VP3 Bitstream Format and Decoding Process
by Mike Melanson (mike at multimedia.cx)
v0.5: December 8, 2004


[December 8, 2004: Note that this document is not complete and likely
will never be completed. However, it helped form the basis of Theora I 
specification available at
  https://www.theora.org/doc/Theora_I_spec.pdf ]


Contents
--------
 * Introduction
 * Underlying Coding Concepts
 * VP3 Coding Overview
 * VP3 Chunk Format
 * Decoding The Frame Header
 * Initializing The Quantization Matrices
 * Hilbert Coding Pattern
 * Unpacking The Block Coding Information
 * Unpacking The Macroblock Coding Mode Information
 * Unpacking The Macroblock Motion Vectors
 * Unpacking The DCT Coefficients
 * Reversing The DC Prediction
 * Reconstructing The Frame
 * Theora Specification
 * Appendix A: Quantization Matrices And Scale Factors
 * Appendix B: Macroblock Coding Mode Alphabets
 * Appendix C: DCT Coefficient VLC Tables
 * Appendix D: The VP3 IDCT
 * Acknowledgements
 * References
 * Changelog


Introduction
------------
A company named On2 (https://www.on2.com/) created a video codec named
VP3. Eventually, they decided to open source it. Like any body of code
that was produced on a deadline, the source code was not particularly
clean or well-documented. This makes it difficult to understand the
fundamental operation of the codec.

This document describes the VP3 bitstream format and decoding process at
a higher level than source code.


Underlying Coding Concepts
-------------------------- 
In order to understand the VP3 coding method it is necessary to
understand the individual steps in the process. Like many multimedia
compression algorithms VP3 does not consist of a single coding method.
Rather, it uses a chain of methods to achieve compression.

If you are acquainted with the MPEG video clique then many of VP3's
coding concepts should look familiar as well. What follows is a list of
the coding methods used in VP3 and a brief description of each.

* Discrete Cosine Transform (DCT): This is a magical mathematical
function that takes a group of numbers and turns it into another group
of numbers. The transformed group of numbers exhibits some curious
properties. Notably, larger numbers are concentrated in certain areas of
the transformed group. 

A video codec like VP3 often operates on 8x8 blocks of numbers. When
these 8x8 blocks are transformed using a DCT the larger numbers occur
mostly in the up and left areas of the block with the largest number
occurring as the first in the block (up-left corner). This number is
called the DC coefficient. The other 63 numbers are called the AC
coefficients.

The DCT and its opposite operation, the inverse DCT, require a lot of
multiplications. Much research and experimentation is focused of
optimizing this phase of the coding/decoding process.

* Quantization: This coding step tosses out information by essentially
dividing a number to be coded by a factor and throwing away the
remainder. The inverse process (dequantization) involves multiplying by
the same factor to obtain a number that is close enough to the original.

* Run Length Encoding (RLE): The concept behind RLE is to shorten runs
of numbers that are the same. For example, the string "88888" is encoded
as (5, 8), indicating a run of 5 '8' numbers. In VP3 (and MPEG/JPEG),
RLE is used to record the number of zero-value coefficients that occur
before a non-zero coefficient. For example:

  0 0 0 0 5 0 2 0 0 0 9

is encoded as:

  (4, 5), (1, 2), (3, 9)

This indicates that a run of 4 zeroes is followed by a coefficient of 5;
then a run of 1 zero is followed by 2; then a run of 3 zeroes is
followed by 9.

* Zigzag Ordering: After transforming and quantizing a block of samples,
the samples are not in an optimal order for run length encoding. Zigzag
ordering rearranges the samples to put more zeros between non-zero
samples.

* Differential (or Delta) Pulse Code Modulation (DPCM): 1 + 1 = 2. Got
that? Seriously, that is what DPCM means. Rather than encoding absolute
values, encode the differences between successive values. For example:

  82 84 81 80 86 88 85

Can be delta-encoded as:

  82 +2 -3 -1 +6 +2 -3

Most of the numbers turn into smaller numbers which require less
information to encode.

* Motion Compensation: Simply, this coding method specifies that a block
from a certain position in the previous frame is to be copied into a new
position in the current frame. This technique is often combined with DCT
and DPCM coding, as well as fractional pixel motion.

* Entropy Coding (a.k.a. Huffman Coding): This is the process of coding
frequently occurring symbols with fewer bits than symbols that are not
likely to occur as frequently.

* Variable Length Run Length Booleans: An initial Boolean bit is
extracted from the bitstream. A variable length code (VLC) is extracted
from the bitstream and converted to a count. This count indicates that
the next (count) elements are to be set to the Boolean value.
Afterwards, the Boolean value is toggled, the next VLC is extracted and
converted to a count, and the process continues until all elements are
set to either 0 or 1.

* YUV Colorspace: Like many modern video codecs, VP3 operates on a YUV
colorspace rather than a RGB colorspace. Specifically, VP3 uses YUV
4:2:0, alias YUV420P, YV12. Note: Throughout the course of this
document, the U and V planes (a.k.a., Cb and Cr planes) will be
collectively referred to as C planes (color or chrominance planes).

* Frame Types: VP3 has intra-coded frames, a.k.a. intraframes, I-frames,
or keyframes. VP3 happens to call these golden frames. VP3 has
interframes, a.k.a. predicted frames or P-frames. These frames can use
information from either the previous interframe or from the previous
golden frame.


VP3 Overview
------------
The first thing to understand about the VP3 coding method is that it
encodes all 3 planes upside down. That is, the data is encoded from
bottom-to-top rather than top-to-bottom as is done with many video
codecs.

VP3 codes a video frame by first breaking each of the 3 planes (Y, U,
and V) into a series of 8x8 blocks called fragments. VP3 also has a
notion of superblocks. Superblocks encapsulate 16 fragments arranged in
a 4x4 matrix. Each plane has its own set of superblocks. Further, VP3
also uses the notion of macroblocks which is the same as that found in
JPEG/MPEG. One macroblock encompasses 4 blocks from the Y plane arranged
in a 2x2 matrix, 1 block from the U plane, and 1 block from the V plane.
While a fragment or a superblock applies to 1 and only 1 plane, a
macroblock extends over all 3 planes.

VP3 compresses golden frames by transforming each fragment with a
discrete cosine transform. Each transformed sample is then quantized and
the DC coefficient is reduced via DPCM using a combination of DC
coefficients from surrounding fragments as predictors. Then, each
fragment's DC coefficient is entropy-coded in the output bitstream,
followed by each fragment's first AC coefficient, then each second AC
coefficient, and so on.

An interframe, naturally, is more complicated. While there is only one
coding mode available for a golden frame (intra coding), there are 8
coding modes that the VP3 coder can choose from for interframe
macroblocks. Intra coding as seen in the keyframe is still available.
The rest of the modes involve encoding a fragment diff, either from the
previous frame or the golden frame, from the same coordinate or from the
same coordinate plus a motion vector. All of the macroblock coding modes
and motion vectors are encoded in an interframe bitstream.


VP3 Chunk Format
----------------
The high-level format of a compressed VP3 frame is laid out as:

 * chunk header
 * block coding information
 * macroblock coding mode information
 * motion vectors
 * DC coefficients
 * 1st AC coefficients
 * 2nd AC coefficients
 * ...
 * 63rd AC coefficients


Decoding The Frame Header
-------------------------
The chunk header always contains at least 1 byte which has the following
format:

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

Further, if the frame is a golden frame, there are 2 more bytes in the
header:

  byte 0: version byte 0
  byte 1:
    bits 7-3: VP3 version number (stored)
    bit 2:    key frame coding method (0 = DCT key frame, only type
              supported)
    bits 1-0: unused, spare bits

All frame headers are encoded with a quality index. This 6-bit value is
used to index into 2 dequantizer scaling tables, 1 for DC values and 1
for AC values. Each of the 3 dequantization tables is modified per these
scaling values.


Initializing The Quantization Matrices
--------------------------------------
VP3 has three static matrices for quantizing and dequantizing fragments.
One matrix is for quantizing golden frame Y fragments, one matrix is for 
quantizing golden frame C fragments, and one matrix is for quantizing both
golden frame and interframe Y or C fragments. While these matrices are
static, they are adjusted according to quality index coded in the header.

The quality index is an index into 2 64-element tables:
dc_scale_factor[] and ac_scale_factor[]. Each quantization factor from
each of the three quantization matrices is adjusted by the appropriate
scale factor according to this formula:

                base quantizer * scale factor
  quantizer  =  -----------------------------
                            100
     
    where scale factor =
        dc_scale_factor[quality_index] for DC dequantizer
        ac_scale_factor[quality_index] for AC dequantizer

The quantization matrices need to be recalculated at the beginning of a
frame decode if the current frame's quality index is different from the
previous frame's quality index.

See Appendix A for the complete VP3 quantization matrices and scale factor
tables.

As an example, this is the base quantization matrix for golden frame Y
fragments:

    16  11  10  16  24  40  51  61
    12  12  14  19  26  58  60  55
    14  13  16  24  40  57  69  56
    14  17  22  29  51  87  80  62
    18  22  37  58  68 109 103  77
    24  35  55  64  81 104 113  92
    49  64  78  87 103 121 120 101
    72  92  95  98 112 100 103  99

If a particular coded frame specifies a quality index of 54. Element 54
of the dc_scale_factor table is 20, thus:

                               16 * 20
  DC coefficient quantizer  =  -------  =  3
                                 100

Element 54 of the ac_scale_factor table is 24. The AC coefficient
quantizers are each scaled using this factor, e.g.:

    11 * 24
    -------   =  2
      100

    100 * 24
    --------  =  24
      100

[not complete; still need to explain how these quantizers are saturated
and scaled with respect to the DCT process]


Hilbert Coding Pattern
----------------------
VP3 uses a Hilbert pattern to code fragments within a superblock. A
Hilbert pattern is a recursive pattern that can grow quite complicated.
The coding pattern that VP3 uses is restricted to this pattern subset,
where each fragment in a superblock is represented by a 'X':

      X -> X    X -> X
           |    ^
           v    |
      X <- X    X <- X
      |              ^
      v              |
      X    X -> X    X
      |    ^    |    ^
      v    |    v    |
      X -> X    X -> X

As an example of this pattern, consider a plane that is 256 samples wide
and 64 samples high. Each fragment row will be 32 fragments wide. The
first superblock in the plane will be comprised of these 16 fragments:

   0   1   2   3  ...  31
  32  33  34  35  ...  63
  64  65  66  67  ...  95
  96  97  98  99  ... 127

The order in which these 16 fragments are coded is:

   0 |  0  1  14 15
  32 |  3  2  13 12
  64 |  4  7   8 11
  96 |  5  6   9 10

All of the image coding information, including the block coding status
and modes, the motion vectors, and the DCT coefficients, are all coded
and decoded using this pattern. Thus, it is rather critical to have the
pattern and all of its corner cases handled correctly. In the above
example, if the bottom row and left column were not present due to the
superblock being in a corner, the pattern proceeds as if the missing
fragments were present, but the missing fragments are omitted in the
final coding list. The coding order would be:

  0, 1, 2, 3, 4, 7, 8, 13, 14


Unpacking The Block Coding Information
--------------------------------------
After unpacking the frame header, the decoder unpacks the block coding
information. The only information determined in this phase is whether a
particular superblock and its fragments are coded in the current frame
or unchanged from the previous frame. The actual coding method is
determined in the next phase.

If the frame is a golden frame then every superblock, macroblock, and
fragment is marked as coded.

If the frame is an interframe, then the block coding information must be
decoded. This is the phase where a decoder will build a list of coded
fragments for which coding mode, motion vector, and DCT coefficient data
must be decoded.

First, a list of partially-coded superblocks is unpacked from the
stream. This list is coded as a series of variable-length run length
codes (VLRLC). First, the code is initialized by reading the next bit in
the stream. Then, while there are still superblocks remaining in the
list, fetch a VLC from the stream according to this table:

  Codeword                Run Length
  0                       1
  10x                     2-3
  110x                    4-5
  1110xx                  6-9
  11110xxx                10-17
  111110xxxx              18-33
  111111xxxxxxxxxxxx      34-4129

For example, a VLC of 1101 represents a run length of 5. If the VLRLC
was initialized to 1, then the next 5 superblocks would be set to 1,
indicating that they are partially coded in the current frame. Then the
bit value is toggled to 0, another VLC is fetched from the stream and
the process continues until each superblock has been marked either
partially coded (1) or not (0).

If any of the superblocks were marked as not partially coded in the
previous step, then a list of fully-coded superblocks is unpacked next
using the same VLRLC as the list of partially-coded superblocks.
Initialize the VLRLC with the next bit in the stream. For each
superblock that was not marked as partially coded, mark it with either a
0 or 1 according to the current VLRLC. By the end of this step, each
superblock will be marked as either not coded, partially coded, or fully
coded.

Let's work through an example with an image frame that is 256x64 pixels.
This means that the Y plane contains 4x2 superblocks and each of the C
planes contains 2 superblocks each. The superblocks are numbered as
follows:

   Y:  0  1  2  3     U:  8  9
       4  5  6  7     V: 10 11

This is the state of the bitstream:

  1100011001101

Which is interpreted as:

 initial   2 1's   1 0   4 1's  5 0's
    1       100     0     1100   1101

Superblocks 0-1 and 3-6 are marked as partially coded. Since there were
blocks that were not marked, proceed to unpack the list of fully-coded
superblocks. This is the state of the bitstream:

  1101101

Which is interpreted as:

 initial  3 1's  3 0's
    1      101    100

Superblocks 2, 7, and 8 are marked as fully coded while superblocks 9,
10, and 11 are marked as not coded. 

If any of the superblocks were marked as partially coded, the next data
in the bitstream will define which fragments inside each partially-coded
superblock are coded. This is the first place where the Hilbert pattern
comes into play.

For each partially-coded superblock, iterate through each fragment
according to the Hilbert pattern. Use the VLRLC method, only with a
different table, to determine which fragments are coded. The VLRLC table
for fragment coding runs is:

  Codeword                Run Length
  0x                      1-2
  10x                     3-4
  110x                    5-6
  1110xx                  7-10
  11110xx                 11-14
  11111xxxx               15-30

Continuing with the contrived example, superblocks 0 and 1 are both
partially coded. This is the state of the bitstream:

  0011001111010001111010...(not complete)

Which is interpreted as:
 initial  2 0's  3 1's  13 0's   1 1   13 0's
    0      01     100   1111010   00   1111010 ...

This indicates that fragments 2-4 in superblock 0 are coded, while
fragments 0, 1, and 5-15 are not. Note that the run of 12 0's cascades
over into the next fragment, indicating that fragment 0 of superblock 1
is not coded. Fragment 1 of superblock 1 is coded, while the rest of the
superblock's fragments are not coded. The example ends there (a real
bitstream should have enough data to describe all of the partially-coded
superblocks). Superblock 2 is fully coded which means all 16 fragments
are coded. Thus, superblocks 0-2 have the following coded fragments:

  0 |  x  x  x  x    x  x  x  x    0  1 14 15
 32 |  3  2  x  x    x  2  x  x    3  2 13 12
 64 |  4  x  x  x    x  x  x  x    4  7  8 11
 96 |  x  x  x  x    x  x  x  x    5  6  9 10

This is a good place to generate the list of coded fragment numbers for
this frame. In this case, the list will begin as:

  33 32 64 37 8 9 41 40 72 104 105 73 ...

and so on through the remaining 8 fragments of superblock 2 and onto the
fragments for the remaining superblocks that are either fully or
partially coded.


Unpacking The Macroblock Coding Mode Information
------------------------------------------------
After unpacking the block coding information, the decoder unpacks the
macroblock coding mode information. This process is simple when
decoding a golden frame-- since the only possible decoding mode is INTRA,
no macroblock coding mode information is transmitted. However, in an
interframe, each coded macroblock is encoded with one of 8 methods:

0, INTER_NO_MV: 
  current fragment = 
    (fragment from previous frame @ same coordinates) +
    (DCT-encoded residual)

1, INTRA: 
  current fragment = DCT-encoded block, just like in a golden frame

2, INTER_PLUS_MV:
  current fragment =
    (fragment from previous frame @ (same coords + motion vector)) +
    (DCT-encoded residual)

3, INTER_LAST_MV:
  same as INTER_PLUS_MV but using the last motion vector decoded from
  the bitstream

4, INTER_PRIOR_LAST;
  same as INTER_PLUS_MV but using the second-to-last motion vector
  decoded from the bitstream

5, USING_GOLDEN:
  same as INTER_NO_MV but referencing the golden frame instead of
  previous interframe

6, GOLDEN_MV:
  same as INTER_PLUS_MV but referencing the golden frame instead of
  previous interframe

7, INTER_FOURMV:
  same as INTER_PLUS_MV except that each of the 4 Y fragments gets its
  own motion vector, and the U and V fragments share the same motion
  vector which is the average of the 4 Y fragment vectors

The MB coding mode information is encoded using one of 8 alphabets. The
first 3 bits of the MB coding mode stream indicate which of the 8
alphabets, 0..7, to use to decode the MB coding information in this frame.
The reason for the different alphabets is to minimize the number of bits
needed to encode this section of information. Each alphabet arranges the
coding modes in a different order, indexing the 8 modes into 8 index
slots. Index 0 is encoded with 1 bit (0), index 1 is encoded with 2 bits
(10), index 2 is encoded with 3 bits (110), and so on up to indices 6 and
7 which are encoded with 6 bits each (1111110 and 1111111, respectively):

  index   encoding
  -----   --------
    0      0
    1      10
    2      110
    3      1110
    4      11110
    5      111110
    6      1111110
    7      1111111

For example, the coding modes are arranged in alphabet 1 as follows:

  index   coding mode
  -----   -----------
    0     MODE_INTER_LAST_MV
    1     MODE_INTER_PRIOR_LAST
    2     MODE_INTER_PLUS_MV
    3     MODE_INTER_NO_MV
    4     MODE_INTRA   
    5     MODE_USING_GOLDEN,      
    6     MODE_GOLDEN_MV   
    7     MODE_INTER_FOURMV

This alphabet arrangement is designed for frames in which motion vectors
based off of the previous interframe dominate.

When unpacking MB coding mode information for a frame, the decoder first
reads 3 bits from the stream to determine the alphabet. In this example,
the 3 bits would be 001 to indicate alphabet 1. Consider this contrived
bitstream following the alphabet number:

  1010000011000011111110...
  
The bits are read as follows:

         10 10 0 0 0 0 110 0 0 0 1111111 0
  index:  1  1 0 0 0 0   2 0 0 0       7 0

This arrangement of indices translates to this series of coding modes:

  index   coding mode
  -----   -----------
    1     MODE_INTER_PRIOR_LAST
    1     MODE_INTER_PRIOR_LAST
    0     MODE_INTER_LAST_MV
    0     MODE_INTER_LAST_MV
    0     MODE_INTER_LAST_MV
    0     MODE_INTER_LAST_MV
    2     MODE_INTER_PLUS_MV
    0     MODE_INTER_LAST_MV
    0     MODE_INTER_LAST_MV
    0     MODE_INTER_LAST_MV
    7     MODE_INTER_FOURMV
    0     MODE_INTER_LAST_MV
  
There are 6 pre-defined alphabets. Consult Appendix B for the complete
alphabets. What happens if none of the 6 pre-defined alphabets fit? The
VP3 encoder can choose to use alphabet 0 which indicates a custom
alphabet. The 3-bit coding mode numbers for each index, 0..7, are stored
after the alphabet number in the bitstream. For example, the sequence:

  000 111 110 101 100 011 010 001 000

would indicate coding alphabet 0 (custom alphabet), index 0 corresponds to
coding mode 7 (INTER_FOURMV), index 1 corresponds to coding mode 6
(GOLDEN_MV), and so on down to index 7 which would correspond to coding
mode 0 (INTER_NO_MV).

There is one more possible alphabet: Alphabet 7. This alphabet is
reserved for when there is such a mixture of coding modes used in a frame
that using any variable-length coding mode would result in more bits than
a fixed-length representation. When alphabet 7 is specified, the decoder
reads 3 bits at a time from the bitstream, and uses those directly as the
macroblock coding modes.

To recap, this is the general algorithm for decoding macroblock coding
mode information:

  if (golden frame)
    all frames are intracoded, there is no MB coding mode information
  else
    read 3 bits from bitstream to determine alphabet
    if alphabet = 0
      this is a custom alphabet, populate index table with 8 3-bit coding
        modes read from bitstream
    foreach coded macroblock, unpack a coding mode:
      if alphabet = 7
        read 3 bits from the bitstream as the coding mode for the
          macroblock
      else
        read a VLC from the bitstream
        use the decoded VLC value to index into the coding mode alphabet
          selected for this frame and assign the indexed coding mode to
          this macroblock
  

Unpacking The Macroblock Motion Vectors
---------------------------------------
After unpacking the macroblock coding mode information, the decoder
unpacks the macroblock motion vectors. This phase essentially assigns a
motion vector to each of the 6 constituent fragments of any coded
macroblock that requires motion vectors.

If the frame is a golden frame then there is no motion compensation and
no motion vectors are encoded in the bitstream.

If the frame is an interframe, the next bit is read from the bitstream
to determine the vector entropy coding method used. If the coding method
is zero then all of the vectors will be unpacked using a VLC method. If
the coding method is 1 then all of the vectors will be unpacked using a
fixed length method.

The VLC unpacking method reads 3 bits from the bitstream. These 3 bits
comprise a number ranging from 0..7 which indicate the next action:

0, MV component = 0
1, MV component = 1
2, MV component = -1
3, MV component = 2, read next bit for sign
4, MV component = 3, read next bit for sign
5, MV component = 4 + (read next 2 bits), read next bit for sign
   range: (4..7, -4..-7)
6, MV component = 8 + (read next 3 bits), read next bit for sign
   range: (8..15, -8..-15)
7, MV component = 16 + (read next 4 bits), read next bit for sign
   range: (16..31, -16..-31)

The fixed length vector unpacking method simply reads the next 5 bits
from the bitstream, reads the next bit for sign, and calls the whole
thing a motion vector component. This gives a range of (-31..31), which
is the same range as the VLC method.

For example, consider the following contrived motion vector bitstream:

  000001011011111000...

The stream is read as:

  0  (000  010)  (110 111 1  100 0)

The first bit indicates the entropy method which, in this example, is
variable length as opposed to fixed length. The next 3 bits are 0 which
indicate a X MV component of 0. The next 3 bits are 2 which indicate a Y
MV component of -1. The first motion vector encoded in this stream is
(0, -1). The next 3 bits are 6 which indicate 8 + next 3 bits (7) with
another bit indicating sign (1 in this case, which is negative). Thus,
the X MV component is -15. The next 3 bits are 4 which indicate a Y MV
component of 3 with one more bit for the sign (0 is positive). So the
second motion vector encoded in this stream is (-15, 3).

As an example of the fixed-length entropy method, consider the following
contrived bitstream:

  1010101101010...

The stream is read as:

  1  01010 1  10101 0

The first bit indicates the fixed length entropy method. The first 5 bits
are 10 followed by a negative sign bit. The next 5 bits are 21 followed by
a positive sign bit. The first motion vector in this stream is (-10, 21).

During this phase of the decoding process, it is traditional to assign all
motion vectors for all coded macroblocks that require them, whether they
are unpacked from the motion vector bitstream or copied from previous
coded macroblocks. It is necessary to track the motion vectors for both
the previous macroblock as well as the next-to-last (prior) macroblock.
The general algorithm for this phase is as follows:

  foreach coded macroblock
    last MV = 0
    prior last MV = 0
    if coding mode = MODE_INTER_PLUS_MV or MODE_GOLDEN_MV
      read current MV pair from the bitstream and set all fragment motion
        vectors to that pair
      prior last MV = last MV
      last MV = current MV
    
    if coding mode = MODE_INTER_FOURMV
      read MV for first Y fragment in macroblock
      read MV for second Y fragment in macroblock
      read MV for third Y fragment in macroblock
      read MV for fourth Y fragment in macroblock
      set U & V fragment motion vectors to average of 4 Y vectors,
        calculated as follows:
        if sum of all 4 X motion components is positive, the X
        motion component for the U & V fragments is (sum + 2) / 4,
        otherwise, it is (sum - 2) / 4; repeat the same process for the
        Y components
      prior last MV = last MV
      last MV = MV for fourth Y fragment from this macroblock
      
    if coding mode = MODE_INTER_LAST_MV
      motion vectors for this macroblock are the same as last MV; note
        that in this case, the last MV remains the last MV and the prior
        last MV remains the prior last MV
      
    if coding mode = MODE_INTER_PRIOR_LAST
      motion vectors for this macroblock are the same as prior last MV
      prior last MV = last MV
      last MV = current MV (effectively, swap last and prior last vectors)


Unpacking The DCT Coefficients
------------------------------
After unpacking the macroblock motion vectors, the decoder unpacks the
fragment DCT coefficient data. Each coded fragment has 64 DCT 
coefficients. Some of the coefficients will be non-zero. Many of the 
coefficients will, or should be 0 as this is where the coding method
derives much of its compression.

During this phase, the decoder will be unpacking DCT coefficients, zero
runs, and end-of-block (EOB) codes. The decoder unpacks the the DC 
coefficients for all fragments, then all of the first AC coefficients, 
and so on until all of the 64 DCT coefficients are unpacked from the
bitstream.

To obtain the DCT coefficients, the decoder unpacks a series of VLCs
from the bitstream which turn into a series of tokens ranging from
0..31. Each of these tokens specifies which action to take next. VP3
defines 80 different 32-element histograms for VLC decoding:

  16 histograms for DC token decoding
  16 histograms for group 1 AC token decoding
  16 histograms for group 2 AC token decoding
  16 histograms for group 3 AC token decoding
  16 histograms for group 4 AC token decoding

The decoder fetches 4 bits from the bitstream that will be used to
select a DC histogram and 4 bits that will be used to select 4 AC
histograms, one for each AC group.

The meaning of each of the 32 possible tokens follows. 'EB' stands for
extra bits read from bitstream directly after the VLC token:

0, DCT_EOB_TOKEN
set the current block to EOB, meaning that the block is marked as being
fully unpacked

1, DCT_EOB_PAIR_TOKEN
set the next 2 blocks to EOB

2. DCT_EOB_TRIPLE_TOKEN
set the next 3 blocks to EOB

3, DCT_REPEAT_RUN_TOKEN
set the next (2 EBs + 4) blocks to EOB

4, DCT_REPEAT_RUN2_TOKEN
set the next (3 EBs + 8) blocks to EOB

5, DCT_REPEAT_RUN3_TOKEN
set the next (4 EBs + 16) blocks to EOB

6, DCT_REPEAT_RUN4_TOKEN
set the next (12 EBs) blocks to EOB

7, DCT_SHORT_ZRL_TOKEN
skip (3 EBs + 1) positions in the output matrix

8, DCT_ZRL_TOKEN
skip (6 EBs + 1) positions in the output matrix

9, ONE_TOKEN
output 1 as coefficient

10, MINUS_ONE_TOKEN
output -1 as coefficient

11, TWO_TOKEN
output 2 as coefficient

12, MINUS_TWO_TOKEN
output -2 as coefficient

13, 14, 15, 16, LOW_VAL_TOKENS
next EB determines coefficient sign; coeff = DCT_VAL_CAT2_MIN (3) +
(token - 13) (this gives a range of +/- 3..6)

17, DCT_VAL_CATEGORY3
next EB determines coefficient sign; coeff = DCT_VAL_CAT3_MIN (7) + next
EB (this gives a range of +/- 7..8)

18, DCT_VAL_CATEGORY4
next EB determines coefficient sign; coeff = DCT_VAL_CAT4_MIN (9) + next
2 EBs (this gives a range of +/- 9..12)

19, DCT_VAL_CATEGORY5
next EB determines coefficient sign; coeff = DCT_VAL_CAT5_MIN (13) +
next 3 EBs (this gives a range of +/- 13..20)

20, DCT_VAL_CATEGORY6
next EB determines coefficient sign; coeff = DCT_VAL_CAT6_MIN (21) +
next 4 EBs (this gives a range of +/- 21..36)

21, DCT_VAL_CATEGORY7
next EB determines coefficient sign; coeff = DCT_VAL_CAT7_MIN (37) +
next 5 EBs (this gives a range of +/- 37..68)

22, DCT_VAL_CATEGORY8
next EB determines coefficient sign; coeff = DCT_VAL_CAT8_MIN (69) +
next 9 EBs (this gives a range of +/- 69..580)

23, 24, 25, 26, 27, DCT_RUN_CATEGORY1
coefficient of +/- 1 preceded by a number of 0s; next EB determines sign
of coefficient; skip (token - 22) 0s in the output matrix before
placing the final coefficient (this gives a range of 1..5 0s)

28, DCT_RUN_CATEGORY1B
coefficient of +/- 1 preceded by a number of 0s; next EB determines sign
of coefficient; skip (next 2 EBs + 6) 0s in the output matrix before
placing the final coefficient (this gives a range of 6..9 0s)

29, DCT_RUN_CATEGORY1C
coefficient of +/- 1 preceded by a number of 0s; next EB determines sign
of coefficient; skip (next 3 EBs + 10) 0s in the output matrix before
placing the final coefficient (this gives a range of 10..17 0s)

30, DCT_RUN_CATEGORY2
coefficient of +/- 2..3 preceded by a single zero; next EB determines
sign of coefficient; coefficient = (next EB + 2)

31, DCT_RUN_CATEGORY2B (not specifically named in VP3 source)
coefficient of +/- 2..3 preceded by 2 or 3 0s; next EB determines
sign of coefficient; coefficient = (next EB + 2); skip (next EB + 2) 0s
before placing coefficient in output matrix

Note: EOB runs can, and often do, cross threshold stages and plane
boundaries. For example, a decoder may have decoded all of the AC #2
coefficients for all fragments and still have an EOB run of 2. That
means that during the AC #3 decode process, the first 2 coded fragments
that are not already EOB will be set to EOB.

Let's work through a highly contrived example to illustrate the
coefficient decoding process.



[not finished]




When the decoder is finished unpacking the DCT coefficients, the entire
encoded VP3 frame bitstream should be consumed.


Reversing The DC Prediction
---------------------------
Now that all of the DCT coefficient data has been unpacked, the DC
coefficients need to be fully reconstructed before the IDCT can be
performed.

VP3 uses a somewhat involved process for DC prediction which uses up to
four DC coefficients from surrounding fragments. For each fragment to be
transformed with the IDCT, the DC coefficient is predicted from weighted
sum of the DC coefficients in the left (l), up-left (ul), up (u), and
up-right (ur) fragments, if they are coded (not unchanged from the
previous frame) in a compatible frame (current, previous, or golden).

In a golden frame, the prediction is quite straightforward since all
fragments will be coded. A fragment's DC prediction will fall into 1 of
5 groups:

     abbbbbbbbb
     cdddddddde
     cdddddddde
     cdddddddde
     cdddddddde

* Group a is the top left corner fragment. There is nothing to predict
from. This DC coefficient has a lot of energy and requires many bits to
code.

* Group b is the remainder of the top row of fragments. These fragments
can only predict from the left fragment.

* Group c is the left column of fragments, not including the top left
fragment. These fragments have the top and top-right fragments from
which to predict.

* Group d is the main body of fragments. These fragments have access to
all 4 predictors.

* Group e is the right column of fragments, not including the top right
fragment. These fragments can predict from the left, up-left and up
fragments.

The process of reversing prediction for interframes grows more complex.
First, the decoder must evaluate which candidate fragments (l, ul, u, or
ur) are available for as predictors. Then, it can only use fragments
that are coded within the same frame (current, previous, or golden).
Further, there are auxiliary predictors for each frame type that are
initialized to 0 at the start of each video frame decode operation. The
decoder falls back on these auxiliary predictors when it can not find
any valid candidate predictors for the current fragment.

To work through some examples, consider the following notation, e.g.:

  ul-C = up-left fragment, coded in the current frame
   u-P = up fragment, coded as a motion residual from the previous frame
  ur-C = up-right fragment, coded in the current frame
   l-G = left fragment, coded as a motion residual from the golden frame   
   x-P = current fragment where DC prediction is being performed, coded
         as a motion residual from the previous frame

This is a simple case:

   ul-C   u-C  ur-C
    l-C   x-C

The current fragment predicts from all four of the candidate fragments
since they are coded in the same frame. 

   ul-P   u-C  ur-C
    l-P   x-P

The current fragment predicts from the left and up-left fragments.

   ul-C   u-P  ur-G
    l-P   x-G

The current fragment predicts from the up-right fragment.

   ul-C   u-C  ur-C
    l-C   x-G

The current fragment does not predict from any of the candidate
fragments since the current fragment is a motion residual from the
golden frame. Rather, add the auxiliary golden frame predictor to the
current fragment's DC coefficient. Save the new DC coefficient as the
new golden frame auxiliary DC predictor.

If the decoder only finds one valid candidate predictor, then it is used
by itself. When the decoder finds multiple valid candidate fragments
from which to predict DC, it applies a weighting function to the
surrounding fragments' DC coefficients. The following table presents all 
16 possible combinations of available/not available predictors and what 
to do in each case:

    ul   u  ur   l
    --  --  --  --
     0   0   0   0    no predictors available:
                        use the last predictor saved for the frame type
                        (either intra, inter, or golden)

     0   0   0   1    left predictor available:
                        pred = l.dc

     0   0   1   0    up-right predictor available:
                        pred = ur.dc

     0   0   1   1    up-right, left predictors available:
                        pred = (53 * ur.dc) + (75 * l.dc)
                               --------------------------
                                         128

     0   1   0   0    up predictor available:
                        pred = u.dc

     0   1   0   1    up, left predictors available:
                        pred = (u.dc + l.dc)
                               -------------
                                     2

     0   1   1   0    up, up-right predictors available:
                        discard up-right predictor
                        pred = u.dc

     0   1   1   1    up, up-right, left predictors available:
                        discard up predictor
                        pred = (53 * ur.dc) + (75 * l.dc)
                               --------------------------
                                         128

     1   0   0   0    up-left predictor available:
                        pred = ul.dc

     1   0   0   1    up-left, left predictors available:
                        discard up-left predictor
                        pred = l.dc

     1   0   1   0    up-left, up-right predictors available:
                        pred = (ul.dc + ur.dc)
                               ---------------
                                      2

     1   0   1   1    up-left, up-right, left predictors available:
                        discard up-left predictor
                        pred = (53 * ur.dc) + (75 * l.dc)
                               --------------------------
                                         128

     1   1   0   0    up-left, up predictors available:
                        discard up-left
                        pred = u.dc

     1   1   0   1    up-left, up, left predictors available:
                        pred = (-26 * ul.dc + 29 * u.dc + 29 * l.dc)
                               -------------------------------------
                                                 32

     1   1   1   0    up-left, up, up-right predictors available:
                        pred = (3 * ul.dc + 10 * u.dc + 3 * ur.dc)
                               -----------------------------------
                                                16

     1   1   1   1    all 4 predictors available:
                        discard up-right predictor
                        pred = (-26 * ul.dc + 29 * u.dc + 29 * l.dc)
                               -------------------------------------
                                                 32

Note that this final prediction case ([ul u l]) risks outranging. The
difference of the predicted DC is checked against u.dc, l.dc, and ul.dc,
in that order, and if the difference is greater than 128 in any case,
the predictor is assigned as that DC coefficient. In pseudocode:

  if (ABSOLUTE_VALUE(pred - u.dc) > 128)
    pref = u.dc
  else if (ABSOLUTE_VALUE(pred - l.dc) > 128)
    pref = l.dc
  else if (ABSOLUTE_VALUE(pred - ul.dc) > 128)
    pref = ul.dc

The predicted value is, at long last, added to the fragment's decoded DC
coefficient. Finally, the new DC coefficient is saved as the frame
type's auxiliary predictor. For example, if this fragment is coded as a
motion residual from the previous frame, save the fragment's DC
coefficient as the previous frame auxiliary predictor.


[still need to mention precise rounding considerations, a.k.a, the
HIGHTBITDUPPED() macro]



Reconstructing The Frame
------------------------
rough outline:
  - foreach fragment:
    - if motion vector
      - copy motion fragment from appropriate frame into current frame
        (don't forget to account for unrestricted motion vectors)
    - dequantize fragment coefficients
    - run coefficients through inverse DCT
    - if INTRA coded fragment
      - output transformed coefficients
    - else
      - apply transformed residual to motion fragment
    
[not finished]


Theora Specification
--------------------
The Theora project leverages the VP3 codec into a new video coding
system. The algorithm and bitstream format are the same as VP3 with a
few minor differences:

1) The frame orientation is reversed-- VP3 is coded from bottom to top
while Theora video is coded from top to bottom.
[nope-- only true in the first few alpha releases; final Theora spec will
be upside-down, the same as VP3]

2) Variable histograms-- VP3 uses a hardcoded set of histograms for DCT
coefficient coding (described in section "Unpacking The DCT
Coefficients"). Theora packs the histogram information in the header of
the transport format (which is meant to be Ogg, but can probably be
coerced into a variety of other multimedia container formats).

3) Variable quantization-- As with the histograms, Theora codes the
quantization tables and quality thresholds (described in section
"Initializing The Quantization Matrices") into the header.

4) [special VLRLC case for encoding unusually large runs of blocks;
necessary for HD resolutions]

[still need coding format of histogram and quantizer information]


Appendix A: VP31 Quantization Matrices And Scale Factors
--------------------------------------------------------
The following quantization matrices and scale factor tables are hardcoded
into the VP31 coding standard. These tables can vary according to the
setup information transported along with a Theora file.

Base quantization matrix for golden frame Y fragments (note that this 
is the same as JPEG):

    16  11  10  16  24  40  51  61
    12  12  14  19  26  58  60  55
    14  13  16  24  40  57  69  56
    14  17  22  29  51  87  80  62
    18  22  37  58  68 109 103  77
    24  35  55  64  81 104 113  92
    49  64  78  87 103 121 120 101
    72  92  95  98 112 100 103  99


Base quantization matrix for golden frame C fragments (note that this 
is the same as JPEG):
    
    17  18  24  47  99  99  99  99
    18  21  26  66  99  99  99  99
    24  26  56  99  99  99  99  99
    47  66  99  99  99  99  99  99
    99  99  99  99  99  99  99  99
    99  99  99  99  99  99  99  99
    99  99  99  99  99  99  99  99
    99  99  99  99  99  99  99  99


Base quantization matrix for interframe Y and C fragments:

    16  16  16  20  24  28  32  40
    16  16  20  24  28  32  40  48
    16  20  24  28  32  40  48  64
    20  24  28  32  40  48  64  64
    24  28  32  40  48  64  64  64
    28  32  40  48  64  64  64  96
    32  40  48  64  64  64  96 128
    40  48  64  64  64  96 128 128


DC coefficient scale factor table:

   220 200 190 180 170 170 160 160
   150 150 140 140 130 130 120 120
   110 110 100 100  90  90  90  80
    80  80  70  70  70  60  60  60
    60  50  50  50  50  40  40  40
    40  40  30  30  30  30  30  30
    30  20  20  20  20  20  20  20
    20  10  10  10  10  10  10  10


AC coefficient scale factor table:

   500 450 400 370 340 310 285 265
   245 225 210 195 185 180 170 160
   150 145 135 130 125 115 110 107
   100  96  93  89  85  82  75  74
    70  68  64  60  57  56  52  50
    49  45  44  43  40  38  37  35
    33  32  30  29  28  25  24  22
    21  19  18  17  15  13  12  10


Appendix B: Macroblock Coding Mode Alphabets
--------------------------------------------
These are the 6 pre-defined alphabets used to decode macroblock coding
mode information:

Alphabet 1:
  index   coding mode
  -----   -----------
    0     MODE_INTER_LAST_MV
    1     MODE_INTER_PRIOR_LAST
    2     MODE_INTER_PLUS_MV
    3     MODE_INTER_NO_MV
    4     MODE_INTRA   
    5     MODE_USING_GOLDEN,      
    6     MODE_GOLDEN_MV   
    7     MODE_INTER_FOURMV

Alphabet 2:
  index   coding mode
  -----   -----------
    0     MODE_INTER_LAST_MV
    1     MODE_INTER_PRIOR_LAST
    2     MODE_INTER_NO_MV
    3     MODE_INTER_PLUS_MV
    4     MODE_INTRA
    5     MODE_USING_GOLDEN
    6     MODE_GOLDEN_MV
    7     MODE_INTER_FOURMV

Alphabet 3:
  index   coding mode
  -----   -----------
    0     MODE_INTER_LAST_MV
    1     MODE_INTER_PLUS_MV
    2     MODE_INTER_PRIOR_LAST
    3     MODE_INTER_NO_MV
    4     MODE_INTRA
    5     MODE_USING_GOLDEN
    6     MODE_GOLDEN_MV
    7     MODE_INTER_FOURMV
    
Alphabet 4:
  index   coding mode
  -----   -----------
    0     MODE_INTER_LAST_MV
    1     MODE_INTER_PLUS_MV
    2     MODE_INTER_NO_MV
    3     MODE_INTER_PRIOR_LAST
    4     MODE_INTRA
    5     MODE_USING_GOLDEN
    6     MODE_GOLDEN_MV
    7     MODE_INTER_FOURMV

Alphabet 5:
  index   coding mode
  -----   -----------
    0     MODE_INTER_NO_MV
    1     MODE_INTER_LAST_MV
    2     MODE_INTER_PRIOR_LAST
    3     MODE_INTER_PLUS_MV
    4     MODE_INTRA
    5     MODE_USING_GOLDEN
    6     MODE_GOLDEN_MV
    7     MODE_INTER_FOURMV
    
Alphabet 6:
  index   coding mode
  -----   -----------
    0     MODE_INTER_NO_MV
    1     MODE_USING_GOLDEN
    2     MODE_INTER_LAST_MV
    3     MODE_INTER_PRIOR_LAST
    4     MODE_INTER_PLUS_MV
    5     MODE_INTRA
    6     MODE_GOLDEN_MV
    7     MODE_INTER_FOURMV


Appendix C: DCT Coefficient VLC Tables
--------------------------------------
- VP31 tables are hardcoded
- Theora tables are transported with video stream

[not finished]


Appendix D: The VP3 IDCT
------------------------

[not finished]


Acknowledgements
----------------
Thanks to Michael Niedermayer (michaelni at gmx dot at) for peer review,
corrections, and recommendations for improvement.

Dan Miller (dan at on2 dot com) for clarifications on pieces of the
format.

Timothy B. Terriberry (tterribe at vt dot edu) for clarification about the
differences between VP3 and Theora, detailed explanation of motion 
vector mechanics.


References
----------
Tables necessary for decoding VP3:
http://mplayerhq.hu/cgi-bin/cvsweb.cgi/~checkout~/ffmpeg/libavcodec/vp3data.h?content-type=text/x-cvsweb-markup&cvsroot=FFMpeg

Official VP3 site:
http://www.vp3.com/

Theora, based on VP3:
https://www.theora.org/

On2, creators of the VP3 format:
https://www.on2.com/


ChangeLog
---------
v0.5: December 8, 2004
- reworked section "Reversing The DC Prediction" to include a tabular 
representation of all 16 prediction modes

v0.4: March 2, 2004
- renamed and expanded section "Initializing The Quantization Matrices"
- outlined section "Reconstructing The Frame"
- moved Theora Differences Appendix to its own section entitled "Theora
Specification"
- added Appendix: Quantization Matrices And Scale Factors
- added Appendix: DCT Coefficient VLC Tables

v0.3: February 29, 2004
- expanded section "Unpacking The Macroblock Coding Mode Information"
- expanded section "Unpacking The Macroblock Motion Vectors"
- added Appendix: Macroblock Coding Mode Alphabets

v0.2: October 9, 2003
- expanded section "Reversing the DC Prediction"
- added Appendix: Theora Differences

v0.1: June 17, 2003
- initial release, nowhere near complete
