Chapter 5	Other Formats



Chapter Overview
File Formats
CGMÐComputer Graphics Metafile
PICT2, ResourcesÐMacintosh Standard
TIFFÐScanner Companies
CDFÐCommon Data Format
FITSÐFlexible Image Transfer System
PHIGS and PHIGS+

.c2.Chapter Overview

This chapter briefly describes several of the currently available 
file formats and compares their features with those of HDF.


File Formats

There are numerous data formats in use today, some designed 
with objectives very similar to those of HDF. We encounter many 
of these at NCSA, and expect to continue to work with them in the 
future. The following sections describe some of the important data 
formats in use and compare their usability to that of HDF.


CGMÐComputer Graphics 
Metafile
CGM is an ANSI standard format for storing graphics in files, 
specifically graphics created with GKS libraries. It covers simple 
vectors, polygons, and raster images up to 24 bits deep. Many 
computing sites are using CGM, but they are quickly finding its 
limitations. For example, CGM does not allow simulation source 
data and an image of that data to be in the same file. Some 
implementations of CGM are not compatible with others, so there is 
now a SLATEC document that explicitly defines a subset of CGM 
for all DOE and NSF sites to implement.


PICT2, 
ResourcesÐMacintosh 
Standard
The PICT format is extensible and already contains most of the 
primitives that are needed, including a one-to-one mapping with 
QuickDraw. Two problems with PICT are (1) it is a serial format 
and (2) it is proprietary. The Macintosh disk format includes what 
are called resources; the resource types are similar to HDF's tags, 
assigned for each new need that arises. This also is proprietary 
and is a difficult standard to use because MacOS defines most of its 
uses for operating system purposes, not generic data storage.


TIFFÐScanner Companies
Microsoft and Aldus developed the TIFF format, which is very 
similar to HDF. TIFF defines a linked list of tag blocks in the file, 
with each tag block describing one image. There is no mechanism 
in place to help the standard grow in a regular fashion. One of the 
side effects is that each block of tags describes one frame which ties 
the file structure in with the data structure. The developers of TIFF 
defined only most of the tags necessary for scanner companies 
and then stopped. The irregular use of this standard by 
commercial companies makes it a moving target.


CDFÐCommon Data 
Format
This calling interface and format was developed at NASA for use 
with scientific data sets. It defines all data sets as multi-
dimensional arrays of multiple variables and allows you to 
include any number of single variables. The NCAR UNIDATA 
project has implemented the Fortran interface and adapted it for 
use in C. At NCAR, they use XDR (from Sun) to store the files 
instead of the original CDF Fortran method of using multiple files 
for one dataset on a VAX. CDF is not a good, transportable file 
format, but a very interesting calling interface for storage of 
scientific data. Its developers are reportedly working on methods 
for storing more than one array element at a time. There are only 
eight data types so far, defined as they are in VAX Fortran.

HDF can support many calling interfaces and the CDF interface 
may be a good one to support, for it has both C and Fortran 
interfaces.


FITSÐFlexible Image 
Transfer System
The FITS file format is primarily used for astronomy; it was one 
of the first tagged formats. Each tag is an ASCII string in the 
header record. The data can be of several types and is stored in 
multi-dimensional arrays. The file is in Fortran records of a 
fixed length. For Fortran-based systems that store only scientific 
data, FITS is still applicable.


PHIGS and PHIGS+
PHIGS and PHIGS+ are being adopted as standards for a calling 
interface to be used by high performance graphics programs. Most 
manufacturers of expensive graphics equipment will be providing 
PHIGS interfaces. They have not, as yet, defined a file storage 
format for PHIGS calls, so you have to send the source code with 
your image. We are interested in adopting a PHIGS compatible 
vector/polygon standard in HDF.

5.1	NCSA HDF Specifications

Other Formats	5.1

National Center for Supercomputing Applications

March 1989

                                                                



5.1	NCSA HDF Specifications

Other Formats	5.1

National Center for Supercomputing Applications

March 1989