cbs_mpeg2: Fix parsing of picture and slice headers

1. The extra information in slice headers was parsed incorrectly:
In the first reading pass to derive the length of the extra information,
one should look at bits n, n + 9, n + 18, ... and check whether they
equal one (further extra information) or zero (end of extra information),
but instead bits n, n + 8, n + 16, ... were inspected. The second pass
of reading (where the length is already known and the bytes between the
length-determining bits are copied into a buffer) did not record what
was in bits n, n + 9, n + 18, ..., presuming they equal one. And during
writing, the bytes in the buffer are interleaved with set bits and
written. This means that if the detected length of the extra information
was greater than the real length, the output was corrupted. Fortunately
no sample is known that made use of this mechanism: The extra information
in slices is still marked as reserved in the specifications. cbs_mpeg2
is now ready in case this changes.

2. Furthermore, the buffer is now padded and slightly different, but
very similar code for reading resp. writing has been replaced by code
used for both. This was made possible by a new macro, the equivalent
to cbs_h2645's fixed().

3. These changes also made it possible to remove the extra_bit_slice
element from the MPEG2RawSliceHeader structure. Said element was always
zero except when the detected length of the extra information was less
than the real length.

4. The extra information in picture headers (which uses essentially the
same syntax as the extra information in slice headers) has simply been
forgotten. This meant that if this extra information was present, it was
discarded during reading; and unfortunately writing created invalid
bitstreams in this case (an extra_bit_picture - the last set bit of the
whole unit - indicated that there would be a further byte of data,
although the output didn't contain said data).

This has been fixed; both types of extra information are now parsed via
the same code and essentially passed through.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/libavcodec/cbs_mpeg2.h b/libavcodec/cbs_mpeg2.h
index 11f93b9df8..2befaab275 100644
--- a/libavcodec/cbs_mpeg2.h
+++ b/libavcodec/cbs_mpeg2.h
@@ -114,6 +114,12 @@ typedef struct MPEG2RawGroupOfPicturesHeader {
     uint8_t broken_link;
 } MPEG2RawGroupOfPicturesHeader;
 
+typedef struct MPEG2RawExtraInformation {
+    uint8_t     *extra_information;
+    AVBufferRef *extra_information_ref;
+    size_t       extra_information_length;
+} MPEG2RawExtraInformation;
+
 typedef struct MPEG2RawPictureHeader {
     uint8_t picture_start_code;
 
@@ -126,7 +132,7 @@ typedef struct MPEG2RawPictureHeader {
     uint8_t full_pel_backward_vector;
     uint8_t backward_f_code;
 
-    uint8_t extra_bit_picture;
+    MPEG2RawExtraInformation extra_information_picture;
 } MPEG2RawPictureHeader;
 
 typedef struct MPEG2RawPictureCodingExtension {
@@ -194,11 +200,7 @@ typedef struct MPEG2RawSliceHeader {
     uint8_t slice_picture_id_enable;
     uint8_t slice_picture_id;
 
-    uint8_t extra_bit_slice;
-
-    size_t extra_information_length;
-    uint8_t *extra_information;
-    AVBufferRef *extra_information_ref;
+    MPEG2RawExtraInformation extra_information_slice;
 } MPEG2RawSliceHeader;
 
 typedef struct MPEG2RawSlice {