FIGURE 8a General Raster Image Data Set Contents
8.2.1 Required General Raster Objects
Every general raster data set must contain three objects. These objects are the image array , the image array name and pixel type . Required objects are automatically created from the information provided at the time the data set is defined. 8.2.1.1 Image Array
An image array is the two-dimensional structure used to store the image's pixel data. 8.2.1.2 Image Array Name
Each image array has an image name consisting of a string of case-sensitive alphanumeric characters. The name must be provided by the calling program; the GR interface does not provide one by default if one isn't specified. Names are assigned when the data set is created and cannot be changed afterwards. Image array names do not have to be unique within a file, but if they are not it can be difficult to distinguish between the general raster data sets in the file. 8.2.1.3 Pixel Type
Another fundamental difference between the SD SDS model and the GR data model is that a GR image data array is defined by data type of its elements and the number of components in each element rather than only the data type. This is because each element in an image array corresponds to one pixel and each element of pixel data can consist of a variable number of color component values. (Red-Blue-Green or RGB, Cyan-Magenta-Yellow-Black or CMYK, etc.) These color component values can be represented by different methods and data lengths (8-bit lookup table or 24-bit direct representation, graphically depicted in Figure 6a in Chapter 6 and Figure 7b in Chapter 7 respectively). The data type of array elements and the number of components in each element are collectively referred to as the pixel type. 8.2.1.4 Dimensions
Image array dimensions specify the size of an image array. There are no routines in the GR library that allow the HDF user to add attributes to a dimension or change the scale, as is possible in the SD interface. 8.2.2 Optional General Raster Objects
There are two types of optional objects available for inclusion in a general raster data set: palettes and attributes. These objects are only created when specifically requested by the calling program; the GR interface doesn't provide predefined palettes or attributes.
Attributes