[FFmpeg-devel] [PATCH] MPEG sequeunce display extension parsing off by 2 error

[Måns Rullgård]

Daniel Kristjansson <danielk at cuymedia.net> writes:

> On Tue, 2010-06-29 at 13:16 +0200, Michael Niedermayer wrote:
>> > 13818-2: 1995(E), p33. 
>> 
>> mine says 1995 too and it doesnt say draft (which doesnt mean that it
>> isnt one ...), can you confirm that yours is not a draft?
>
> Mine doesn't say draft, but I don't really know if it is. I bought a
> copy at some point, but the copy I'm looking at doesn't have my name
> embedded in it like most of the ones I've bought.
>
>> more precissely mines says:
>> 6.2.2.4     Sequence display extension
>> 
>> |sequence_display_extension() {                |No. of  |Mnemonic|
>> |                                              |bits    |        |
>> | extension_start_code_identifier              |4       |uimsbf  |
>> | video_format                                 |3       |uimsbf  |
>> | colour_description                           |1       |uimsbf  |
>> | if ( colour_description ) {                  |        |        |
>> |  colour_primaries                            |8       |uimsbf  |
>> |  transfer_characteristics                    |8       |uimsbf  |
>> |  matrix_coefficients                         |8       |uimsbf  |
>> | }                                            |        |        |
>> | display_horizontal_size                      |14      |uimsbf  |
>> | marker_bit                                   |1       |bslbf   |
>> | display_vertical_size                        |14      |uimsbf  |
>> | next_start_code()                            |        |        |
>> |}                                             |        |        |
>
> Reading your copy it looks like those 3 bits at the end are totally
> unaccounted for. Things are byte aligned before display_horizontal_size
> and the next_start_code will be byte aligned, but we've only got
> 29 bits in between. Those three bits are the steganography bits. ;)

See section 5.2.3:

  The next_start_code() function removes any zero bit and zero byte
  stuffing and locates the next start code.

-- 
M?ns Rullg?rd
mans at mansr.com