[FFmpeg-devel] [PATCH 4/4] examples: adding device_get_capabilities example

[Diederick C. Niehorster]

On Sat, Jun 5, 2021 at 8:51 AM Andreas Rheinhardt
<andreas.rheinhardt at outlook.com> wrote:
>
> Diederick Niehorster:
> > +
> > +    // since there is no equivalent of avformat_alloc_output_context2 for an input context,
> > +    // so we get this dirty code that users shouldn't have to write....
> > +    fmt_ctx = avformat_alloc_context();
> > +    fmt_ctx->url = av_strdup(dshow_input_name);
>
> Missing checks.
>
> > +    fmt_ctx->iformat = fmt;
> > +    if (fmt_ctx->iformat->priv_data_size > 0) {
>
> priv_data_size is not part of the public API; so the code is indeed
> dirty and so this example is unacceptable as-is. This might be a
> scenario where AVFMT_FLAG_PRIV_OPT might have been useful, see
> 2ff40b98ecbd9befadddc8fe665a391f99bfca32.
>
> > +        if (!(fmt_ctx->priv_data = av_mallocz(fmt_ctx->iformat->priv_data_size))) {
> > +            ret = AVERROR(ENOMEM);
> > +            goto end;
> > +        }
> > +        if (fmt_ctx->iformat->priv_class) {
> > +            *(const AVClass**)fmt_ctx->priv_data = fmt_ctx->iformat->priv_class;
> > +            av_opt_set_defaults(fmt_ctx->priv_data);
> > +        }
> > +    }
> > +    // end dirty code

I have fixed the rest, but I believe it is not possible to fix the
above problem with the current API? The code above shows the state i
need the format context and attached input context to be in when
calling av_opt_query_ranges. The device cannot be opened (that is
read_header should not be called) before calling av_opt_query_ranges,
as it is not possible to reliably query possible settings of the
device once it is opened (without temporarily closing it anyway, which
would be undesirable). One solution i see is to:
1. implement a avformat_alloc_input_context()
2. analogous to avformat_alloc_output_context2, add a few
if-statements in avformat_open_input() to skip steps if they are
already done on the passed-in AVFormatContext or its iformat.

It would e.g. allow something along the lines of:
AVFormatContext* fmt_ctx = NULL;
avformat_alloc_input_context(&fmt_ctx, NULL, "dshow",
"video=\"Integrated Webcam\"");
// ...
// query device capabilities, built up option dict for setting up
device as wanted based on query results
// ....
avformat_open_input(&fmt_ctx,NULL,NULL,opts);
// NB: filename and fmt inputs above are NULL as already set by
avformat_alloc_input_context

Would such an API extension be considered?

NB: The constraint of my implementation of the AVDevice Capabilties
API should also be documented. I propose, more generally: the
avdevice_capabilities_create/av_opt_query_ranges API has the
constraint that while it is always valid to query capabilities on an
unopened avdevice (as provided by avformat_alloc_input_context), some
devices may not allow querying capabilities once avformat_open_input()
has been called on them. In that case, upon av_opt_query_ranges these
devices will return AVERROR(EIO).

All the best,
Dee