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i NCSA HDF Specifications
Introduction i
National Center for Supercomputing Applications
March 1989
March 1989
Introduction
Overview
At the National Center for Supercomputing Applications, diverse
groups are working on supercomputing projects of many kinds.
The Hierarchical Data Format (HDF) was designed to make the
sharing of data between different people, different projects, and
different types of computers easy and self-describing. An
extensible header, along with carefully crafted internal layers,
provides a system that can grow along with the software that NCSA
develops. This chapter provides a brief overview of HDF
capabilities and design.
What Is HDF?
HDF is a growth-oriented approach to file format design. Rather
than try to address all of the short-term issues in a fixed format, or
attempt to solve all of the hard problems in an all-purpose format,
HDF defines a framework for continued future growth. New
calling interfaces can be defined that are compatible with old ones.
Files can be made upwardly compatible for years to come without
giving up added power in the future. HDF also makes it easy for
the user to include annotations, titles, and specific descriptions of
the data in the file, so that files can be archived with human-
readable information about their origins.
HDF uses the concept of a tagged, or object oriented, file
organization. The idea is to store both a known format description
and the data in the same file. HDF tags describe the format of the
data because each tag is assigned a specific meaning╤one tag is
assigned to "File Identifier," another is assigned to "Raster
Image," and so on (see Figure 1). A program that has been written
to understand a certain list of tag types can scan the file for those
tag types and process the data. This program also can ignore any
data that is beyond its scope.
Figure 1 Raster Image Sets in an HDF File
HDF files never need to become out of date. For example, suppose a
site falls far behind in the HDF standard, so its users can only
work with the portions of the specification that are three years old.
Users at this site might want to import files from NCSA. Even with
the more advanced data files, they can list the types of data in the
file. All of the older types of data objects that they understand are
still accessible, despite the fact that the older types of data objects
are mixed in with new kinds of data. In addition, if the more
advanced site uses the text annotation facilities of HDF effectively,
the files will arrive with complete human-readable descriptions of
how to decipher the new tag types.
To present a convenient user interface made up of something other
than a list of tag types with their associated data requirements,
HDF supports multiple calling interfaces. The low level calling
interface for manipulating tags and raw data is designed to be
used by systems programmers who are providing the higher level
interfaces for applications like raster image storage or scientific
data archiving.
An important issue in data file design is that of machine
independence or transportability. The HDF design is not machine
independent, but it defines the data completely. HDF requires you
to fully specify all number types used, so conversion programs can
identify what number formats are being used and do the
conversions when needed.
Use of This Manual
This manual is designed for software developers who are
designing applications or routines for use with HDF files and for
users who need detailed information about HDF.
Manual Contents
The manual is organized into the following chapters:
Chapter 1, "The Basic Structure of HDF Files," introduces and
describes the components and organization of Hierarchical Data
Format files.
Chapter 2, "The NCSA HDF General Purpose Interface," contains
a list of the routines that make up the low level interface described
in detail in the HDF manual.
Chapter 3, "Sets and Groups," describes raster image sets and
scientific datasets, explains the role of sets and groups in an HDF
file, and discusses the routines that are available for storing and
retrieving scientific datasets.
Chapter 4, "HDF Conventions," presents guidelines regarding the
use of HDF that are not discussed elsewhere.
Chapter 5, "Other Formats," briefly describes the following
formats and compares their features with those of HDF: CGM,
PICT2, TIFF, CDF, FITS, PHIGS, and PHIGS+.
Appendix A, "NCSA HDF Tags," presents a list of brief
descriptions of the tags assigned at NCSA for general use.
Appendix B, "Header Files," includes the general header files
used in compiling all HDF libraries.
Form of Presentation
The material in this manual is presented in text or screen displays.
Text
In explaining various features and commands, this manual often
presents a word within a paragraph in italics to indicate that the
word is defined within the paragraph.
Portions of this manual refer to other portions of the manual where
the other portions explain related topics. These cross references
usually mention the title of sections or chapters enclosed in
quotation marks, such as, See Chapter 1, "The Basic Structure of
HDF Files."
Screen Displays
Screen displays in this manual are presented in courier type.