You can use ImageMagick's identify command to identify details about image files. The welcoming splash image used for the GRUB bootloader is located in /boot/grub and is a gzipped .xpm image. If you run identify on the image, you'll discover that it's a 640x480 xpm image with 16-bit color depth. That's all you need to know to construct a replacement image of your own. Using the GIMP or another graphics tool, crop or resize your chosen image to 640x480 and change the color depth to 16 bits. Save the image as splash.xpm and then gzip the resulting file. Replace the original Red Hat file, and you now have a custom boot image. The use of identify helped you duplicate the parameters of the original image to comply with the requirements of GRUB.
The identify command is useful to identify unknown image files and to determine whether they're corrupt.
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. Because of this, it is 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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