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, 202 insertions, 0 deletions

diff --git a/libavformat/rtpdec_h261.c b/libavformat/rtpdec_h261.c
new file mode 100644
index 0000000000..4988300831
--- /dev/null
+++ b/libavformat/rtpdec_h261.c
@@ -0,0 +1,202 @@
+/*
+ * RTP parser for H.261 payload format (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 "libavcodec/get_bits.h"
+#include "avformat.h"
+#include "rtpdec_formats.h"
+
+#define RTP_H261_PAYLOAD_HEADER_SIZE 4
+
+struct PayloadContext {
+    AVIOContext *buf;
+    uint8_t      endbyte;
+    int          endbyte_bits;
+    uint32_t     timestamp;
+};
+
+static av_cold PayloadContext *h261_new_context(void)
+{
+    return av_mallocz(sizeof(PayloadContext));
+}
+
+static void h261_free_dyn_buffer(AVIOContext **dyn_buf)
+{
+    uint8_t *ptr_dyn_buffer;
+    avio_close_dyn_buf(*dyn_buf, &ptr_dyn_buffer);
+    av_free(ptr_dyn_buffer);
+    *dyn_buf = NULL;
+}
+
+static av_cold void h261_free_context(PayloadContext *pl_ctx)
+{
+    /* return if context is invalid */
+    if (!pl_ctx)
+        return;
+
+    /* free buffer if it is valid */
+    if (pl_ctx->buf) {
+        h261_free_dyn_buffer(&pl_ctx->buf);
+    }
+
+    /* free context */
+    av_free(pl_ctx);
+}
+
+static av_cold int h261_init(AVFormatContext *ctx, int st_index,
+                             PayloadContext *data)
+{
+    if (st_index < 0)
+        return 0;
+
+    ctx->streams[st_index]->need_parsing = AVSTREAM_PARSE_FULL;
+
+    return 0;
+}
+
+static int h261_handle_packet(AVFormatContext *ctx, PayloadContext *rtp_h261_ctx,
+                              AVStream *st, AVPacket *pkt, uint32_t *timestamp,
+                              const uint8_t *buf, int len, uint16_t seq,
+                              int flags)
+{
+    int sbit, ebit, gobn, mbap, quant;
+    int res;
+
+    /* drop data of previous packets in case of non-continuous (lossy) packet stream */
+    if (rtp_h261_ctx->buf && rtp_h261_ctx->timestamp != *timestamp) {
+        h261_free_dyn_buffer(&rtp_h261_ctx->buf);
+        rtp_h261_ctx->endbyte_bits = 0;
+    }
+
+    /* sanity check for size of input packet: 1 byte payload at least */
+    if (len < RTP_H261_PAYLOAD_HEADER_SIZE + 1) {
+        av_log(ctx, AV_LOG_ERROR, "Too short RTP/H.261 packet, got %d bytes\n", len);
+        return AVERROR_INVALIDDATA;
+    }
+
+    /*
+     * decode 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
+     */
+    sbit  =  (buf[0] >> 5) & 0x07;
+    ebit  =  (buf[0] >> 2) & 0x07;
+    gobn  =  (buf[1] >> 4) & 0x0f;
+    mbap  = ((buf[1] << 1) & 0x1e) | ((buf[2] >> 7) & 0x01);
+    quant =  (buf[2] >> 2) & 0x1f;
+
+    /* pass the H.261 payload header and continue with the actual payload */
+    buf += RTP_H261_PAYLOAD_HEADER_SIZE;
+    len -= RTP_H261_PAYLOAD_HEADER_SIZE;
+
+    /* start frame buffering with new dynamic buffer */
+    if (!rtp_h261_ctx->buf) {
+        /* sanity check: a new frame starts with gobn=0, sbit=0, mbap=0, quant=0 */
+        if (!gobn && !sbit && !mbap && !quant) {
+            res = avio_open_dyn_buf(&rtp_h261_ctx->buf);
+            if (res < 0)
+                return res;
+            /* update the timestamp in the frame packet with the one from the RTP packet */
+            rtp_h261_ctx->timestamp = *timestamp;
+        } else {
+            /* frame not started yet, need more packets */
+            return AVERROR(EAGAIN);
+        }
+    }
+
+    /* do the "byte merging" at the boundaries of two consecutive frame fragments */
+    if (rtp_h261_ctx->endbyte_bits || sbit) {
+        if (rtp_h261_ctx->endbyte_bits == sbit) {
+            rtp_h261_ctx->endbyte     |= buf[0] & (0xff >> sbit);
+            rtp_h261_ctx->endbyte_bits = 0;
+            buf++;
+            len--;
+            avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
+        } else {
+            /* ebit/sbit values inconsistent, assuming packet loss */
+            GetBitContext gb;
+            init_get_bits(&gb, buf, len*8 - ebit);
+            skip_bits(&gb, sbit);
+            if (rtp_h261_ctx->endbyte_bits) {
+                rtp_h261_ctx->endbyte |= get_bits(&gb, 8 - rtp_h261_ctx->endbyte_bits);
+                avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
+            }
+            while (get_bits_left(&gb) >= 8)
+                avio_w8(rtp_h261_ctx->buf, get_bits(&gb, 8));
+            rtp_h261_ctx->endbyte_bits = get_bits_left(&gb);
+            if (rtp_h261_ctx->endbyte_bits)
+                rtp_h261_ctx->endbyte = get_bits(&gb, rtp_h261_ctx->endbyte_bits) <<
+                                        (8 - rtp_h261_ctx->endbyte_bits);
+            ebit = 0;
+            len  = 0;
+        }
+    }
+    if (ebit) {
+        if (len > 0)
+            avio_write(rtp_h261_ctx->buf, buf, len - 1);
+        rtp_h261_ctx->endbyte_bits = 8 - ebit;
+        rtp_h261_ctx->endbyte      = buf[len - 1] & (0xff << ebit);
+    } else {
+        avio_write(rtp_h261_ctx->buf, buf, len);
+    }
+
+    /* RTP marker bit means: last fragment of current frame was received;
+       otherwise, an additional fragment is needed for the current frame */
+    if (!(flags & RTP_FLAG_MARKER))
+        return AVERROR(EAGAIN);
+
+    /* write the completed last byte from the "byte merging" */
+    if (rtp_h261_ctx->endbyte_bits)
+        avio_w8(rtp_h261_ctx->buf, rtp_h261_ctx->endbyte);
+    rtp_h261_ctx->endbyte_bits = 0;
+
+    /* close frame buffering and create resulting A/V packet */
+    res = ff_rtp_finalize_packet(pkt, &rtp_h261_ctx->buf, st->index);
+    if (res < 0)
+        return res;
+
+    return 0;
+}
+
+RTPDynamicProtocolHandler ff_h261_dynamic_handler = {
+    .enc_name          = "H261",
+    .codec_type        = AVMEDIA_TYPE_VIDEO,
+    .codec_id          = AV_CODEC_ID_H261,
+    .init              = h261_init,
+    .alloc             = h261_new_context,
+    .free              = h261_free_context,
+    .parse_packet      = h261_handle_packet,
+    .static_payload_id = 31,
+};