[Libav-user] list all available formats of webcam

[Philippe Gorley]

On 2019-04-17 9:03 a.m., Hristo Ivanov wrote:
> Hi.
> 
> The implementation of 
> 'avdevice_capabilities_create'(https://ffmpeg.org/doxygen/trunk/avdevice_8c_source.html#l00143) 
> starts with the following lines:
> 
> 1 int ret; 2 av_assert0(s && caps); 3 av_assert0(s->iformat || 
> s->oformat); 4 if ((s->oformat && 
> !s->oformat->create_device_capabilities) || 5 (s->iformat && 
> !s->iformat->create_device_capabilities)) 6 return AVERROR(ENOSYS);
> 
> My guess is the -40(ENOSYS) error is returned in line 6. Can you check 
> in your code if 'm_pFormatCtxInCam->iformat->create_device_capabilities' 
> is different from null.
> The next step would be to understand why is 
> 'm_pFormatCtxInCam->iformat->create_device_capabilities' null. With 
> ffmpeg you just have to dive in the source code and try to understand it.
> Compiling a debug version of the ffmpeg libs/dlls is extremely useful 
> for this purpose.

It is null because, while the API exists, it is not implemented. AFAIK, 
no device implements it.

Source: I grepped the latest source for avdevice_capabilities_create and 
found nothing of interest.

The way to go is to either parse the logs or directly use the DShow/V4L2 
libraries.

You can also implement the API yourself and send the patch to the 
ffmpeg-devel mailing list. ;)

> 
>  > Maybe I did not initialize everything correctly or bad ordering of 
> open input?
> 
>> Please see my code:
> 
> 
> The code seems correct, my only problem with the above code is in the 
> next line:
> 
> 
>  > int ret = avformat_open_input(&m_pFormatCtxInCam, szUrl, m_pInFrmt, 
> &m_pInOptions);
> 
> Try using no(NULL) options:
> 
> 
> int ret = avformat_open_input(&m_pFormatCtxInCam, szUrl, m_pInFrmt, NULL);
> 
> Maybe opening an input with options shuts down the querying capabilities 
> of the input, I don't really know, it's just a guess.
> 
> 
> Regards.
> 
> 
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-- 
Philippe Gorley
Free Software Consultant | Montréal, Qc
Savoir-faire Linux