ffmpeg: support setting AVStream.discard

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -1146,6 +1146,30 @@ requested by @command{ffserver}.
 The option is intended for cases where features are needed that cannot be
 specified to @command{ffserver} but can be to @command{ffmpeg}.
 
+@item -discard (@emph{input})
+Allows discarding specific streams or frames of streams at the demuxer.
+Not all demuxers support this.
+
+@table @option
+@item none
+Discard no frame.
+
+@item default
+Default, which discards no frames.
+
+@item noref
+Discard all non-reference frames.
+
+@item bidir
+Discard all bidirectional frames.
+
+@item nokey
+Discard all frames excepts keyframes.
+
+@item all
+Discard all frames.
+@end table
+
 @end table
 
 As a special exception, you can use a bitmap subtitle stream as input: it

ffmpeg.h

@@ -204,6 +204,8 @@ typedef struct OptionsContext {
     int        nb_guess_layout_max;
     SpecifierOpt *apad;
     int        nb_apad;
+    SpecifierOpt *discard;
+    int        nb_discard;
 } OptionsContext;
 
 typedef struct InputFilter {
@@ -240,6 +242,7 @@ typedef struct InputStream {
     int file_index;
     AVStream *st;
     int discard;             /* true if stream data should be discarded */
+    int user_set_discard;
     int decoding_needed;     /* true if the packets must be decoded in 'raw_fifo' */
     AVCodecContext *dec_ctx;
     AVCodec *dec;

ffmpeg_opt.c

@@ -572,6 +572,8 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
         char *framerate = NULL, *hwaccel = NULL, *hwaccel_device = NULL;
         char *codec_tag = NULL;
         char *next;
+        char *discard_str = NULL;
+        const AVOption *discard_opt = av_opt_find(dec, "skip_frame", NULL, 0, 0);
 
         if (!ist)
             exit_program(1);
@@ -601,6 +603,14 @@ static void add_input_streams(OptionsContext *o, AVFormatContext *ic)
         ist->reinit_filters = -1;
         MATCH_PER_STREAM_OPT(reinit_filters, i, ist->reinit_filters, ic, st);
 
+        MATCH_PER_STREAM_OPT(discard, str, discard_str, ic, st);
+        ist->user_set_discard = AVDISCARD_NONE;
+        if (discard_str && av_opt_eval_int(dec, discard_opt, discard_str, &ist->user_set_discard) < 0) {
+            av_log(NULL, AV_LOG_ERROR, "Error parsing discard %s.\n",
+                    discard_str);
+            exit_program(1);
+        }
+
         ist->filter_in_rescale_delta_last = AV_NOPTS_VALUE;
 
         ist->dec_ctx = avcodec_alloc_context3(ist->dec);
@@ -1165,7 +1175,7 @@ static OutputStream *new_output_stream(OptionsContext *o, AVFormatContext *oc, e
     if (source_index >= 0) {
         ost->sync_ist = input_streams[source_index];
         input_streams[source_index]->discard = 0;
-        input_streams[source_index]->st->discard = AVDISCARD_NONE;
+        input_streams[source_index]->st->discard = input_streams[source_index]->user_set_discard;
     }
     ost->last_mux_dts = AV_NOPTS_VALUE;
 
@@ -1804,7 +1814,7 @@ static int open_output_file(OptionsContext *o, const char *filename)
                     if(ost->st->codec->codec_type == AVMEDIA_TYPE_AUDIO) ost->avfilter = av_strdup("anull");
                     if(ost->st->codec->codec_type == AVMEDIA_TYPE_VIDEO) ost->avfilter = av_strdup("null");
                     ist->discard = 0;
-                    ist->st->discard = AVDISCARD_NONE;
+                    ist->st->discard = ist->user_set_discard;
                     break;
                 }
             }
@@ -2852,6 +2862,9 @@ const OptionDef options[] = {
         "print timestamp debugging info" },
     { "max_error_rate",  HAS_ARG | OPT_FLOAT,                        { &max_error_rate },
         "maximum error rate", "ratio of errors (0.0: no errors, 1.0: 100% errors) above which ffmpeg returns an error instead of success." },
+    { "discard",        OPT_STRING | HAS_ARG | OPT_SPEC |
+                        OPT_INPUT,                                   { .off = OFFSET(discard) },
+        "discard", "" },
 
     /* video options */
     { "vframes",      OPT_VIDEO | HAS_ARG  | OPT_PERFILE | OPT_OUTPUT,           { .func_arg = opt_video_frames },