Subject: Re: FLI file-format?
From: [email-masked] (Kevin Darling)
Date: 12 Mar 91 20:20:26 GMT
Newsgroups: comp.graphics

>Has someone a description of the FLI-format?

Autodesk Animator files explanation (.FLI only excerpted).  I believe
that the original programmer wrote up this doc.  It's correct, as I've
used the info to realtime playback stock .FLIs on a 680x0 machine.
All numbers in a .FLI file are in Intel format, so you may have to
compensate for that, of course. - kevin

8.1 Flic Files (.FLI)

     The details of a FLI file are moderately complex, but the
idea behind it is simple: don't bother storing the parts of a
frame that are the same as the last frame.  Not only does this
save space, but it's very quick.  It's faster to leave a pixel
alone than to set it.

     A FLI file has a 128-byte header followed by a sequence of
frames. The first frame is compressed using a bytewise run-length
compression scheme.  Subsequent frames are stored as the
difference from the previous frame.  (Occasionally the first
frame and/or subsequent frames are uncompressed.)  There is one
extra frame at the end of a FLI which contains the difference
between the last frame and the first frame.

     The FLI header:

     byte size name      meaning
     offset

     0    4    size      Length of file, for programs that want
                         to read the FLI all at once if possible.
     4    2    magic     Set to hex AF11.  Please use another
                         value here if you change format (even to
                         a different resolution) so Autodesk
                         Animator won't crash trying to read it.
     6    2    frames    Number of frames in FLI.  FLI files have
                         a maxium length of 4000 frames.
     8    2    width     Screen width (320).
     10   2    height    Screen height (200).
     12
     14   2    flags     Must be 0.
     16   2    speed     Number of video ticks between frames.
     18   4    next      Set to 0.
     22   4    frit      Set to 0.
     26   102  expand    All zeroes -- for future enhancement.

     Next are the frames, each of which has a header:

     byte size name      meaning
     offset
     0    4    size      Bytes in this frame.  Autodesk Animator
                         demands that this be less than 64K.
     4    2    magic     Always hexadecimal F1FA
     6    2    chunks    Number of 'chunks' in frame.
     8    8    expand    Space for future enhancements. All zeros.

     After the frame header come the chunks that make up the
frame.  First comes a color chunk if the color map has changed
from the last frame.  Then comes a pixel chunk if the pixels have
changed.  If the frame is absolutely identical to the last frame
there will be no chunks at all.

     A chunk itself has a header, followed by the data.  The
chunk header is:

     byte size name meaning
     offset
     0    4    size Bytes in this chunk.
     4    2    type Type of chunk (see below).

     There are currently five types of chunks you'll see in a FLI
file:

     number    name      meaning
     11        FLI_COLOR Compressed color map
     12        FLI_LC    Line compressed -- the most common type
                         of compression for any but the first
                         frame.  Describes the pixel difference
                         from the previous frame.
     13        FLI_BLACK Set whole screen to color 0 (only occurs
                         on the first frame).
     15        FLI_BRUN  Bytewise run-length compression -- first
                         frame only
     16        FLI_COPY  Indicates uncompressed 64000 bytes soon
                         to follow.  For those times when
                         compression just doesn't work!

     The compression schemes are all byte-oriented.  If the
compressed data ends up being an odd length a single pad byte is
inserted so that the FLI_COPY's always start at an even address
for faster DMA.

FLI_COLOR Chunks

     The first word is the number of packets in this chunk. This
is followed directly by the packets.  The first byte of a packet
says how many colors to skip.  The next byte says how many colors
to change.  If this byte is zero it is interpreted to mean 256. 
Next follows 3 bytes for each color to change (one each for red,
green and blue).

FLI_LC Chunks

     This is the most common, and alas, most complex chunk.   The
first word (16 bits) is the number of lines starting from the top
of the screen that are the same as the previous frame. (For
example, if there is motion only on the bottom line of screen
you'd have a 199 here.)  The next word is the number of lines
that do change.  Next there is the data for the changing lines
themselves.  Each line is compressed individually; among other
things this makes it much easier to play back the FLI at a
reduced size.

     The first byte of a compressed line is the number of packets
in this line.  If the line is unchanged from the last frame this 
is zero.  The format of an individual packet is:

 skip_count
 size_count
 data

     The skip count is a single byte.  If more than 255 pixels
are to be skipped it must be broken into 2 packets. The size
count is also a byte.  If it is positive, that many bytes of data
follow and are to be copied to the screen.  If it's negative a
single byte follows, and is repeated -skip_count times.

     In the worst case a FLI_LC frame can be about 70K.  If it
comes out to be 60000 bytes or more Autodesk Animator decides
compression isn't worthwhile and saves the frame as FLI_COPY.

FLI_BLACK Chunks

     These are very simple.  There is no data associated with
them at all. In fact they are only generated for the first frame
in Autodesk Animator after the user selects NEW under the FLIC menu.

FLI_BRUN Chunks

     These are much like FLI_LC chunks without the skips.  They
start immediately with the data for the first line, and go line-
by-line from there.  The first byte contains the number of
packets in that line.  The format for a packet is:

 size_count
 data

     If size_count is positive the data consists of a single byte
which is repeated size_count times. If size_count is negative
there are -size_count bytes of data which are copied to the
screen. In Autodesk Animator if the "compressed" data shows signs
of exceeding 60000 bytes the frame is stored as FLI_COPY instead.

FLI_COPY Chunks

     These are 64000 bytes of data for direct reading onto the
screen.

 -eof-

Notes: Since these are animations, the last frame will delta into a
copy of the first one (which was usually a large BRUN chunk).  Therefore,
looping should go back to the _second_ frame chunk (usually a LC
or COLOR chunk) instead of all the way back to the file beginning, to
avoid a "stutter" caused by unnecessarily redecoding the original frame.
Also, a very few files may have palette animation, so write your code
so that COLOR chunks can be found at any time.  - kevin