View source for On2 VP6

* FOURCCs: VP60, VP61, VP62
* Company: [[On2]]
* Whitepaper: http://www.on2.com/cms-data/pdf/1125607149174329.pdf ([[Mirrored Files|mirrored]])
* Specification: http://multimedia.cx/mirror/vp6_format.pdf
* Samples: [http://samples.mplayerhq.hu/V-codecs/VP6/ http://samples.mplayerhq.hu/V-codecs/VP6/]
* Samples: {{DexvertSamples|video/vp6EncodedVideo}}
* Nice article form author of the codec (Paul Wilkins): http://www.dspdesignline.com/211100053?printableArticle=true

== Implementations ==

An early open source implementation could be found at http://libvp62.sourceforge.net/, but
was driven underground by On2 on copyright infringement claims.

This specification is said to be incomplete with regard to the On2 VP6 specification.

A decoder implementation may be found in the FFMPEG source file [http://svn.mplayerhq.hu/ffmpeg/trunk/libavcodec/vp6.c?view=markup vp6.c]

== Format ==

The aim here is to open this standard with a full description of the bitstream format and decoding process.  Contributors from On2 especially encouraged here, but it is anticipated that this section will be completed through reverse engineering and by people who saw libvp62 source code before it was censored.  

Please do not submit any copyrighted text or code here.

=== Introduction ===

VP6 uses unidirectional ("P-frame") and intra-frame (within the current frame) prediction. Entropy coding is performed using arithmetic  (range?) coding and an 8x8 iDCT is used.  The format supports dynamic adjustment of encoded video resolution. There are three variants of the VP6 codec, VP60 (Simple Profile), VP62 (Advanced Profile) and VP62 (Heightened Sharpness Profile).


=== Macroblocks ===

Each video frame is composed of an array of 16x16 macroblocks, just like [[MPEG-2]], [[MPEG-4]] parts 2 and 10.  Each [[MB]] (macroblock) takes one of the following modes ("[[MV]]" means "motion vector"):

* Intra MB
* Inter MB, null MV, previous frame reference
* Inter MB, differential MV, previous frame reference
* Inter MB, four MVs, previous frame reference
* Inter MB, MV 1, previous frame reference
* Inter MB, MV 2, previous frame reference
* Inter MB, null MV, bookmarked frame reference
* Inter MB, differential MV, bookmarked frame reference
* Inter MB, MV 1, bookmarked frame reference
* Inter MB, MV 2, bookmarked frame reference

=== Frame Header ===

The frame header commences with a section that is encoded using conventional big-endian bit packing.

{| border="1"
! Syntax !! Number of bits !! Type !! Semantics
|- 
| frame_mode || 1 || Enum || 0x0 signifies an intra frame
|-
| qp || 6 || Unsigned || Quantization parameter valid range 0..63
|-
| marker || 1 || Constant || 0=VP61/62, 1=VP60
|-
| if (frame_mode == 0) { || || ||0 equals to INTRA_FRAME
|-
| version || 5 || Constant || 6=VP60/61, 7=VP60(Electronic Arts), 8=VP62
|-
| version2 || 2 || Constant || 0=VP60, 3=VP61/62
|-
| interlace || 1 || Boolean || true (1) means interlace will be used
|-
| if (marker==1 or version2==0) {
|-
| offset || 16 || Unsigned || secondary buffer offset (bytes releative to start of buffer)
|-
| }
|-
| dim_y || 8 || Unsigned || Macroblock height of video 
|-
| dim_x || 8 || Unsigned || Macroblock width of video 
|-
| render_y || 8 || Unsigned || Display height of video 
|-
| render_x || 8 || Unsigned || Display width of video
|-
| }else{
|-
| if (marker==1 or version2==0) {
|-
| offset || 16 || Unsigned || secondary buffer offset (bytes releative to start of buffer)
|-
| }
|-
| }
|}

If dim_x or dim_y have values different from the previous intra frame, then the resolution of the encoded image has changed.

Arithmetic coding commences at the next bit (which should be on a byte boundary):

{| border="1"
! Syntax !! Type !! Semantics
|-
| if (frame_mode == 0) {
|-
| marker1 || Equiprobable 2-bit || Ignored
|-
| } else {
|-
| bookmark || Equiprobable 1-bit || bookmark == 0x1 means this frame will be the next bookmark frame
|-
| filter1 || Equiprobable 1-bit || 
|-
| if (filter1 == 0x1) {
|-
| filter2 || Equiprobable 1-bit || 
|-
| } 
|-
| filter_info || Equiprobable 1-bit ||
|-
| }
|-
| if (frame_mode == 0 <nowiki>||</nowiki> filter_info == 0x1) {
|-
| filter_mode1 || Equiprobable 1-bit ||
|-
| if (filter_mode1 == 0x1) {
|-
| filter_threshold1 || Equiprobable 5-bit ||
|-
| filter_motion_param || Equiprobable 3-bit ||
|-
| } else {
|-
| filter_mode2 || Equiprobable 1-bit ||
|-
| }
|-
| filter_mode3 || Equiprobable 4-bit ||
|-
| } 
|-
| marker2 || Equiprobable 1-bit || Secondary buffer encoding algorithm. 0=Range coding, 1=Huffman coding.
|}

If the secondary buffer is present, coeffient symbols are read from the secondary buffer using the algorithm indicated by marker2. VP60 has "the ability to switch to a faster entropy encoding strategy to ensure smooth playback" and "the ability to decode different parts of the bitstream on different sub-processors (for instance the vlx and the core), to ensure better overall system utilization" [http://www.on2.com/cms-data/pdf/1125607149174329.pdf].

=== Entropy Coding ===

Described here is the decoding process for the arithmetically-coded (AC) parts of the bitstream.  VP6 uses a 16-bit [http://en.wikipedia.org/wiki/Range_encoding range coding] scheme to code binary symbols.

The AC decoder maintains three state variables:  ''code'', ''mask'' and ''high''.  

====Initialization====

At initialization, the first two bytes of the AC bitstream are shifted into ''code''.  The variable ''high'' is set to 0xff00.  The variable ''mask'' is set to 0xffff.

====Decoding a Binary Value====

Each binary symbol has an associated probability ''p'' in the range 0 to 0xff.  

A threshold, ''t'', is computed thus:

: ''t'' = 0x100 + ( 0xff00 & ( ( (''high''-0x100) * ''p'' ) >> 8 ) )

Equiprobable binary symbols are treated somewhat differently:

: ''t'' = 0xff00 & ( (''high''+0x100) >> 1 )

The binary value may then be decoded by comparing ''code'' and ''t''.  If ''code'' is less than ''t'', the binary value is decoded as 0.  If ''code'' is equal to or greater than ''t'', the binary value is decoded as 1.

If a 1 was decoded, then

: ''high'' = ''high'' - ''t''
: ''code'' = ''code'' - ''t''

If a 0 was decoded, then

: ''high'' = ''t''

The following renormalization is now repeated while (''high'' & 0x8000) is non-zero.

: ''high'' = 2 * ''high''
: ''code'' = 2 * ''code''
: ''mask'' = 2 * ''mask''
: if ((''mask'' & 0xff) == 0x00) {
:: ''code'' = ''code'' | <i>next byte from bitstream</i>
:: ''mask'' = ''mask'' | 0xff
:}

====Decoding an Equiprobable n-bit Integer Value====

Integer values are coded as a big-endian sequence of equiprobable binary values.  To decode an n-bit equiprobable integer value, n equiprobable binary values should be decoded using the sequence above and left-shifted into an integral result variable.

=== Inverse DCT ===

Inverse DCT is performed on 8x8 blocks of pixels. The algorithm used is the same (or a small variation) of the one used for the [[On2 VP3|VP3]] decoder in [[FFmpeg]] [http://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/libavcodec/vp3dsp.c?rev=1.9&content-type=text/x-cvsweb-markup&cvsroot=FFMpeg], the original vp3 iDCT code is here [http://svn.xiph.org/trunk/vp32/CoreLibs/CDXV/Vp31/dx/generic/IDctPart.c].

[[Category:Video Codecs]]