UNDERSTANDING THE FORMATS OF WEB IMAGE FILES

Formats of Web Image files

    The format of a file is indicated by its extension, or suffix, which follows the "dot." Image files (clip art and photographs) used in desktop publishing are converted to the gif or jpg format before they are used on the Web.

    Why?

      An image (graphic) file is automatically downloaded to the user's hard drive--to the cache folder, on a temporary basis.

      Image files are compressed to cut down on the loading time to enable the transfer of the file.

The two main formats of compressed files on the Web are gif and jpg.

GIF IMAGES

The gif format can be used for any image needing normal resolution and a maximum of 256 colors.

  1. This format sometimes makes a smaller file than the jpg version of the same image, particularly when the number of colors can be reduced, for example, to 16.
     

  2. Gifs can be interlaced. The loading process seems to be quicker because the image appears in alternate lines of pixels, so the viewer sees the whole (but incomplete) image quicker than if it loads in sequence from the top down.


    3.  

  3. The background of a gif can be coded as transparent.

EXAMPLES OF GIF CLIP ART
 

This is a 13 K 
gif file.

A 17 K gif file

4 KB gif file

 3 KB gif file

The table below illustrates the optional transparent coding of a gif file; the first image does not have a transparent background, the second does.
 

Animated Gifs

JPG IMAGES

The JPG (JPEG) format can be used for any image needing high resolution and millions of colors. In some situations, the gif format is sometimes more efficient for "weight" and also more versatile.  With certain compression ratios, however, jpg files can be smaller than gif files.  See the section on photographs.

 

Page Changed 09/09/2005

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