Understanding Image Definitions in IDL
An understanding of basic image definitions is necessary before proceeding with image processing tasks. Some routines are specifically designed for certain types of images. Binary, grayscale, and indexed images are two-dimensional arrays, while RGB images are three-dimensional arrays. In which group an image belongs is determined by its contents and how it relates to its color information.
Within IDL, an image can be categorized as follows:
Table 1-1: Image Definitions
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Image Type
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Descriptions
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Binary Images
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Binary images contain only two values (off or on). The off value is usually a zero and the on value is usually a one. This type of image is commonly used as a multiplier to mask regions within another image.
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Grayscale Images
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Grayscale images represent intensities. Pixels range from least intense (black) to most intense (white). Pixel values usually range from 0 to 255 or are scaled to this range when displayed.
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Indexed Images
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Instead of intensities, a pixel value within an indexed image relates to a color value within a color lookup table. Since indexed images reference color tables composed of up to 256 colors, the data values of these images are usually scaled to range between 0 and 255.
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RGB Images
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Within the three-dimensional array of an RGB image, two of the dimensions specify the location of a pixel within an image. The other dimension specifies the color of each pixel The color dimension always has a size of 3 and is composed of the red, green, and blue color bands (channels) of the image.
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Note
Grayscale and binary images can actually be treated as indexed images with an associated grayscale color table.
Color information can also be represented in other forms, which are described in Color Systems (Using IDL).
