Книга: Fedora™ Unleashed, 2008 edition

Working with Graphics Formats

Working with Graphics Formats

Image file formats are developed to serve a specific technical purpose (lossless compression, for example, where the file size is reduced without sacrificing image quality) or to meet a need for a proprietary format for competitive reasons. Many file formats are covered by one or more patents. For example, the GIF format had fallen into disfavor with the open-source crowd because the patent holder waited a while before deciding to enforce his patent rights.

If you want to view or manipulate an image, you need to identify the file format to choose the proper tool for working with the image. The file's extension is your first indicator of the file's format. The graphics image formats supported by the applications included with Fedora include the following:

? .bmp — Bitmapped graphics, commonly used in Microsoft Windows

? .gif — CompuServe Graphics Interchange Format

? .jpg — Joint Photographic Experts Group

? .pcx — IBM Paintbrush

? .png — Portable Network Graphics

? .svg — Scalable Vector Graphics

? .tif — Tagged Image File format

An extensive list of image file extensions can be found in the man page for ImageMagick, an excellent application included with Fedora, which you learn more about in upcoming sections of this chapter.

Often, a file you want to manipulate in some way is in a format that cannot be used by either your graphics application or the final application. The solution is to convert the image file — sometimes through several formats. The convert utility from ImageMagick is useful, as is the netpbm family of utilities. If it is not already installed, ImageMagick can be installed with the Add Remove Software GUI found in the System Settings menu; the netpbm tools are always installed by default.

The convert utility converts between image formats recognized by ImageMagick. Color depth and size also can be manipulated during the conversion process. You can use ImageMagick to append images, surround them with borders, add labels, rotate and shade them, and perform other manipulations well suited to scripting. Commands associated with ImageMagick include display, animate, identify, and import_. The application supports more than 130 different image formats (all listed in the man page for ImageMagick).

The netpbm tools are installed by default because they compose the underpinnings of graphics format manipulation. The man page for each image format lists related conversion utilities; the number of those utilities gives you some indication of the way that format is used and shows how one is built on another:

? The man page for ppm, the portable pixmap file format, lists 47 conversion utilities related to ppm. This makes sense because ppm, or portable pixmap, is considered the lowest common denominator for color image files. It is therefore often used as an intermediate format.

? The man page for pgm, the portable graymap file format, lists 22 conversion utilities. This makes sense because pgm is the lowest common denominator for grayscale image files.

? The man page for pnm, the portable anymap file format, lists 31 conversion utilities related to it. However, there is no format associated with PNM because it operates in concert with ppm, pgm, and pbm.

? An examination of the man page for pbm, the portable bitmap file format, reveals no conversion utilities. It's a monochrome format and serves as the foundation of the other related formats.

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