JHV User's Guide

HDF Data Objects

HDF is a library and platform independent data format for the storage and exchange of scientific data. It may contain many types of information that a scientist might need. Currently there are five types of HDF objects that can be viewed by JHV. These data objects include file annotation, generic raster image, scientific data set, vdata and vgroup. This section gives an introduction to the HDF data objects. More details can be found at the HDF Homepage.


1. File annotation

HDF annotations are text strings used to describe a file or any of the data elements the HDF file contains. Annotations come in two forms; labels and descriptions. Labels are short annotations used for assigning things like titles or time stamps to a file or its data objects. Longer annotations are called descriptions, and they typically contain more extensive information such as a source code module or mathematical formulae.

JHV displays both the file description and data labels. Click the file annotation object (HDF hierarchy tree node), the JHV displays the file annotation in the text area. Click any of the HDF object folders, the JHV will display the labels of that object. For example,

2. Generic Raster (GR) Image

The general raster data model is designed to provide a flexible means of storing raster image data including 8-bit raster or 24-bit raster images. The raster image data is stored in a two-dimensional array and attributes can be created for the image, the file or both. Palettes (Color Look-up Tables) can be created also and attached to the image as well as compression method information.

For backward compatibility, the HDF library allows users to access images as a generic raster or as an 8-bit or 24-bit raster image. In such a case, the image is stored once under two identities. This may cause the JHV to show two objects in the hierarchy, although there is really only one image in the file.

3. Scientific Data Set

The scientific data set (SDS) model supports four primary data objects: arrays, dimensions, dimension scales, and dimension attributes. As in the multifile SD SDS model, the fundamental object of the data model is the SDS array. Unlike the SD multifile SDS model the DFSD SDS model has, in addition to dimension attributes, attributes that refer to the SDS array itself.

The current JHV can only browser SDS data with up 3 dimensions. It does not handle data arrays with more than 3 dimensions. A 2D data array is displayed as a single images. A 3D array is displayed as animated images sliced along the axis chosen by the user.

4. HDF Vdata (table)

An HDF Vdata object is a collection of records whose values are stored in fixed-length fields. A vdata is like a table in which each row has the same structure and each column contains data of the same data type. The Vdata model allows multiple entries per field. The number of entries or components in a field is more commonly called the order of the field. The current displays a vdata table in spreadsheet.

5. HDF Vgroup

In HDF, a vgroup object is a structure designed to associate related objects. Data organization within a vgroup resembles the Unix file system. The general structure of vgroups is similar to Unix directories or subdirectories in that a vgroup may contain references to other vgroups or data objects. Any HDF object can be included within a vgroup.

The following is an example of Vgroup hierarchy.


NCSA
The National Center for Supercomputing Applications

University of Illinois at Urbana-Champaign

hdfhelp@ncsa.uiuc.edu