The actual image frames are stored after the "movi" string in the AMF header. If the
image compression type is LZC, LZG, LZIV, or LZ8, then timing information is
inserted between image data. Timing chunks are stored as JUNK chunks, consisting of
the following:
The string "JUNK".
A four-byte value indicating the length of this chunk, stored least
significant byte (LSB) first. AMF version 1.0 files have length values of 4,
versions 2.0 and 3.0 files have length values of 8.
A four-byte value. In a version 1.0 file, this is a floating point number
indicating the time at this frame. On UNIX platforms using version 1.0, this
value must be byte swapped after reading. In version 2.0 and 3.0 files, this
is an integer, stored LSB first, indicating the number identifier of this
frame.
A four-byte floating point number indicating the time at this frame (version
2.0 and higher only).
Image data follows the timing chunk. The data varies depending on what compression
method is used. As of version 3.0, the following types are supported:
LZC: Lempel-Ziv compression on a 24 bit color image.
LZIV or LZG: Lempel-Ziv compression on a grayscale image.
JPEG: JPEG compression.
LZ8: Lempel-Ziv compression on an 8 bit (256 color) image.
Below is a HEX dump of a frame chunk for an LZC encoded file. The LZVI, LZG, and LZ8
compressions follow a similar format:Figure 1.
Legend
Light Green
Size of the JUNK chunk used for timing info. This is a 4-byte long
integer value stored least significant byte (LSB) first.
Royal Blue
The time value for this frame. This value is only read in version 1.0
files. It is present but ignored in version 2.0 or higher, as the timing
value for those versions comes from the AMF footer structure elsewhere
in the file.
Purple
Frame number. A 4-byte integer stored LSB first.
Cyan
The string value "00db".
Red
The size of the image chunk, in bytes, as stored in this file. A
four-byte value stored LSB first.
Yellow
The compressed size of the image data. A four-byte value stored LSB
first.
Dark Blue
The uncompressed size of the image data. A four-byte value stored LSB
first.
Dark Green
Denotes the beginning of the image data.
A JPEG compressed AMF does not use timing JUNK chunks, as this compression type was
added for version 2.0 when timing information was already available from the AMF
footer structure. Following is a HEX dump of a frame chunk for a JPEG encoded
file:Figure 2.
Legend
Cyan
The string value "00db".
Red
The size of the image chunk, in bytes, as stored in this file. A
four-byte value stored LSB first.
