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Cyber Paint Sequence *.SEQ (low resolution only)

This format, while fairly complex, yields excellent compression of animated
images while offering reasonably fast decompression times.

1 word magic number [$FEDB or $FEDC]
1 word version number
1 long number of frames
1 word speed (high byte is vblanks per frame)
118 bytes reserved
---------
128 bytes total for .SEQ header

for each frame {
1 word type (ignored?)
1 word resolution [always 0]
16 words palette
12 bytes filename [usually " . "]
1 word color animation limits [not used]
1 word color animation speed and direction [not used]
1 word number of color steps [not used]
1 word x offset for this frame [0 - 319]
1 word y offset for this frame [0 - 199]
1 word width of this frame, in pixels (may be 0, see below)
1 word height of this frame, in pixels (may be 0, see below)
1 byte operation [0 = copy, 1 = exclusive or]
1 byte storage method [0 = uncompressed, 1 = compressed]
1 long length of data in bytes (if the data is compressed, this
will be the size of the compressed data BEFORE decompression)
60 bytes reserved
--------
128 bytes total for frame header

? bytes data
}

Frames are "delta-compressed," meaning that only the changes from one
frame to the next are stored. On the ST, .SEQ files are always full-screen
low resolution animations, so the sequence resulting from expanding all the
data will be n 320 by 200 pixel low resolution screens, where n is given in
the .SEQ header.

Since only the changes from frame to frame are stored, image data for a
frame will rarely be 320x200 (except for the very first frame, which will
always be a full screen). Instead what is stored is the smallest rectangular
region on the screen that contains all the changes from the previous frame to
the current frame. The x offset and y offset in the frame header determine
where the upper left corner of the "change box" lies, and the width and height
specify the box's size.

Additionally, each "change box" is stored in one of five ways. For each
of these, the screen is assumed to have the full-screen image from the last
frame on it.

o uncompressed copy: The data for this frame is uncompressed image data,
and is simply copied onto the screen at position (x, y) specified
in the frame header.

o uncompressed eor: The data for this frame is exclusive or'ed with the
screen at position (x, y).

o compressed copy: The data for this frame must be decompressed (see
below), and then copied onto the screen at position (x, y) specified
in the frame header.

o compressed eor: The data for this frame must be decompressed (see
below), and then exclusive or'ed with the screen RAM at position (x, y).

o null frame: The width and/or height of this frame is 0, so this
frame is the same as the previous frame.

Of the 5 methods above, the one that results in the smallest amount
of data being stored for a particular is used for that frame.

Compression Scheme:

Compression is similar to that employed by Tiny, but is not quite as
space-efficient.

Control word meanings:

For a given control word, x:

x < 0 Absolute value specifies the number of unique words to
take from the data section (from 1 to 32767).
x > 0 Specifies the number of times to repeat the next word
taken from the data section (from 1 to 32767).

Note that a control word of 0 is possible but meaningless.

Format of expanded data:

The expanded data is not simply screen memory bitmap data; instead the four
bitplanes are separated, and the data within each bitplane is presented
vertically instead of horizontally. (This results in better compression.)

To clarify, data for a full screen would appear in the following order:

bitplane 0, word 0, scanline 0
bitplane 0, word 0, scanline 1
...
bitplane 0, word 0, scanline 199
bitplane 0, word 1, scanline 0
bitplane 0, word 1, scanline 1
...
bitplane 0, word 1, scanline 199
...
bitplane 0, word 79, scanline 199
bitplane 1, word 0, scanline 0
...
bitplane 3, word 79, scanline 199

Note however, that the data does not usually refer to an entire screen, but
rather to the smaller "change box," whose size is given in the frame header.
</pre>
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