Week 3 - DQ 1 Thursday, May 27, 2010

Based on Kabachinski’s (2007) article, what are the differences between the various graphic formats? Give an example of when you would use each.

When it comes to digital images, there are two broad categories: bitmap and vector. Bitmap images are based on pixels, and show images as the representation of a digital image as a matrix of picture elements (pixels) on a grid. In contrast, vector images are based on shapes, and geometric principles. Kabachinski's (2007) article focuses on the differences between some of the bitmap images: TIFF, GIF, JPEG, and PNG. All three are popular to use on web pages, with unique strengths and weaknesses.

TIFF means Tag Image File Format (TIFF). It was designed for flexibility in image date exchange between machines and applications. TIFF is popular for high color depth pictures and is compatible (supported by) graphic and publishing applications as well as document processors and scanners. Advantages of using TIFF is that it is flexible and adaptable to both machines and applications. The biggest strength of TIFF images is that they are lossless, meaning they retain all original data, even if edited. It is a logical choice for archiving images due to this lossless quality.

GIF stands for Graphical Interchange Format (GIF), and became popular due to its ease of portability, file compression, and is the only option for online animation (besides vector-based formats). It can specify a different color palette for every frame. It is best suited for things like clip art images and logos with large areas of solid color. However, due to the ability to store a new palette each time, it can be used fairly successfully for pictures as well. It is also useful for sharp-edged line art due to its compressibility and smallish color palette. The biggest advantage to GIF is its relatively small file size, loading quickly as well as saving memory space. GIF is a good choice for web sites because of this quality. It's biggest disadvantage is that it is a lossy file, meaning when it is compressed, original image data is lost. Changes cannot be reversed, and each change needs to be saved.

Generally, GIF files should be used for logos, line drawings and icons. It is not the best choice for photographic images, and graphics which have long stretches of continuous-tone in them.

Portable Network Graphics (PNG) was developed as a patent-free answer to the GIF format but is also an improvement on the GIF technique. It sought to utilize the medium while minimizing it's deficiencies. A unique feature of PNG is it's "alpha transparency." PNG builds on the idea of transparency in GIF images and allows the control of the degree of transparency, known as opacity.This leads to interesting effects when overlaying pictures.

It's greatest advantage is it's scalability of color depth, as it allows 16, 24, and 32-bit images. Saving, restoring and re-saving a PNG image will not degrade its quality. However, this ability comes at the cost of larger file size and longer download times. While PNG has more features and a higher compression utility, one of it's biggest drawbacks is lack of animation. Because it lacks animation, it has gained only limited favor with web users.

The JPEG format, started by the Joint Photographic Experts Group, with its support for 16.7 million colors, is primarily intended for photographic images and complex graphics. The internal compression algorithm of the JPEG format, unlike the GIF format, actually throws out information. This is it's biggest disadvantage, which is also called "JPEG rot". Because JPEG has lossy compression, that means every time you save the file you lose some data. Every time you save a JPEG file, it gets a little worse. "JPEG rot"is referrering to this quality of JPEG images.

The user typically has to compromise on either the quality of the image or the size of the file. JPG does not work well on line drawings, lettering or simple graphics because there is not a lot of the image that can be thrown out in the lossy process, so the image loses clarity and sharpness. As a rule, the JPEG format should be used on photographic images, and images which do not look as good with only 256 colors.

Here is a table that summarises the key differences between the GIF, JPEG and PNG image formats:

GIF	JPEG	PNG-8	PNG-24
Better for clipart and drawn graphics with few colours, or large blocks of colour	Better for photographs with lots of colours or fine colour detail	Better for clipart and drawn graphics with few colours, or large blocks of colour	Better for photographs with lots of colours or fine colour detail
Can only have up to 256 colours	Can have up to 16 million colours	Can only have up to 256 colours	Can have up to 16 million colours
Images are "lossless" - they contain the same amount of information as the original (but with only 256 colours)	Images are "lossy" - they contain less information than the original	Images are "lossless" - they contain the same amount of information as the original (but with only 256 colours)	Images are "lossless" - they contain the same amount of information as the original
Can be animated	Cannot be animated	Cannot be animated	Cannot be animated
Can have transparent areas	Cannot have transparent areas	Can have transparent areas	Can have transparent areas

Table accessed at: Elated.com:  Helping People Make Web Sites since 1997