#LyX 1.3 created this file. For more info see http://www.lyx.org/
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\layout Title
\noindent 
Description of the 4XM Video Codec
\layout Author

by Michael Niedermayer <michaelni@gmx.at>
\layout Standard


\begin_inset LatexCommand \tableofcontents{}

\end_inset 


\layout Section

Introduction
\layout Standard

The 4XM video codec is a mixture between a very simplified JPEG scheme and
 rectangular block based fullpel motion compensation with DC difference
 coding.
 The codec uses 4:2:0 YCbCr colorspace for the JPEG part but converts it
 to RGB16 before using it for motion compensation.
\layout Standard

The latest version of this document is available at http://www.mplayerhq.hu/~micha
el/4xm.{lyx,txt,html,ps}
\layout Standard

4XM video is normally encapsulated in the proprietary 
\begin_inset LatexCommand \url[4XM format]{http://www.pcisys.net/~melanson/codecs/4xm-format.txt}

\end_inset 

.
\layout Standard
\noindent 
This document assumes familiarity with mathematical and coding concepts
 such as the discrete cosine transform, quantization, YCbCr colorspaces,
 macroblocks, and variable length codes (VLCs).
 A familiarity with the standard JPEG coding method is also helpful.
\layout Section

Terms and Definitions
\layout List
\labelwidthstring 00.00.0000

AC Any DCT coefficient for which the frequency in one or both dimensions
 is non-zero.
\layout List
\labelwidthstring 00.00.0000

DC The DCT coefficient for which the frequency is zero in both dimensions
\layout List
\labelwidthstring 00.00.0000

(I)DCT (Inverse) Discrete Cosine Transform
\layout List
\labelwidthstring 00.00.0000

VLC Variable Length Code
\layout List
\labelwidthstring 00.00.0000

AAN\SpecialChar ~
IDCT IDCT algorithm by Arai, Agui, and Nakajima
\layout List
\labelwidthstring 00.00.0000

JPEG Joint Photographic Expert Group
\layout Section

High-level Description
\layout Standard

The 4XM video coding method embodies 3 types of frames: I-frames, P-frames,
 and C-frames.
 I-frames are intraframes and stand on their own.
 P and C-frames are Interframes.
\layout Subsection

I-Frame
\layout Standard

I-frames are practically the same as JPEG images.
 Differences include just a single Huffman table, different headers, and
 a bitstream split into 2 partitions with one partition written in 32-bit
 byteswapped order.
 There are also no parameters for rate or quality control.
\layout Standard

The picture is split into macroblocks which are coded left->right, top->bottom.
\layout Subsubsection

Macroblock
\layout Standard

16x16 luma + 8x8 chroma as 4 8x8 luma blocks and 2 8x8 chroma blocks :
\layout Standard

Y:
\begin_inset  Tabular
<lyxtabular version="3" rows="2" columns="2">
<features>
<column alignment="center" valignment="top" leftline="true" width="0">
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard

1
\end_inset 
</cell>
</row>
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

2
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard

3
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 

 Cb:
\begin_inset  Tabular
<lyxtabular version="3" rows="1" columns="1">
<features>
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
<row topline="true" bottomline="true">
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\begin_inset Text

\layout Standard

4
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 

 Cr:
\begin_inset  Tabular
<lyxtabular version="3" rows="1" columns="1">
<features>
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard

5
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 


\layout Subsubsection

DC Prediction
\layout Standard

DC values are predicted from the last coded block.
 The initial prediction value used for the first top left luma block is
 0.
 No special handling is done between luma and chroma blocks or at the right
 border, so the DC value of the rightmost 8x8 Cr block of the first row
 will be used as the predictor for the first/top-left 8x8 luma block of
 the second MB row.
 
\layout Subsubsection

Dequantization and IDCT
\layout Standard

4XM uses an AAN IDCT with the premultiply table merged with the quantization
 table.
 The quantization table is the default luma table used in JPEG.
\layout Paragraph

default luma quantization table used in JPEG
\layout LyX-Code

16,  11,  10,  16,  24,  40,  51,  61,
\layout LyX-Code

12,  12,  14,  19,  26,  58,  60,  55,
\layout LyX-Code

14,  13,  16,  24,  40,  57,  69,  56,
\layout LyX-Code

14,  17,  22,  29,  51,  87,  80,  62,
\layout LyX-Code

18,  22,  37,  56,  68, 109, 103,  77,
\layout LyX-Code

24,  35,  55,  64,  81, 104, 113,  92,
\layout LyX-Code

49,  64,  78,  87, 103, 121, 120, 101,
\layout LyX-Code

72,  92,  95,  98, 112, 100, 103,  99
\layout Paragraph

AAN premultiply table
\layout LyX-Code

16384, 22725, 21407, 19266, 16384, 12873,  8867,  4520,
\layout LyX-Code

22725, 31521, 29692, 26722, 22725, 17855, 12299,  6270,
\layout LyX-Code

21407, 29692, 27969, 25172, 21407, 16819, 11585,  5906,
\layout LyX-Code

19266, 26722, 25172, 22654, 19266, 15137, 10426,  5315,
\layout LyX-Code

16384, 22725, 21407, 19266, 16384, 12873,  8867,  4520,
\layout LyX-Code

12873, 17855, 16819, 15137, 12873, 10114,  6967,  3552,
\layout LyX-Code

8867 , 12299, 11585, 10426,  8867,  6967,  4799,  2446,
\layout LyX-Code

4520 ,  6270,  5906,  5315,  4520,  3552,  2446,  1247
\layout Paragraph

merged table used in 4XM
\layout LyX-Code

16, 15, 13, 19, 24, 31, 28, 17,
\layout LyX-Code

17, 23, 25, 31, 36, 63, 45, 21,
\layout LyX-Code

18, 24, 27, 37, 52, 59, 49, 20,
\layout LyX-Code

16, 28, 34, 40, 60, 80, 51, 20,
\layout LyX-Code

18, 31, 48, 66, 68, 86, 56, 21,
\layout LyX-Code

19, 38, 56, 59, 64, 64, 48, 20,
\layout LyX-Code

27, 48, 55, 55, 56, 51, 35, 15,
\layout LyX-Code

20, 35, 34, 32, 31, 22, 15,  8,
\layout Standard

This is simply the element-wise product of the quantization table and the
 AAN table divided by 
\begin_inset Formula $2^{16}$
\end_inset 

.
\layout Standard

4XM's AAN IDCT uses (a*const)>>16 to approximate multiplications and simply
 shifts the transformed result 16 bits to the right.
 The scaled constants are:
\layout Standard


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<column alignment="center" valignment="top" leftline="true" width="0">
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

exact
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard

scaled constant
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

1.082392200...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard


\family roman 
\series medium 
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70936
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1
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.
\family roman 
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\color none
414213562
\family default 
\series default 
\shape default 
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\emph default 
\bar default 
\noun default 
\color default
...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard


\family roman 
\series medium 
\shape up 
\size normal 
\emph off 
\bar no 
\noun off 
\color none
92682
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

1.847759065...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard

121095
\end_inset 
</cell>
</row>
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

2.613125930...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" rightline="true" usebox="none">
\begin_inset Text

\layout Standard


\family roman 
\series medium 
\shape up 
\size normal 
\emph off 
\bar no 
\noun off 
\color none
171254
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</cell>
</row>
</lyxtabular>

\end_inset 


\layout Standard

which are simply the exact constants multiplied by 
\begin_inset Formula $2^{16}$
\end_inset 

 and rounded to the nearest integer.
\layout Standard

see 
\begin_inset LatexCommand \htmlurl[4xm.c]{http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/ffmpeg/ffmpeg/libavcodec/4xm.c?rev=HEAD&content-type=text/vnd.viewcvs-markup}

\end_inset 


\layout Subsubsection

YCbCr 4:2:0 -> RGB565 colorspace transform
\layout Standard

Chroma is first upsampled by sample replication / nearest neighbor scaling,
 so that the same Cb and Cr samples are used for each 2x2 Y samples
\layout Standard

R= (Y + Cr + 128)>>3
\layout Standard

G= (Y - ((Cb+Cr)>>1) + 128)>>2
\layout Standard

B= (Y + 2Cb + 128)>>3
\layout Standard

There is no check or protection against overflow, so values will wrap around
 if they are too large or small.
\layout Subsection

P-Frame
\layout Standard

A P-frame picture is split into blocks which are coded left->right, top->bottom.
 Each block contains 8x8 samples in RGB565 format (5 bits for red, 6 bits
 for green, 5 bits for blue).
 Each block can be recursively split into 2, down to 2x1/1x2 sized blocks.
\layout Standard

A P-frame block can be coded using 1 of 7 methods:
\layout Enumerate

motion compensated with 1 vector
\layout Enumerate

horizontally split in the middle
\layout Enumerate

vertically split in the middle
\layout Enumerate

skipped (block data copied from the frame before the last)
\newline 
Example: Intraframe, Interframe1, Interframe2, Interframe3
\newline 
a skipped block in Interframe3 will use the data from Interframe1; skipped
 blocks in Interframe1 are dissallowed as there is no source frame
\layout Enumerate

motion vector + DC difference (the 16-bit words of the DC and the source
 block are simply added, there is no special handling of overflows)
\layout Enumerate

DC only, the whole block is filled with the DC color
\layout Enumerate

hardcoded pixel values (left->right, top->bottom)
\layout Standard

Block splitting is only available if the resulting blocks are larger than
 1x2/2x1.
 Hardcoded pixel values are only available for 1x2/2x1 sized blocks
\layout Standard

Motion compensation assumes that the number of words (16-bit RGB565 pixels)
 per line is equal to the width, so that motion vectors which point right
 or left outside of the picture use the pixels from the other side.
 There is no subpel motion compensation or filtering which means that motion
 compensation can simply be done by copying the pixels from the motion block.
\layout Subsubsection

Motion Vector table
\layout Standard

See mv[256][2] at 
\begin_inset LatexCommand \htmlurl[4xm.c]{http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/ffmpeg/ffmpeg/libavcodec/4xm.c?rev=HEAD&content-type=text/vnd.viewcvs-markup}

\end_inset 

.
\layout Subsection

C-Frame
\layout Standard

A C-frame is essentially a partial P frame.
 It has all the same coding options but a different header.
\layout Section

Bitstream
\layout Standard

All 32-bit values are in little endian byte order.
\layout Subsection

I-Frame
\layout List
\labelwidthstring 00.00.0000

32bit 'ifrm'
\layout List
\labelwidthstring 00.00.0000

32bit chunk length
\layout List
\labelwidthstring 00.00.0000

32bit 0 (unknown)
\layout List
\labelwidthstring 00.00.0000

32bit bitstream size
\layout List
\labelwidthstring 00.00.0000

n\SpecialChar ~
byte bitstream
\layout List
\labelwidthstring 00.00.0000

32bit prefixstream size / 4
\layout List
\labelwidthstring 00.00.0000

32bit token_count
\layout List
\labelwidthstring 00.00.0000

n\SpecialChar ~
byte prefixstream
\layout Subsubsection

Prefix stream
\layout LyX-Code

start             8bit
\layout LyX-Code

end               8bit
\layout LyX-Code

do{
\layout LyX-Code

  for(i=start; i<=end; i++)
\layout LyX-Code

    frequency[i]  8bit
\layout LyX-Code

  start           8bit
\layout LyX-Code

  if(start==0) break;
\layout LyX-Code

  end             8bit
\layout LyX-Code

}
\layout LyX-Code

while(not 32bit aligned)
\layout LyX-Code

  0               8bit
\layout LyX-Code

for(i=0; i<token_count; i++)
\layout LyX-Code

  prefix[i]       prefix_vlc
\layout LyX-Code

256               prefix_vlc
\layout Standard

Note: The prefix_vlc are stored so that each aligned 32-bit word is stored
 in byteswapped order.
 This byteswapping is not done to the bitstream, just the prefix stream
\layout Standard

Frequency values which are not explicitly set are 0 except that frequency[256]=1.
 This is the 
\begin_inset Quotes eld
\end_inset 

end of picture
\begin_inset Quotes erd
\end_inset 

 code.
\layout Subsubsection

Macroblock
\layout Standard

A macroblock bitstream is simply a bitstream of 6 blocks.
\layout Subsubsection

Block
\layout LyX-Code

dc_prefix        prefix_vlc         prefix stream
\layout LyX-Code

dc_suffix        dc_prefix bits     bitstream
\layout LyX-Code

i=1;
\layout LyX-Code

while(i<64){
\layout LyX-Code

  ac_prefix      prefix_vlc         prefix_stream
\layout LyX-Code

  if(ac_prefix == 0xF0)
\layout LyX-Code

    i+=16;
\layout LyX-Code

  else if(ac_prefix == 0x00)
\layout LyX-Code

    break;
\layout LyX-Code

  else{
\layout LyX-Code

    i+= ac_prefix>>4;
\layout LyX-Code

    level_prefix= ac_prefix&0xF;
\layout LyX-Code

    level_suffix level_prefix bits  bitstream
\layout LyX-Code

    block[ zigzag[i] ]= level;
\layout LyX-Code

    i++;
\layout LyX-Code

  }
\layout LyX-Code

}
\layout Subsection

P-Frame
\layout List
\labelwidthstring 00.00.0000

32bit 'pfrm'
\layout List
\labelwidthstring 00.00.0000

32bit chunk size
\layout List
\labelwidthstring 00.00.0000

32bit 0 (unknown)
\layout List
\labelwidthstring 00.00.0000

32bit unknown, perhaps a checksum
\layout List
\labelwidthstring 00.00.0000

32bit unknown
\layout List
\labelwidthstring 00.00.0000

32bit bitstream size
\layout List
\labelwidthstring 00.00.0000

32bit wordstream size
\layout List
\labelwidthstring 00.00.0000

32bit bytestream size
\layout List
\labelwidthstring 00.00.0000

n\SpecialChar ~
bytes bitstream, stored in byteswapped 32-bit words
\layout List
\labelwidthstring 00.00.0000

n\SpecialChar ~
bytes RGB16 wordstream, stored in little endian order
\layout List
\labelwidthstring 00.00.0000

n\SpecialChar ~
bytes bytestream
\layout Subsubsection

Block
\layout LyX-Code

block(){
\layout LyX-Code

  mode    vlc     bitstream
\layout LyX-Code

  if(mode==h_split || mode==v_split){
\layout LyX-Code

    block()
\layout LyX-Code

    block()
\layout LyX-Code

  }
\layout LyX-Code

  if(mode==mc || mode==mcdc)
\layout LyX-Code

    mv    8bit    bytestream
\layout LyX-Code

  if(mode==dc || mode==mcdc)
\layout LyX-Code

    dc    16bit   wordstream
\layout LyX-Code

  if(mode==esc){
\layout LyX-Code

    col1  16bit   wordstream
\layout LyX-Code

    col2  16bit   wordstream
\layout LyX-Code

  }  
\layout LyX-Code

}
\layout Subsection

C-Frame
\layout List
\labelwidthstring 00.00.0000

32bit 'cfrm'
\layout List
\labelwidthstring 00.00.0000

32bit chunk size
\layout List
\labelwidthstring 00.00.0000

32bit 0 (unknown)
\layout List
\labelwidthstring 00.00.0000

32bit frame number / frame id, this is the frame number where the frame
 will be shown, it is also the frame number at which the last cframe part
 of this frame will be; note, all parts of the same cframe contain the same
 id here
\layout List
\labelwidthstring 00.00.0000

32bit whole frame size
\layout List
\labelwidthstring 00.00.0000

* p frame, this is (unk, unk, bitstream size, wordstream size, ...) for the
 first c frame chunk of a c frame
\layout Section

VLC Codes
\layout Subsection

prefix_vlc in I frames
\layout Standard

The prefix_vlc table is generated from the frequencies stored in the prefix
 stream.
 Additionally, the element 256 is added with an implicit frequency of 1.
 For the exact algorithm see libavcodec/4xm.c read_huffman_tables().
\layout Subsection

level vlc in I Frames
\layout Standard

Identical to JPEG
\layout Standard


\begin_inset  Tabular
<lyxtabular version="3" rows="6" columns="3">
<features>
<column alignment="center" valignment="top" leftline="true" width="0">
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
<column alignment="center" valignment="top" rightline="true" width="0">
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

prefix
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

vlc
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

level
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

1
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0/1
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

-1/1
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

2
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0X/1X
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

-3..-2/2..3
\end_inset 
</cell>
</row>
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

3
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0XX/1XX
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

-7..-4/4..7
\end_inset 
</cell>
</row>
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

...
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

...
\end_inset 
</cell>
</row>
</lyxtabular>

\end_inset 


\layout Standard

One way to decode this is:
\layout LyX-Code

if(prefix){
\layout LyX-Code

  v= get_bits(prefix);
\layout LyX-Code

  if((v & (1<<(prefix-1))) == 0)
\layout LyX-Code

    v= (-1 <<prefix)|(v+1);
\layout LyX-Code

}else
\layout LyX-Code

  v= 0;
\layout Subsection

Block Mode Codes in P frames
\layout Paragraph

For blocks 8x8, 8x4, 8x2, 4x8, 4x4, 4x2, 2x8, 2x4, 2x2
\layout Standard


\begin_inset  Tabular
<lyxtabular version="3" rows="6" columns="2">
<features>
<column alignment="center" valignment="top" leftline="true" width="0">
<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

0
\end_inset 
</cell>
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

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11110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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mcdc
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11111
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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0
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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mc
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10
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<row topline="true">
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110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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<row topline="true">
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1110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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1111
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mc
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110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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mcdc
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<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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1111
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

dc
\end_inset 
</cell>
</row>
</lyxtabular>

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For blocks 2x1, 1x2
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\begin_inset  Tabular
<lyxtabular version="3" rows="5" columns="2">
<features>
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<column alignment="center" valignment="top" leftline="true" rightline="true" width="0">
<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\layout Standard

0
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\layout Standard

mc
\end_inset 
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<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\layout Standard

10
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\layout Standard

skip
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<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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\layout Standard

mcdc
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</cell>
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<row topline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

1110
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
\begin_inset Text

\layout Standard

dc
\end_inset 
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</row>
<row topline="true" bottomline="true">
<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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1111
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<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
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esc
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\layout Section

Applications and Platforms
\layout Standard

The 4XM video codec is intended for gaming applications.
 It is known to operate on these computing platforms:
\layout Itemize

PC/Microsoft Windows
\layout Itemize

Apple Macintosh
\layout Itemize

Sega Dreamcast
\layout Itemize

Nintendo Gameboy Advance
\layout Standard

While the Dreamcast and the targeted PC/Mac platforms have quite a bit of
 computing power (at least 200 MHz), the GBA has an ARM RISC CPU running
 at 16-17 MHz.
\layout Standard

The 4XM coding method seems a little odd in its mixture of YCbCr and RGB
 colorspaces.
 In the end, all of the output data is RGB565.
 It is useful to note that many video game consoles can efficiently manuipulate
 this colorspace with video hardware.
 By contrast, many video consoles have no, or very limited, facilites for
 direct YCbCr rendering, particularly planar YCbCr modes.
\layout Standard

One more note about interframe block addition: One possible approach to
 implementing this part of the method on console hardware, at least the
 Sega Dreamcast, would be to fill a texture with all zero values, skip all
 blocks that are not coded, and fill the coded blocks with the coded RGB565
 difference.
 Then, the final texture could be added to the current frame.
 This also has the implicit side effect of saturating the addition so that
 the resulting pixels do not wrap around.
\layout Section

Changelog
\layout List
\labelwidthstring 00.00.0000

0.01 2003-06-01
\newline 
initial version by Michael Niedermayer
\layout List
\labelwidthstring 00.00.0000

0.02 2003-06-07
\newline 
minor changes
\layout List
\labelwidthstring 00.00.0000

0.03 2003-06-08
\newline 
peer review, grammar/spelling/punctuation fixes and 
\begin_inset Quotes eld
\end_inset 

Applications and Platforms
\begin_inset Quotes erd
\end_inset 

 section by Mike Melanson
\newline 
minor changes by Michael
\layout List
\labelwidthstring 00.00.0000

0.04 2003-06-08
\newline 
minor changes by Mike Melanson and Michael Niedermayer
\layout Section

Copyright
\layout Standard

Copyright 2003 Michael Niedermayer <michaelni@gmx.at> 
\newline 
This text can be used under the GNU Free Documentation License or GNU General
 Public License.
 See 
\begin_inset LatexCommand \htmlurl{http://www.gnu.org/licenses/fdl.txt}

\end_inset 

.
\the_end