Keywords: sic

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### GILDAS Images

Variables are normally allocated using virtual memory, and hence are lost once the program terminates. It is possible to allocate variables as disk files, called Images, which are mapped into the virtual memory of the program, using command

            DEFINE IMAGE Variable File Keyword

where

  `Variable` is the desired variable name. This variable name should be at most 3 characters to hold the header variables.
  `File` is the name of the file holding the image. Default extension is `.gdf` (GILDAS Data Format), but any extension is valid.
  `Keyword` indicates how the file must be used and may be
    `READ` use an existing image as Read-Only variable
    `WRITE` use an existing image as Read-Write variable. The user must have Write access to the file.
    `EXTEND` Allows to extend the last dimension of an existing image. The user must have write access to it.
    `REAL` create a new image of type `REAL`
    `DOUBLE` create a new image of type `DOUBLE`
    `INTEGER` create a new image of type `INTEGER`.

For already existing images (`READ, WRITE`), the type and size of variable are determined by the program. For new images, the size must be specified in the variable name, and the variable is always declared as Read-Write. To `EXTEND` image, the new value of the last dimension must be specified in the variable name. Header variables (see next section) are defined with the same status (Read or Write) as the image.

Images (i.e. files in the GILDAS data format) are used by most programs distributed by the Groupe d'Astrophysique. See the GILDAS (Grenoble Image and Line Data Analysis Software) documentation. Tables usually refer to images with 2 dimensions, but more generally to images which do not define a coordinate system.

The `DEFINE IMAGE` command has two variants, `DEFINE TABLE` and `DEFINE HEADER`. `DEFINE TABLE` only defines a single variable pointing to the image content. On the opposite `DEFINE HEADER` defines only the header variables, as described in next section.

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