JHV can be used to view several types of HDF data objects. Due the differing natures of these types of data sets, the manner in which they open for display differs from one type to another. For example, an 8-bit raster image will open a preview copy of the image, a 3D SDS first asks how the data is to be sliced for viewing, and a Vdata opens as a data table. This chapter describes opening each of the data types in a JHV session.
Currently JHV can be used with five types of HDF objects:
The last four types of data above are the essential data sets; HDF was created to store these types of information in a widely-accessible format. Annotations, the first item, are a fundamentally different sort of data; they exist solely to describe the data stored in the HDF file.
Complete information regarding these data structures is available from The HDF Home Page.
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. Descriptions are longer annotations; they typically contain more extensive information, such as a source code module or mathematical formulae.
There are also two types of annotations: file annotations and data object annotations. A file annotation describes an entire HDF file. A data object annotation describes a particular data object within the file.
File and data object annotations can each be either labels or descriptions. Thus an annotation might be a file label, a file description, an object label, or an object description.
The general raster, or GR, data model is designed to
provide a flexible means of storing raster image data, including 8-bit raster
or 24-bit raster images. Raster image data is stored in a two-dimensional
array and attributes may be associated with the image, the file, or both.
Palettes (also known as CLUTs or color look-up tables) and compression method
information can also be associated with an image.
For backward compatibility, the HDF library allows users to access images as
a generic raster image or as an 8-bit or 24-bit raster image. In such a case,
the image is stored once, but under two identities; JHV will show two objects
in the hierarchy, although there is really only one image in the file.
To examine a portion of the image at greater resolution (up to the
resolution of the original image), drag the mouse on the preview image to
select a subsample or click on the image to select the full image.
The selected subsample will be displayed in a separate image window.
JHV can display 2- or 3-dimensional SDS arrays. A 2D array is displayed
as a single image; a 3D array is displayed as an animated sequence of
2D images sliced along the third axis.
See Chapter 7, Animations, for
further information.
A 2D SDS array is displayed as a 2D image in the preview image panel.
A 3D SDS array is displayed as a sequence of 2D images; each 2D image
is a slice of the 3D image taken along an axis selected by the user.
These 2D images can be viewed singly or as an animated sequence.
See Chapter 7, Animations, for
further information.
3.2 Raster Images
Image Name : Raster Image #1
Image Index: 1
Image Size : 625 by 271
Data Number Type: 8-bit unsigned char
Num of Components: 1
Interlace : 0
Num of Attributes: 0
Figure 3.2 General raster image attributes displayed in the message window
Figure 3.3 GR preview image in the main window (upper panels);
selected full image and zoomed subset in the image window
(lower panels and described in a later chapter)
3.3 Scientific Data Set
The scientific data set model, or SDS, supports four
primary data objects: arrays, dimensions, dimension scales, and dimension
attributes. The fundamental object of the model is the SDS array; the
remaining objects describe the array.
Dataset name : Data-Set-2
Rank : 3, 128 X 128 X 65
Data number type : 16-bit signed integer
Number of Attributes : 6
Figure 3.4 SDS attributes displayed in the message window
Figure 2.5 SDS Preview Image
3.4 Vdatas, or Tables
An HDF Vdata object is a collection of records whose values are stored in
fixed-length fields. Thus, a vdata is like a table in which each row has the
same structure and each column contains data of the same type. The Vdata
model allows multiple entries per field; the number of entries or components
in a field is called the order of the field.
VStag : 1963
ref : 2
Vdata Class : Data
Vdata Name : Sea Ice Motion Vectors - 17766010
Vdata records : 291
Interlace : 0
Field number : 8
Field list : REFLAT,REFLON,DSPLAT,DSPLON,DELTAX,DELTAY,ROTANGLE,MATCH
Record size : 45 bytes
Field name Field order Field type
REFLAT 1 64-bit floating point
REFLON 1 64-bit floating point
DSPLAT 1 64-bit floating point
DSPLON 1 64-bit floating point
DELTAX 1 32-bit floating point
DELTAY 1 32-bit floating point
ROTANGLE 1 32-bit floating point
MATCH 1 8-bit unsigned integer
Figure 3.6 A Vdata spreadsheet
3.5 Vgroups
An HDF vgroup object is a structure designed to associate related objects.
The general structure of Vgroups is similar to the Unix file system of
directories, subdirectories, and files in that a Vgroup may contain
references to other Vgroups and/or to data objects. Any HDF object can
be included within a Vgroup.
Figure 3.7 A Vgroup
AVHRR 1KM Global 10 day Composite
and
avhrr-ndvi-060192.hdf
.
Tag/Ref: 1965/2
Name: AVHRR 1KM Global 10 day Composite
Class Name: PlanetaryGrid
Entries: 2
Tag/Ref: 1965/42
Name: avhrr-ndvi-060192.hdf
Class Name: CDF0.0
Entries: 5
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University of Illinois at Urbana-Champaign