Image Formats -- ~HTML TEST BOARD~

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Date Posted: 20:27:01 01/07/03 Tue
Author: admin
Subject: Image Formats

GIF Image Formats
So, why wasn't the Bitmap chosen as the King of all Internet Images? Because Bill Gates hadn't yet gotten into the fold when the earliest browsers started running inline images.
GIF, which stands for "Graphic Interchange Format," was first standardized in 1987 by CompuServe, although the patent for the algorithm (mathematical formula) used to create GIF compression actually belongs to Unisys. The first format of GIF used on the Web was called GIF87a, representing its year and version. It saved images at 8 pits-per-pixel, capping the color level at 256. That 8-bit level allowed the image to work across multiple server styles, including CompuServe, TCP/IP, and AOL. It was a graphic for all seasons, so to speak.

CompuServe updated the GIF format in 1989 to include animation, transparency, and interlacing. They called the new format, you guessed it: GIF89a.

There's no discernable difference between a basic (known as non-interlaced) GIF in 87 and 89 formats.
Even the bytes are the same. It's the transparency, animation, and non-interlacing additions to GIF89a that really set it apart.

JPEG Image Formats
JPEG is a compression algorithm developed by the people the format is named after, the Joint Photographic Experts Group. JPEG's big selling point is that its compression factor stores the image on the hard drive in less bytes than the image is when it actually displays. The Web took to the format straightaway because not only did the image store in fewer bytes, it transferred in fewer bytes. As the Internet adage goes, the pipeline isn't getting any bigger so we need to make what is traveling through it smaller.
For a long while, GIF ruled the Internet roost. I didn't really like the new JPEG format when it came out. It was less grainy than GIF, but it also caused computers without a decent amount of memory to crash the browser. (JPEGs have to be "blown up" to their full size. That takes some memory.) There was a time when people only had 8 or 4 megs or memory in their boxes. Really. It was way back in the Dark Ages.

JPEGs are "lossy." That's a term that means you trade-off detail in the displayed picture for a smaller storage file.

I save my JPEGs at 50% or medium compression.
The difference between the 1% and 50% compression is not too bad, but the drop in bytes is impressive. The numbers I am showing are storage numbers, the amount of hard drive space the image takes up.

You've probably already surmised that 50% compression means that 50% of the image is included in the algorithm. If you don't put a 50% compressed image next to an exact duplicate image at 1% compression, it looks pretty good. But what about that 99% compression image? It looks horrible. Bytes are lost at the expense of detail.

Progressive JPEGs
You can almost guess what this is all about. A progressive JPEG works a lot like the interlaced GIF89a by filling in every other line, then returning to the top of the image to fill in the remainder. The example is again presented three times at 1%, 50%, and 99% compression.
Rule of thumb: If you're going to use progressive JPEG, keep the compression up high, 75% or better.

Which image do I use where?
There's just not a good answer to this question. No matter what I say, someone else can give you just as compelling a reason why you should do the opposite. I'll tell you the rules I follow:

Small images, like icons and buttons: GIF (usually non-interlaced)

Line art, grayscale (black and white), cartoons: GIF (usually non-interlaced)

Scanned images and photographs: JPEG. (I prefer sequential. I'm not a fan of progressive.)

Large images or images with a lot of detail: JPEG (I prefer sequential)

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Replies:

Re: Image Formats -- jannel, 09:08:31 03/04/03 Tue