VAG at 09:20, 31 March 2012

2012-03-31T09:20:52Z

New page

* Extension: smv
* Company: [[Psygnosis]]
* Samples: http://cd.textfiles.com/cdaction/cdaction04/FILM/

SMV is a short-lived FMV format used in the PC game [http://www.mobygames.com/game/dos/wipeout Wipeout] by Psygnosis. It employs LZ-like compression to encode 8-bit palettized video and audio.

== File Format ==
An SMV file consist of a number of chunks with unique [[TwoCC]]. All numbers are little-endian.

u16 chunkid
u16 chunklen
u8 payload[chunklen]

=== ST chunk ===
File header. Encodes basic movie properties.

u16 width -- movie width
u16 height -- and height
u16 mbwidth -- macroblock width
u16 mbheight -- and height
u16 frames -- number of video frames
u16 colors -- number of used colors
u16 nibbles -- offset of frame's nibble data
-- if chunk size is >= 15
u8 codec -- video codec format

* Codec field may be absent, in such case assume codec 0.
* mbwidth is always 16, mbheight may be either 16 or 8.
* Movie playback rate is 12 FPS.

=== GP chunk ===
New palette. Contains a number of 6-bit [[RGB]] entries, as specified in ST chunk.

=== MU chunk ===
Sound chunk. Contains audio fragment in 15862Hz, 8-bit, unsigned, mono format. If the payload size is equal to the amount of audio data for a frame (i.e. the size is freq / fps, which is equal to 1321) then the audio data is stored in raw [[PCM]] format. Otherwise, it's additionally LZ-word-packed.

=== FR chunk ===
Frame chunk. Contains encoded video frame.

=== FE chunk ===
End of the movie. No payload.

== Video Compression ==

=== Video Codec 0 ===

LZUnpack(payload, temp, words)
DrawBlocks(temp)

=== Video Codec 1 ===

LZUnpack(payload, temp, bytes)
DrawBlocks(temp)

=== Video Codec 2 ===

LZUnpack(payload, temp, bytes)
UnpackPartitions(temp, temp2)
DrawBlocks(temp2)

UnpackPartitions decodes Macroblock's nibbles.

Source frame is broken on to several equal partitions, 2 across by 4 down. Encoded partitions format:

u8 pixels[16 * num_mblocks] -- passed as is to DrawBlocks()
for each partition
u8 codebook[256][2] -- nibbles codebook for each partition
for each partition
for each macroblock
u8 index[mb_width*mb_height/4] -- index of codebook entry

Each entry of codebook encodes 4 nibbles. First byte encodes two nibbles of upper macroblock's line, second - next line, and so on.

=== Video Codec 3 ===
This codec is never used, but the code is still in place.

if GetByte()
LZUnpack(payload, temp, bytes)
UnpackSpecial(temp, temp2)
DrawBlocks(temp2)
else
same as codec 1

UnpackSpecial performs advanced decompression. '''TODO''' ''this may be some common algorithm''

=== Macroblocks drawing ===
Macroblocks are stored in the following format:

u8 pixels[16 * num_mblocks]
u4 nibbles[num_mblocks]

Size of pixels[] is equal to ST chunk's ''nibbles''.
For each macroblock draw it by indexing its pixels by nibbles. First nibble stored in top 4 bits of byte.

== Common Algorithms ==
=== LZ decompression ===
This is a more-or-less common LZ77-like algorithm. It can operate in two modes: byte mode and words mode. Mode affects backchain offset encoding.
* "next bit" means next bit from the internal 8-bit bitreader queue. Queue refilled with the input stream bytes, bits are consumed starting from the highest.
* "next byte" means raw octet from the input stream

repeat

if next bit == 0
output next byte
else
if next bit == 0
offset = next byte
else
if byte mode
offset = next 3 bits || next byte + 1
if words mode
offset = next 2 bits || next byte + 1 *2

if next bit == 0 length = 2
else if next bit == 0 length = 3 + next bit
else if next bit == 0 length = 5 + next bit
else if next bit == 0 length = 7
else if next byte != 0 length = 7 + this byte
else finish unpacking

copy length bytes from output - offset to output
note that, bytes may overlap to repeat itself

[[Category:Game Formats]]
[[Category:Video Codecs]]
[[Category:Formats missing in FFmpeg]]

Psygnosis SMV - Revision history

VAG at 09:20, 31 March 2012