David Lash DePaul University SNL 262 Web Page Design Notes on gifs

David Lash 2 How the gif controversy happened zIn 1983 an IEEE paper presented a compression algorithm call LWZ (lempel Ziv Welch). yCompression algorithms at the time valuable for transferring networking data yFrom what I understand, paper did not say their method was patented. zIn CompuServe announced a format to digitize an image called GIF (grafical Interchange Format). yIt used LWZ algorithm but apparently CompuServe did not know about patent zJust a little later Mosaic browser (by UofI Graduate student Marc Ansderson who later founded Netscape) built into it capability to display gif images yFormat became very popular zAround 1995, Unisys started enforcing it patent rights yOften to the surprise of Web Administrators. yLarge sites had to pay hefty royalties, yPeople wrote “hacks” to disable the compression yImages were pulled off. zJuly 2003 LWZ patent expired in US.

David Lash 3 What is digital image? zA format for encoding a way to represent a picture using RGB color format yEach pixel to display needs to have a RGB value yLess colors you display the smaller the image x(since can compress the image more) zGif format specification allows you to describe more than 1 image yThere is a delay to show each image giving an animation effect. (but no sound) yWant to know how to create animated gifs? xTry this site:

David Lash 4 What is digital image? zA format for encoding a way to represent a picture using RGB color format yEach pixel to display needs to have a RGB value yLess colors you display the smaller the image x(since can compress the image more) zGif format specification allows you to discribe more than 1 image yThere is a delay to show each image giving an animation effect. (but no sound)

David Lash 5 How Does Compression Work? zLWZ is a dictionary based system of compression zSuppose you are compressing the following statement: ask not what your country can do for you ask what you can do for your country zYou might build a dictionary that looks like: 1.ask 2.what 3.your 4.country 5.can 6.do 7.for 8.you zYou might therefore encode our sentence as: 1 not zA program can be written to translate to and from that format. zCompression method look for even more patterns and build dictionaries to compress smaller

David Lash 6 Which to use gif or jpg? zTypically might look at image in both formats to see best for your site zTake a look at this site jpggifs High-quality images with lots of colorsUsed for logos, ads with uniform colors > 16 million colorsUp to 256 colors No transparencies or animationTransparencies and annimation