[FFmpeg-devel] libavformat/rtpdec_jpeg.c patch to allow fragmentation offset to include q header and table data

[Hayden Myers]

>From fb2b280e44b785b99b27c24503c61574168701b9 Mon Sep 17 00:00:00 2001
From: Hayden Myers <hmyers at skylinenet.net>
Date: Fri, 10 Sep 2021 14:35:28 -0400
Subject: [PATCH] libavformat/rtpdec_jpeg.c: Allow fragmentation offset to
 include q header and data

Some jpeg over rtp streams include the quantization header and table data
in the
reported fragmentation offset value of the jpeg header.  ffmpeg stores q
table
data separate from the jpeg stream data.  When the fragmentation offset is
compared
against the current position in the jpeg frame, it's off by q header + q
data bytes.
This results in all packets being dropped.  It thinks there's data loss,
but there
really isn't.  Added a conditional to check if the offset differs by q
header +
q data bytes, before dropping the packet.

Signed-off-by: Hayden Myers <hmyers at skylinenet.net>
---
 libavformat/rtpdec_jpeg.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/libavformat/rtpdec_jpeg.c b/libavformat/rtpdec_jpeg.c
index b32d074136..4ec9fc8bd9 100644
--- a/libavformat/rtpdec_jpeg.c
+++ b/libavformat/rtpdec_jpeg.c
@@ -211,6 +211,7 @@ static void create_default_qtables(uint8_t *qtables,
uint8_t q)
     }
 }

+
 static int jpeg_parse_packet(AVFormatContext *ctx, PayloadContext *jpeg,
                              AVStream *st, AVPacket *pkt, uint32_t
*timestamp,
                              const uint8_t *buf, int len, uint16_t seq,
@@ -350,10 +351,27 @@ static int jpeg_parse_packet(AVFormatContext *ctx,
PayloadContext *jpeg,
         return AVERROR_INVALIDDATA;
     }

-    if (off != avio_tell(jpeg->frame) - jpeg->hdr_size) {
-        av_log(ctx, AV_LOG_ERROR,
-               "Missing packets; dropping frame.\n");
-        return AVERROR(EAGAIN);
+    if (off != (avio_tell(jpeg->frame) - jpeg->hdr_size)) {
+
+        /* Take into account that the fragment offset may include the quant
+         * table data. Allow the offset to differ by the size of the
+         * q header and table.
+         */
+
+        //Default to 2 * 64 byte tables for 8 bit precision.
+        uint8_t qtable_len =  128;
+
+        // Use the q table len value stored in the ctx
+        if (q >= 127 && q < 255)
+            qtable_len = jpeg->qtables_len[q-128];
+
+        // account for MBZ, Precision, and Length bytes.
+        qtable_len += 4;
+
+        if (off != (avio_tell(jpeg->frame) - (jpeg->hdr_size -
qtable_len))) {
+            av_log(ctx, AV_LOG_ERROR, "Missing packets; dropping
frame.\n");
+            return AVERROR(EAGAIN);
+        }
     }

     /* Copy data to frame buffer. */
-- 
2.25.1