lavc: add a null bitstream filter

It is useful for testing/debugging and will also be used as the default
filter in the following commit adding pre-decode filtering to avoid
having a separate non-filtered codepath.

doc/bitstream_filters.texi

@@ -90,6 +90,9 @@ avconv -i frame_%d.jpg -c:v copy rotated.avi
 
 @section noise
 
+@section null
+This bitstream filter passes the packets through unchanged.
+
 @section remove_extradata
 
 @c man end BITSTREAM FILTERS

libavcodec/Makefile

@@ -764,6 +764,7 @@ OBJS-$(CONFIG_MJPEG2JPEG_BSF)             += mjpeg2jpeg_bsf.o
 OBJS-$(CONFIG_MJPEGA_DUMP_HEADER_BSF)     += mjpega_dump_header_bsf.o
 OBJS-$(CONFIG_MOV2TEXTSUB_BSF)            += movsub_bsf.o
 OBJS-$(CONFIG_NOISE_BSF)                  += noise_bsf.o
+OBJS-$(CONFIG_NULL_BSF)                   += null_bsf.o
 OBJS-$(CONFIG_REMOVE_EXTRADATA_BSF)       += remove_extradata_bsf.o
 OBJS-$(CONFIG_TEXT2MOVSUB_BSF)            += movsub_bsf.o
 

libavcodec/bitstream_filters.c

@@ -34,6 +34,7 @@ extern const AVBitStreamFilter ff_imx_dump_header_bsf;
 extern const AVBitStreamFilter ff_mjpeg2jpeg_bsf;
 extern const AVBitStreamFilter ff_mjpega_dump_header_bsf;
 extern const AVBitStreamFilter ff_mov2textsub_bsf;
+extern const AVBitStreamFilter ff_null_bsf;
 extern const AVBitStreamFilter ff_text2movsub_bsf;
 extern const AVBitStreamFilter ff_noise_bsf;
 extern const AVBitStreamFilter ff_remove_extradata_bsf;

libavcodec/null_bsf.c

@@ -0,0 +1,43 @@
+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * Null bitstream filter -- pass the input through unchanged.
+ */
+
+#include "avcodec.h"
+#include "bsf.h"
+
+static int null_filter(AVBSFContext *ctx, AVPacket *out)
+{
+    AVPacket *in;
+    int ret;
+
+    ret = ff_bsf_get_packet(ctx, &in);
+    if (ret < 0)
+        return ret;
+    av_packet_move_ref(out, in);
+    av_packet_free(&in);
+    return 0;
+}
+
+const AVBitStreamFilter ff_null_bsf = {
+    .name           = "null",
+    .filter         = null_filter,
+};