On2 VP6

From MultimediaWiki
Jump to navigation Jump to search

Other Implementations

Early Open source implementation could be found here http://libvp62.sourceforge.net/. (broken link, project removed)

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 suspected that this section will be completed through reverse engineering and by people who saw libvp62 source code before it was censored.

Introduction

VP6 uses unidirectional ("P-frame") and intra-frame (within the current frame) prediction. Entropy coding is performed using arithmetic coding. The format supports dynamic adjustment of encoded video resolution.

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, bookmared frame reference

Frame Header

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

Syntax Number of bits Type Semantics
frame_mode 1 Enum 0x1 signifies an intra frame
qp 6 Unsigned Quantization parameter valid range 0..63
marker_bit 1 Constant Value should be 0x1
if (frame_mode == 0x01) {
version 7 Constant Value should be 0x23
interlace 1 Boolean true (1) means interlace will be used
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
}

If dim_x or dim_y have different values to 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):

Syntax Type Semantics
if (frame_mode == 0x1) {
marker1 Equiprobable 2-bit Ignored
} else {
bookmark Equiprobable 1-bit Bookmark this frame for use as a future reference
filter1 Equiprobable 1-bit
if (filter1 == 0x1) {
filter2 Equiprobable 1-bit
}
filter_info Equiprobable 1-bit
}
if (frame_mode == 0x1 || filter_info == 0x1) {
filter_mode1 Equiprobable 1-bit
if (filter_mode1 == 0x1) {
filter_threshold1 Equiprobably 5-bit
filter_motion_param Equiprobable 3-bit
} else {
filter_mode2 Equiprobable 1-bit
}
filter_mode3 Equiprobable 4-bit
}
marker2 Equiprobable 1-bit