fileformats
There are three major graphics formats for web files. Two of these formats, GIF and JPEG, are in common use today, and the third, PNG, is a newly designed format that should become a standard in the near future.
CompuServe GIF Format:
The CompuServe Graphics Interchange Format (GIF, pronounced with either a hard or soft g) is the most commonly used on the Internet, primarily because it was the first format that early web browsers used as a standard.
GIF Format Specifications
GIF images can contain up to a maximum of 256 colors, and as few as two (black and white). The format uses LZW image compression to reduce the file size. This compression, unlike some other compression methods, is not “lossy”—when you re-open a GIF image, it appears exactly as it did before you saved it.

GIF images can be interlaced, which displays the image in gradually increasing detail as it is downloaded, rather than all at once with maximum detail. This option allows the viewer to get a rough idea what the image looks like before the download is complete, so they can choose to save time by canceling the download.

The GIF89a format allows you to specify areas of the image as transparent. This is useful for placing images with irregular edges on a web page background without a rectangular border around the image. Photoshop allows you to use three different methods to specify which areas are transparent, depending on how you create the image. You will work with each method in the next section.

Lastly, you can use additional software to combine multiple GIF images into one animated file. The animated GIF image can play repeatedly, like a film loop, or can be set to run once and then stop.

Best Uses for GIF Images
Because of the way LZW compression works, GIF images are highly efficient for saving illustrations and graphics with large flat color areas.

GIF is the format of choice to compress lettering, simple cartoons, and line drawings. GIF images are often made up of Vector graphics, such as lines and curves, which are mathematically defined, so they produce a high quality result, regardless of how much they are scaled. GIF files are often composed of few colors and compress well.

The compression algorithm scans across rows of pixels, and if any consecutive pixels in one row match exactly, the value is only written to the file once. Therefore, large images with flat colors are not much larger in file size than images with significantly fewer pixels. This principle comes into play in images like the vertical bar background you created. Even though the image was 1300 pixels wide, most of the pixels in each row were white, so the file size would not be large if it were saved in GIF format.

For this reason, it is also best to create gradients that run vertically, so all of the pixels in one horizontal row are the same color. All of the heading text images in the Cilantro’s site including the main logo use vertical gradients to conserve file space.

The GIF format is not as good as JPEG for photographic images, since it only supports up to 256 colors. Photographic images appear best in RGB mode, which allows up to 16.7 million colors. The main reason to use the GIF format for photographic images is when you need a transparent area surrounding the photograph. In this case, the JPEG format cannot be used, so you must use the GIF89a format.

== About PNG ==

What can I say? PNG rocks.
Honestly, I can't think of any reason not to use PNG as your standard image format except that it has a funny name and may be a little unfamiliar. But all the main image editors love it, so why not you?

Here are just some of PNG's advantages:
1) Support for 16-bit RGB+Alpha images. Lower color spaces also supported.

2) Very tight, lossless compression.

3) Ubiquitous. Even if you've never heard of PNG, chances are you have programs that can read PNG files. Virtually every commercial image editor supports PNG, and so does Apple's QuickTime. Most modern web browsers support embedded PNG files, some even support alpha transparency.

3a) User Friendly. Even though some programs such as Adobe Photoshop and After Effects can't natively write 16-bit PNG files, they can read them just fine. Even 8-bit RGB programs can usually read 16-bit PNGs. So feel free to send 16-bit PNGs to anyone, even if they don't have SuperPNG.

4) Public, open source format. Free of patents. 5) Fun to say. Everything you ever wanted to know about PNG is available at libpng.org/ and fnordware.com

Most of this information is from a course I participated some years ago but the company does not exist any more. The information's about NPG is from fnordware.com

Updated last time January 22 Year 2003.


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