format was set directly on the VP8Context, but now that the >> VP8 decoder uses ff_get_format() that initial value is not used. This >> change adds a get_format() callback on the AVCodecContext used by the VP8 >> decoder to set the format appropriately. > > But this is semantically wrong, the formats supported by the decoder > implementation are choosen by the code calling get_format(). > the get_format callback chooses amongth these formats depending on the > users preferrance/usefullness.
Yes. It's a hack to make the VP8 decoder allocate a non-native frame format (YUVA420P) which is compatible with its native format (YUV420P) for any use it makes of it. This is a hack which already exists, we are just moving it to a different place now that the VP8 decoder uses ff_get_format(). >> Now, because that is the same AVCodecContext as the one supplied by the >> user, the user could themselves have stored something into the get_format >> field (even though it isn't used by the WebP decoder) and expect to be able >> to read it back correctly later. I think this would be madness, but is I >> suppose technically allowed; saving and restoring the value across the VP8 >> calls would fix that case. > > Why do you try to misuse the API ? To make the minimal modifications to existing hacks for fate to pass for WebP. I do not care at all about WebP, but the existing hacks abusing the VP8 decoder block the other changes. > i mean i think we agree that this violates the API. Making sure it > works doesnt solve that it violates the API. And anyone working on > the API would likely assume that code conforms to the API documentation. > -> the developer would think one thing and the code would do another > thats a recipe for creating bugs. > > I think the code calling *get_format() should pass the correct list > of formats to the callback and not some other part override the users > get_format calback to work around that the list passed is wrong. The user's get_format callback is never used. (There is only one output format possibility for each input, so it wouldn't do anything anyway.) > am i missing something ? > is what i suggested difficult to do or do you see a issue with that > solution ? The list passed by the VP8 decoder to ff_get_format() is not wrong - the VP8 decoder does not support alpha so including YUVA420P would be incorrect. - Mark _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
