GIF and JPG Image Formats

GIF (Graphics Interchange Format) is a compressed image format designed to minimize file size. Images must contain 256 colors or less before they can be saved in the GIF format.

The optimal way to create a GIF file for the web is to change your image to indexed color, select adaptive palette, and adjust the color depth as low as your image allows without sacrificing quality. The pixel depth (ranging from 3 to 8) determines the numbers of color in your palette (3=8 colors and 8=256 colors).

The ideal pixels per inch (ppi) settting for images, GIF or JPEG is 72 dpi. An image with a resolution of 72 ppi will display just as good as an image saved at a higher resolution on current monitors. Therefore, saving images for the web at 72 ppi provides an optimal image for viewing and for minimizing file size.

NOTE: In Adobe Photoshop, change the mode of the picture to Indexed color, determine pixel depth, select adaptive palette, and then save it as a GIF, or export as a GIF89a image.

Non-interlaced and Interlaced Gif Images

GIFs can be save as interlaced or non-interlaced.

Non-interlaced waits to bring in the picture until it's fully loaded into cache. Interlaced brings in the picture in a four stage decoding process. The user will not see a fully "fleshed-out" image after the first of the four passes: the next three passes gradually enhance the quality of the image until the fourth pass.

JPEG

JPEG (Joint Photographic Experts Group) compression can help maintain a 24 bit color photorealistic picture and a small file size. Making images smaller is a definite plus when transmitting across networks. Just remember that JPEG works better on photographs, highly detailed artwork, and art with a big range in tonal value. JPEG compression is not as effective for type, simple cartoons, or line drawings.

Experiment with the amount of compression (range is from 1-10) to apply to your image:

Higher value = better quality = less compression = larger file size

Most images can be compressed to some degree without a noticeable change in image quality.

For further information contact Karen Chinn.