level II images

Images

Image formats commonly used for web content are JPG or GIF. GIF and JPEG are compressed images viewed through any web browser.  The GIF and the JPEG are used because they are small and are therefore quicker to load on the internet.  Below are some differences between GIF and JPEG graphic image format types:

GIF:

• Means Graphics Interchange Format
• GIF is a “lossless” compression algorithm
• Limited by 256 indexed color (so called 8-bit color)
• Uses a variant of the Lempel-Ziv Welch compression algorithm LZW
• Does not work well with images created using HIGH COLOR depth.
• Best to use for few colors; line art graphics
• Other characteristics specific to GIF relate to ANIMATION and TRANSPARENCY
  

GIF is a lossless compression scheme.  This means that nothing is lost when the GIF is saved.  What is meant by GIF animation? GIF Animation is a GIF based movie format.  24 GIF images are displayed per second creating an animation like those in a cartoon movie (see Figure 3c). These animations are quicker to load because of their size.  Like the GIF pictures, these are good for the internet because people don't like to wait for the graphics to load.

JPEG:

• Means Joint Photographic Experts Group
• JPEG is a "lossy" compression algorithm
• Supports 24-bit called TRUE COLOR Variable and high compression levels
• Best used on high quality images without distinct edges and gradual color changes
• Difficult to edit
• Every time a JPEG is saved, it loses quality

JEPG is used mostly for high quality images without distinct edges and gradual color changes.  JPEG is "lossy", meaning that the decompressed image isn't quite the same as the one you started with.  (There are lossless image compression algorithms, but JPEG achieves much greater compression rate than possible with a lossless methods).  JPEG is designed to exploit known limitations of the human eye, notably the fact that small color changes are perceived less accurately than small changes in brightness.  Thus, JPEG is intended for compressing images that will be looked at by humans.  JPEG works by converting the spatial image representation into a frequency map.  A mathematic function (Discrete Cosine Transform) separates the high- and low-frequency information present in the image.  The high frequency information is then selectively discarded depending on the quality setting.  The greater the compression level, the greater the degree of information loss.  Data is lost each time a file is JPEG encoded, depending on the smoothing and compression level, but also at the highest quality levels.  The data loss can be set to a minimum and is usually invisible to the human eye.  The data loss may therefore only be a problem if the files are encoded many times (see Figure 3d-e) or if you plan to machine-analyze your images.

Figure 3a. Logo, GIF, image matrix 100*100 pixels, border index transparency, 64colors, size 5.76k, load time1sec at 28 kbps (kilobytes per second ).

Figure 3b. Drawing, GIF, 32colors, size6.5k, load time 2sec at 28 kbps (kilobytes per second ).

Figure 3c. Animated GIF, vertebroplasty puncture.

Figure 3d. Picture, JPEG, quality 80 %, smoothing 0%, 24bitscolor, size9.18k, load time 2sec at 28 kbps (kilobytes per second ).

Figure 3e. Image degradation example: same picture as above after 50 JPEG encoding processes (quality 60% smoothing 20%).