On Sat, Jun 5, 2021 at 8:51 AM Andreas Rheinhardt
<andreas.rheinha...@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
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