ASUS V1/V2 Codecs
1 Introduction
ASV1/2 are purely intracoded DCT-based codecs, with a per file quantization parameter (QP), use 4:2:0 YCbCr colorspace and operate on macroblocks.
The latest version of this document is available at http://www.mplayerhq.hu/~michael/asv1.{lyx,txt,html,ps}
This document assumes familiarity with mathematical and coding concepts such as the discrete cosine transform, quantization, YUV (YCbCr) colorspaces, macroblocks, and variable length codes (VLCs).
2 Terms and definitions
AC Any DCT coefficient for which the frequency in one or both both dimensions in non-zero.
DC The DCT coefficient for which the frequency is zero in both dimensions
QP Quantization Parameter
(I)DCT (Inverse) Discrete Cosine Transform
VLC Variable Length Code
3 Highlevel Description
3.1 Picture
The picture is split into macroblocks which are coded left->right, top->bottom, as long as MBs are completely within the width/height area that is divisible by 16. If width is not divisible by 16, the right column is coded after the main body. If height is not divisible by 16, the bottom strip is coded after the right column.
Example: 56x56 sized image
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0
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1
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2
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9
|
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3
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4
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5
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10
|
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6
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7
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8
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11
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12
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13
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14
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15
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3.2 Macroblock
16x16 luma + 8x8 chroma as 4 8x8 luma blocks and 2 8x8 chroma blocks
Y:
Cb:
Cr:
Contains 64(=8x8) coefficients in 16(=4x4) coefficient groups with 4(=2x2) coefficients each
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0
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1
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5
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9
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2
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3
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8
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12
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4
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7
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11
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14
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6
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10
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13
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15
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Coefficients in the coefficient groups 10..15 cannot be coded (they must be 0) in ASV1.
The first coefficient (DC coefficient) in the first coefficient group must be coded as 0. Note it is coded seperately in the 8 bits prior to the AC coefficients.
3.4 Coefficient group
3.5 DC coefficient Dequantization
c00’ = 8c00
c00 quantized and coded DC coefficient
c00’ unquantized DC coefficient
3.6 AC coefficient Dequantization
cxy’ = (cxy)/(16)⌊(Dqxy)/(QP)⌋
QP Quantization Parameter from the global header
D 64 for ASV1, 128 forASV2
c quantized and coded coefficient
c’ unquantized coefficient
q ISO MPEG1 intra quantization matrix
4 Bitstream
The bitstream in ASV1 is stored with byte-swapped 32bit words (24..31, 16..23, 8..15, 0..7, 56..63, 48..55, 40..47, 32..39, ...)
The bitstream in ASV2 is stored with the bits in each byte reversed so (7..0, 15..8, 23..16, 31..24, 39..32, ...)
4.1 Fourcc
ASV1 / ASV2
4.2 Global (per file) Header
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Length in bits
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Value
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8
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QP
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24
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unkown, 0 for ASV1
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8
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’A’
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8
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’S’
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8
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’U’
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8
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’S’
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Note, the biSize parameter (in the case of storage in AVI) is 48
4.3 Macroblock
Simply the bitstream of the 6 blocks
4.4 ASV1 Block
8-bit DC coefficient
. for each coefficient group
5bit End Of Block
4.5 ASV2 Block
4-bit coefficient group count
8-bit DC coefficient
2-4bit First Coded coefficient pattern
. for each coded coefficient
. for each coefficient group
5 VLC Codes
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0011
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-3
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011
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-2
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11
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-1
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000xxxxxxxx
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xxxxxxxx
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10
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1
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010
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2
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0010
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3
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xxxxxxxx is a twos complement signed 8 bit nubmer (=int8_t)
5.1.2 Coded coefficient pattern
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00001
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14
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00010
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13
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...
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...
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01101
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2
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01110
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1
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01111
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EOB
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10
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0
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11
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15
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5.2.1 First Coded coefficient pattern
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VLC
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CCP
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00
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0111
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01
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0000
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100
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0110
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101
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0100
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1100
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0011
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1101
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0001
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1110
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0101
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1111
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0010
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5.2.2 Coded coefficient pattern
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VLC
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CCP
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00
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0000
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010
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0100
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011
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1000
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1000
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1010
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1001
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1100
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1010
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0010
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1011
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1101
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1100
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1111
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1101
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1110
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111000
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0111
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111001
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0101
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111010
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0011
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111011
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0001
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111100
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0110
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111101
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1001
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11111
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1011
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0000111111
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-31
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...
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...
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001111
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-7
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001011
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-6
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001101
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-5
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001001
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-4
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0111
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-3
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0101
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-2
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11
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-1
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00000xxxxxxxx
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xxxxxxxx
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10
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1
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0100
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2
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0110
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3
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001000
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4
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001100
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5
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001010
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6
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001110
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7
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...
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...
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0000111110
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31
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xxxxxxxx is a twos complement signed 8 bit nubmer (=int8_t)
6 Example ASV1 decoder
-
foreach macroblock, decode Y0 Y1 Y2 Y3 U V blocks, Y blocks are arranged as:
Y0 Y1
Y2 Y3
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foreach block:
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DC coeff is the next 8 bits in the stream * 8 (this gives a range of 0..2040)
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foreach of up to 11 coefficient groups:
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get vlc from ccp table, value should range from 0..16
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if value is 16, coefficient decode is done
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if 11th iteration, 16 should occur, else error
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ccp contains 4 bits at this point (3-0)
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each bit corresponds to a quantized coeff. #(iter. * 4 + bit)
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if 0, coeff. is 0 else get_level():
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get vlc from level vlc table
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if vlc is 3, use the next 8 bits (as a signed number) as level
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else, use vlc - 3
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store coefficient
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dequantize, idct
7 Changelog
0.01 2003-05-21
initial version by Michael Niedermayer with some stuff from Mike Melanson
0.02 2003-05-21
spelling, punctuation, example decoder, various minor changes by Mike Melanson
various minor changes by Michael
0.03 2003-09-01
minor fixes and ASV2 by Michael
0.04 2004-08-24
clarify bitstream order by Michael
8 Copyright
Copyright 2003 Michael Niedermayer <michaelni@gmx.at>
This text can be used under the GNU Free Documentation License or GNU General Public License
see http://www.gnu.org/licenses/fdl.txt