Representing Image Data in IDL

Pixel values in an image file can be stored in many different data types. IDL maintains 15 different data types. The original data type of an image is reflected in IDL when importing the image, but the type can be converted once the image is stored in an IDL variable. The following types are commonly used for images:

Byte — An 8-bit unsigned integer ranging in value from 0 to 255. Pixels in images are commonly represented as byte data.

Unsigned Integer — A 16-bit unsigned integer ranging from 0 to 65535.

Signed Integer — A 16-bit signed integer ranging from -32,768 to +32,767.

Unsigned Longword Integer — A 32-bit unsigned integer ranging in value from 0 to approximately four billion.

Longword Integer — A 32-bit signed integer ranging in value from approximately minus two billion to plus two billion.

Floating-point — A 32-bit, single-precision, floating-point number in the range from -10³⁸ to 10³⁸, with approximately 6 or 7 decimal places of significance.

Double-precision — A 64-bit, double-precision, floating-point number in the range from -10³⁰⁸ to 10³⁰⁸ with approximately 14 decimal places of significance.

While pixel values are commonly stored in files as whole numbers, they are usually converted to floating-point or double-precision data types prior to performing numerical computations. See the examples section of "REFORM" (IDL Reference Guide) and "Calculating Image Statistics" in Chapter 4 for more information.

IDL provides predefined routines to convert data from one type to another. These routines are shown in the following table:

Table 1-2: Some IDL Data Type Conversion Functions
Function	Description
BYTE	Convert to byte
BYTSCL	Scale data to range from 0 to 255 and then convert to byte
UINT	Convert to 16-bit unsigned integer
FIX	Convert to 16-bit integer, or optionally other type
ULONG	Convert to 32-bit unsigned integer
LONG	Convert to 32-bit integer
FLOAT	Convert to floating-point
DOUBLE	Convert to double-precision floating-point