Graphics Basics

Graphic File Formats

File Formats

Anyone using a computer should be familiar with the concept of file formats. File formats help to identify what kind of file is being worked with and are usually distinguished by the file extension. Most computer applications have a native file format, i.e., a default format for files created in that program. (For example, by default, Adobe InDesign saves files with an .indd extension, while Adobe Illustrator saves files with an .ai extension.) In addition, many applications allow the user to save files in formats other than the application's native format. It is a good idea to save files in the native format in addition to any desired non-native format.

There are three primary graphic file formats for Web images: GIF (Graphic Interchange Format), JPEG (Joint Photographic Experts Group), and PNG (Portable Network Graphics). Each of these graphic file formats is cross-platform and uses some form of compression to be optimized for use on the Web.

GIF (Graphic Interchange Format)

GIF is the preferred file format for images with large areas of solid color, such as logos, text as graphics, cartoons, etc. The GIF file format supports 8-bit images (up to 256 colors).

Here are some other important features of GIF images:

  1. GIFs use a lossless compression scheme, i.e., images do not lose data when compressed and, therefore, don't lose image quality.
  2. GIF 89a images allow for one transparent color.
  3. GIF interlaced images can be displayed as low-resolution images initially and then develop clarity and detail gradually.
  4. GIF images can be used to create simple animations.
  5. GIFs are saved with a .gif extension.

JPEG (Joint Photographic Experts Group)

JPEG is the preferred file format for photographic images. In addition, the JPEG format works well with subtle transitions in color, such as watercolors, pencil or charcoal drawings, etc. The JPEG file format supports 24-bit images (over 16 million colors).

Here are some other important features of JPEG images:

  1. JPEGs use a lossy compression scheme; data is removed from the image to make the file size smaller when compressed.
  2. JPEG images allow for various compression levels, providing for the adjustment of compression to best display the image without losing too much of the image quality. Due to the various compression levels, it is suggested that the original image be saved and used for editing purposes; don't edit a JPEG image that has been compressed.
  3. JPEG images are not interlaced; however, progressive JPEG images can be interlaced.
  4. JPEGs are saved with a .jpeg or .jpg extension.

PNG (Portable Network Graphics)

PNG is a relatively new image file format that has many advantages over GIF and JPEG. The PNG file format can be 24-bit or 8-bit, eliminating some of the limitations of the other formats.

Here are some other important features of PNG images:

  1. PNG images use a lossless compression scheme.
  2. PNG images are interlaced.
  3. PNG images support 8-bit transparency.
  4. PNG images include the ability to correct for gamma information based on the computer monitor on which they are being viewed.
  5. PNGs are saved with a .png extension.

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange for better compression rates.wiki