cmdutils: Add a stream specifier to map usable streams

It drops everything that cannot be used for re-encoding and/or
streamcopy.

cmdutils.c

@@ -1587,6 +1587,28 @@ int check_stream_specifier(AVFormatContext *s, AVStream *st, const char *spec)
 
         av_freep(&key);
         return ret;
+    } else if (*spec == 'u') {
+        AVCodecContext *avctx = st->codec;
+        int val;
+        switch (avctx->codec_type) {
+        case AVMEDIA_TYPE_AUDIO:
+            val = avctx->sample_rate && avctx->channels;
+            if (avctx->sample_fmt == AV_SAMPLE_FMT_NONE)
+                return 0;
+            break;
+        case AVMEDIA_TYPE_VIDEO:
+            val = avctx->width && avctx->height;
+            if (avctx->pix_fmt == AV_PIX_FMT_NONE)
+                return 0;
+            break;
+        case AVMEDIA_TYPE_UNKNOWN:
+            val = 0;
+            break;
+        default:
+            val = 1;
+            break;
+        }
+        return avctx->codec_id != AV_CODEC_ID_NONE && val != 0;
     } else if (!*spec) /* empty specifier, matches everything */
         return 1;
 

doc/avtools-common-opts.texi

@@ -48,6 +48,9 @@ Match the stream by stream id (e.g. PID in MPEG-TS container).
 Matches streams with the metadata tag @var{key} having the specified value. If
 @var{value} is not given, matches streams that contain the given tag with any
 value.
+@item u
+Matches streams with usable configuration, the codec must be defined and the
+essential information such as video dimension or audio sample rate must be present.
 
 Note that in @command{avconv}, matching by metadata will only work properly for
 input files.