Indeo 5

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Revision as of 01:39, 15 August 2006 by FyKnight (talk | contribs) (Block header: might or might not be huffman but is definitely a vlc)
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General description

Frame layout

The general indeo5 frames layout is composed of one global header, followed by the content of the three YUV plans.

In this document, the global header is split into 3 parts:

  • Frame header: describe the kind of frame (I/P/B)
  • GOP header: some data which is true for all the frame in this GOP (present only in the first (I) frame of the GOP)
  • More header: some more data which is true only for this single frame

Each YUV plan begin with a Plan header, containing values which are valid only for this single plan.

Encoding

This codec is based on the slant transform. Other used standard techniques are huffman coding and motion compensation.

Conventions

Headers are described in some tables. Each row of those tables describes a value which may be read from the frame. Those tables and rows are presented in the order of appearance in the frame.

Here are the meaning of each columns:

  • size: The size of this value in bits. Bits are counted in LSB to MSB order. As an example, with the byte 01110000b, reading 3 bits then 5 bits will return 000b then 01110b. Reading more than 8 bits thus reads as a little-endian value. Think of the get_bits function as filling up the return value from its LSB, using the bits from each byte starting from their LSB.
  • name: Kind of variable name, used to reference the value. When a value is named valueX, it generally means we don't know it's purpose. Lines named alignmentX means that bits reader need to skip bits until next byte boundary.
  • condition: The value is present in the frame only if this condition is matched. No condition means that the value is always present.
  • nb times: How many times the value is repeated.
  • comments: Some details about the content of the value. It may also explain that a value is repeated until a certain condition is reached.


Headers

Frame header

size name condition nb times comments
5 const1 = 0x1F always
3 frame_flags
  • frame_flags & 0x01 => backward predictive
  • frame_flags & 0x02 => forward predictive
  • frame_flags & 0x04 => null frame

common values:

  • 0 => I frame
  • 1 => P frame
  • 3 => B frame
  • 6 and 7 are illegal
8 id_in_gop frame number in GOP (0 for I frame)

null frames don't contain anything else than this header.

GOP header

This header is present in I frames only. The values in this header are valid during the whole GOP starting at this frame.

size name condition nb times comments
8 gh_flags gh_flags & 0x02 => YV12 (default YVU9)
16 value1 gh_flags & 0x01 discard in decoding
32 value2 gh_flags & 0x20
2 value3 gh_flags & 0x40
3 value4 legal values are 0, 1, 2 and 6
4 res_id see Resolution table
13 height res_id == 15 frame height
13 width frame width
6 value5 2*n (n == 1 always?)
2 value6 value5 >> 3 need to be = 0
4 value7 gh_flags & 0x08
24 value8
?? alignment1 align bits reader on next byte
24 value9
16 value10 value9 & 0x800000 loops while value10 & 0x8000 (probably some kind of VLC ?)

More header

This header is present in all kinds of frame except null.

size name condition nb times comments
8 mh_flags
24 frame_size mh_flags & 0x01 tolal size of frame data
16 value11 mh_flags & 0x10
8 counter1 mh_flags & 0x20 this whole block loops while counter1 != 0
8 value12 counter1
3 value13 mh_flags & 0x40
4 counter2 value13 == 7
4 value14 counter2
3 value15
?? alignment2 align bits reader on next byte

Plane header

This header is present at the beginning of every plane.

size name condition nb times comments
8 ph_flags
24 plan_size mh_flags & 0x80 tolal size of plan data
8 counter3 ph_flags & 0x10 must be < 0x3E
8 value16 counter3
8 value17
3 value18 ph_flags & 0x40
3 table1_id ph_flags & 0x80 see Table 1
4 counter4 table1_id == 7 used instead of Table1
4 value19 counter4
1 value20
16 value21 value20
5 value22
?? alignment3 align bits reader on next byte
8 counter5 ph_flags & 0x20 all of this is repeated as long as value23 is true
8 skip1 counter5
1 value23
?? alignment4 align bits reader on next byte


Planes

Plane data

Needs more analysis. Follows plane header.

size name condition nb times comments
1 value24
1 value25 ! value24 plan_data_size = value25
8 value26 value25 == 1 plan_data_size = value26
24 value27 value26 == 0xFF plan_data_size = value27

Block header

One of these for each block after plane data. Conditions/loops not yet investigated.

size name condition nb times comments
1 value28
vlc value29
1 value30
4 value31
1 value32
vlc value33
vlc value34
vlc value35

Block data

Follows block header. One of these for each plane that has 'plane_flags&1'. The variable 'run' starts at -1 and carries over from one coded plane to the next. I don't really know what I'm doing with vlc's so the names might not be correct... but their functional description is.

size name condition nb times comments
vlc vlc while (vlc != vlcEnd)
vlc run_add vlc == vlcEsc run += run_add + 1
vlc lindex_lo lindex = lindex_lo | (lindex_hi<<6)
vlc lindex_hi


If vlc != vlcEsc then run_add is run_table[vlc], lindex is lindex_table[vlc].

After each loop, stored coefficient is: block[ scan_table[run] ] = level_tables[run][lindex-1].

The values of vlcEnd and vlcEsc are variable, as is the vlc table itself. However, they are all fixed for all the planes in the same block. run_table, lindex_table, scan_table are also fixed-per-block. level_tables is per-plane.

Annexes

Resolution table

res_id 0 1 2 3 4 5 6 7 8 9 10 11
width 640 320 160 704 352 352 176 240 640 704 80 88
height 480 240 120 224 240 288 144 180 240 240 60 72

Table 1

table1_id 0 1 2 3 4 5 6 default
counter4 10 11 12 13 11 13 13 9
value19
1
2
3
4
4
7
5
5
4
1
2
3
4
4
4
7
5
4
3
3
2
2
4
5
5
5
5
6
4
4
3
1
1
3
3
4
4
5
6
6
4
4
3
2
1
1
3
4
4
5
5
5
6
5
4
2
2
3
4
5
5
5
5
6
4
3
3
2
1
1
3
4
5
5
5
6
5
4
3
3
2
1
1
3
4
4
5
5
5
6
5
5

default is used when !(ph_flags & 0x80)

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