[theora-dev] Understanding the Theora Bitstream

[Mike Melanson]

Hi,
        I checked out the Theora source code for the first time as I
figured that's the only way the Theora quirks will get documented (and
subsequently re-implemented in libavcodec). I see that most of the code
cribs the original VP3 source code, including the misspelled comments.
Moving right along, I can't reconcile the way bits are packed in the
stream. In VP3, the first byte of an encoded frame is supposed to contain:

  bit 7: 0 = golden frame, 1 = interframe
  bit 6: unused
  bits 5-0: Quality index (0..63)

According to {theora}/lib/decode.c, it goes frame type (1 bit), Q index (6
bits), then spare bit (1 bit). With some strategically-placed printf()'s,
I'm seeing data like this:

 hey! Theora packet has 9 bytes
   first 8 bytes: 32 00 0B D7 69 F6 FF DC
     header: frame type 0, Q index = 0x32, spare bits = 0
(I would have expected 0110 0100 = 0x64 for the first byte)

 hey! Theora packet has 9 bytes
   first 8 bytes: 5A 3F 09 E7 E1 3D E0 00
     header: frame type 1, Q index = 0x1A, spare bits = 0
(I would have expected 1011 0100 = 0xB4 for the first byte)

 hey! Theora packet has 497 bytes
   first 8 bytes: 50 38 CF 1E 36 68 B7 9C
     header: frame type 1, Q index = 0x10, spare bits = 0
(I would have expected 1010 0000 = 0xA0 for the first byte)

I remember reading in a spec somewhere that Ogg uses some nutty logic for
packing bitstreams. Is this a manifestation of said logic?

        Thanks...

--
	-Mike Melanson
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