rtp: Initial H.261 support

The packetizer only supports splitting at GOB headers - if
such aren't available frequently enough, it splits at any
random byte offset (not at a macroblock boundary either, which
would be allowed by the spec) and sends a payload header pretend
that it starts with a GOB header.

As long as a receiver doesn't try to handle such cases cleverly
but just drops broken frames, this shouldn't matter too much
in practice.

Signed-off-by: Martin Storsjö <martin@martin.st>

1 files changed, 102 insertions, 0 deletions

diff --git a/libavformat/rtpenc_h261.c b/libavformat/rtpenc_h261.c
new file mode 100644
index 0000000000..514e7bf9ca
--- /dev/null
+++ b/libavformat/rtpenc_h261.c
@@ -0,0 +1,102 @@
+/*
+ * RTP packetization for H.261 video (RFC 4587)
+ * Copyright (c) 2014 Thomas Volkert <thomas@homer-conferencing.com>
+ *
+ * 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
+ */
+
+#include "avformat.h"
+#include "rtpenc.h"
+
+#define RTP_H261_HEADER_SIZE 4
+
+static const uint8_t *find_resync_marker_reverse(const uint8_t *restrict start,
+                                                 const uint8_t *restrict end)
+{
+    const uint8_t *p = end - 1;
+    start += 1; /* Make sure we never return the original start. */
+    for (; p > start; p--) {
+        if (p[0] == 0 && p[1] == 1)
+            return p;
+    }
+    return end;
+}
+
+void ff_rtp_send_h261(AVFormatContext *ctx, const uint8_t *frame_buf, int frame_size)
+{
+    int cur_frame_size;
+    int last_packet_of_frame;
+    RTPMuxContext *rtp_ctx = ctx->priv_data;
+
+    /* use the default 90 KHz time stamp */
+    rtp_ctx->timestamp = rtp_ctx->cur_timestamp;
+
+    /* continue as long as not all frame data is processed */
+    while (frame_size > 0) {
+        /*
+         * encode the H.261 payload header according to section 4.1 of RFC 4587:
+         * (uses 4 bytes between RTP header and H.261 stream per packet)
+         *
+         *    0                   1                   2                   3
+         *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+         *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+         *   |SBIT |EBIT |I|V| GOBN  |   MBAP  |  QUANT  |  HMVD   |  VMVD   |
+         *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+         *
+         *      Start bit position (SBIT): 3 bits
+         *      End bit position (EBIT): 3 bits
+         *      INTRA-frame encoded data (I): 1 bit
+         *      Motion Vector flag (V): 1 bit
+         *      GOB number (GOBN): 4 bits
+         *      Macroblock address predictor (MBAP): 5 bits
+         *      Quantizer (QUANT): 5 bits
+         *      Horizontal motion vector data (HMVD): 5 bits
+         *      Vertical motion vector data (VMVD): 5 bits
+         */
+        rtp_ctx->buf[0] = 1; /* sbit=0, ebit=0, i=0, v=1 */
+        rtp_ctx->buf[1] = 0; /* gobn=0, mbap=0 */
+        rtp_ctx->buf[2] = 0; /* quant=0, hmvd=5 */
+        rtp_ctx->buf[3] = 0; /* vmvd=0 */
+        if (frame_size < 2 || frame_buf[0] != 0 || frame_buf[1] != 1) {
+            /* A full, correct fix for this would be to make the H261 encoder
+             * support inserting extra GOB headers (triggered by setting e.g.
+             * "-ps 1"), and including information about macroblock boundaries
+             * (such as for h263_rfc2190). */
+            av_log(ctx, AV_LOG_WARNING,
+                   "RTP/H261 packet not cut at a GOB boundary, not signaled correctly\n");
+        }
+
+        cur_frame_size = FFMIN(rtp_ctx->max_payload_size - RTP_H261_HEADER_SIZE, frame_size);
+
+        /* look for a better place to split the frame into packets */
+        if (cur_frame_size < frame_size) {
+            const uint8_t *packet_end = find_resync_marker_reverse(frame_buf,
+                                                                   frame_buf + cur_frame_size);
+            cur_frame_size = packet_end - frame_buf;
+        }
+
+        /* calculate the "marker" bit for the RTP header */
+        last_packet_of_frame = cur_frame_size == frame_size;
+
+        /* complete and send RTP packet */
+        memcpy(&rtp_ctx->buf[RTP_H261_HEADER_SIZE], frame_buf, cur_frame_size);
+        ff_rtp_send_data(ctx, rtp_ctx->buf, RTP_H261_HEADER_SIZE + cur_frame_size, last_packet_of_frame);
+
+        frame_buf  += cur_frame_size;
+        frame_size -= cur_frame_size;
+    }
+}