[FFmpeg-devel] Possible bug in h264.c ff_h264_decode_nal

[David R Robison]

I am trying to play video from a Pelco camera and it is not decoding right. It appears that the entire SPS is not being parsed. In the h264.c file the ff_h264_decode_nal function the following is defined.

#define STARTCODE_TEST \
          if (i + 2 < length && src[i + 1] == 0 && src[i + 2] <= 3) {     \
              if (src[i + 2] != 3) {                                      \
                  /* startcode, so we must be past the end */             \
                  length = i;                                             \
} \
break; \
          }

However, it seems to me from the documentation that the start code will either end with a 1 or a 3. If I change the function to

#define STARTCODE_TEST \
          if (i + 2 < length && src[i + 1] == 0 && (src[i + 2] == 1 ||
src[i + 2] == 3)) {     \
              if (src[i + 2] != 3) {                                      \
                  /* startcode, so we must be past the end */             \
                  length = i;                                             \
}                                                           \
break;                                                      \
          }

Then my video plays fine. Any thoughts on this? Here is the patch file:

Index: ffmpeg/libavcodec/h264.c
===================================================================
--- ffmpeg/libavcodec/h264.c    (revision 4489)
+++ ffmpeg/libavcodec/h264.c    (working copy)
@@ -178,7 +178,7 @@
       length--;

   #define STARTCODE_TEST                                                  \
-        if (i + 2 < length && src[i + 1] == 0 && src[i + 2] <= 3) {     \
+        if (i + 2 < length && src[i + 1] == 0 && (src[i + 2] == 1 ||
src[i + 2] == 3)) {     \
               if (src[i + 2] != 3) {                                      \
                   /* startcode, so we must be past the end */             \
                   length = i;                                             \


One more piece of information. In speaking with the company that makes our RTSP library, they stated that they remove the emulation prevention bytes from the data it copies into the media type. When they leave them in it seems to play. However, it make me wonder if this patch is still valid for ffmpeg.

Any thoughts?

David

-- 

David R Robison
Open Roads Consulting, Inc.
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