avconv: allow no input files.

It is now possible to use lavfi sources.

avconv.c

@@ -2431,11 +2431,6 @@ int main(int argc, char **argv)
         exit_program(1);
     }
 
-    if (nb_input_files == 0) {
-        av_log(NULL, AV_LOG_FATAL, "At least one input file must be specified\n");
-        exit_program(1);
-    }
-
     ti = getutime();
     if (transcode() < 0)
         exit_program(1);

doc/avconv.texi

@@ -805,6 +805,9 @@ the matching type.
 Output link labels are referred to with @option{-map}. Unlabeled outputs are
 added to the first output file.
 
+Note that with this option it is possible to use only lavfi sources without
+normal input files.
+
 For example, to overlay an image over video
 @example
 avconv -i video.mkv -i image.png -filter_complex '[0:v][1:v]overlay[out]' -map
@@ -827,6 +830,11 @@ graph will be added to the output file automatically, so we can simply write
 @example
 avconv -i video.mkv -i image.png -filter_complex 'overlay' out.mkv
 @end example
+
+To generate 5 seconds of pure red video using lavfi @code{color} source:
+@example
+avconv -filter_complex 'color=red' -t 5 out.mkv
+@end example
 @end table
 @c man end OPTIONS