File IO for AVS .fld format data files
Author JeffFessler
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Updated Last
25 Days Ago
Started In
May 2021


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File IO for AVS format "field" data files with extension .fld for the Julia language, in conjunction with the FileIO package. This data format supports N-dimensional arrays of real numbers.


Following the FileIO API, this package provides (but does not export) methods

  • AVSfldIO.load(filename)
  • AVSfldIO.save(filename, data)

It does export the following methods:

  • header, is_external_file, fid = fld_open(file ; dir="", chat=false)
  • header = fld_header(file::String ; dir="", chat=false)
  • data = fld_read(file::String ; dir="", chat=false)
  • fld_write(file, data ; kwargs...)

Use chat=true for verbose debugging output. Use dir=somepath to prepend a path to file. See docstrings for more details.


Most users will simply use the FileIO load and save methods as follows.

using FileIO # need version ≥ 1.9
data1 = rand(6,7,8) # random 3D data
file = "test.fld"
save(file, data1) # will throw error if the file exists already
data2 = load(file)
@assert data1 == data2

File format overview

The AVS .fld data format comes in two flavors.

In the AVS "internal" format:

  • an ASCII header is at the top of the file,
  • the header is followed by two "form-feed" (^L) characters,
  • which are then followed by the data in binary format.

In the AVS "external" format, the header and the data are in separate files, and the ASCII header file includes the name of the data file. The data file can contain either ASCII or binary data.

AVS internal format

For a 128 × 64 array (first dimension varies fastest) consisting of short integers (Int16 in Julia), the format of the AVS internal header would be:

# AVS field file

followed by the two form feeds, and then the 128 × 64 short integers in binary format.

For a 3D array of size 128 × 64 × 20 of 32-bit floating point numbers with the host CPU endianness, the header is

# AVS field file

The save method in this library writes to the AVS internal format by default, and the filename must end with the extension .fld.

AVS external format

Now suppose you have stored the above array data in a binary file named, say, sino.dat with some home-brew header in it that consists of, say, 1999 bytes. And suppose you do not want to convert from home-brew format to AVS internal format. Then you can use the AVS external format by creating an ASCII file named, say, sino.fld containing:

# AVS field file
variable 1 file=sino.dat filetype=binary skip=1999

You can add additional comments to these headers using lines that begin with #. The skip=1999 option indicates that there is a 1999 byte header to be skipped before reading the binary data.

This format does not allow for additional headers buried within the data.

If there is no binary header, then you can omit the skip=0 line altogether. If your data is in ASCII format (hopefully not), then replace filetype=binary with filetype=ascii. However, for ASCII data, the skip= option refers to ASCII entries, not bytes.

See table below for supported types in the data=... line.

The complete AVS .fld format includes other features that almost certainly are not supported by this IO library.

This library supports some extensions that are not standard AVS but are very handy, like a single 3D header file that points to multiple 2D files that get treated as a single entity. More documentation coming on request.

Magic bytes

It is convention that an AVS .fld file begins with # AVS in the first line, as illustrated in the examples above, so the interface to this library in FileIO.jl uses that 5-byte string as the "magic bytes" or "magic number" for this file type. If you have a file that does not have that string as the start of its header, then simply add it with an editor (including a newline at the end).

Data types

The following table shows the supported options for the data= field in the header. The options that end in _le or _be or _sun are "extensions" designed for portability, because options like int are not portable between hosts with different endianness.

format Julia type endian bytes
byte UInt8 n/a 1
short_be UInt16 be 2
short_sun UInt16 be 2
xdr_short UInt16 be 2
short_le UInt16 le 2
int Int32 ? 4
int_le Int32 le 4
int_be Int32 be 4
xdr_int Int32 be 4
float Float32 ? 4
float_le Float32 le 4
float_be Float32 be 4
xdr_float Float32 be 4
double Float64 ? 8
double_le Float64 le 8
double_be Float64 be 8
xdr_double Float64 be 8

Entries with ? are native to the host CPU and thus not portable. The byte format is unsigned 8 bits.

The following table shows the saved output data= field for various input Julia data fields, assuming little endian (:le) default to AVSfldIO.fld_write.

Julia type data note
Bool byte
UInt8 byte
Int16 short_le
Int32 int_le
Int64 int_le (downgraded)
Float16 float_le (upgraded)
Float32 float_le
Float64 double_le
BigFloat double_le (downgraded)


The "application visualization system" (AVS) https://www.avs.com was an interactive graphics and computation framework developed in the early 1990s and used widely in the medical imaging community. See the article at https://doi.org/10.1109/38.31462 for an overview.

AVS data files have the extension .fld and many software frameworks provide file IO support for this format.


Jeff Fessler and his group at the University of Michigan.

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