[FFmpeg-devel] [PATCH 1/4] vp8: Add hwaccel hooks

[wm4]

On Mon, 20 Feb 2017 11:20:48 +0000
Mark Thompson <sw at jkqxz.net> wrote:

> 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 <sw at 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.
> 

Would it be possible to add some field to the private context which
controls this instead?