Formats Designed For High Compression

JPEG (Joint Photographic Experts Group) (21,798 bytes)

• JPEG compression economizes on the way data is stored and also identifies and discards extra data, that is, information beyond what the human eye can see.
• Because it discards data, the JPEG algorithm is referred to as "lossy". This means that once an image has been compressed and then decompressed, it will not be identical to the original image. In most cases, the difference between the original and compressed version of the image is indistinguishable.
• In general, compressed JPEG images have compression ratios of between 5:1 and 15:1. A trade-off does exist between the image quality and the amount of compression. You do not need to decompress images saved in the JPEG format. They are automatically decompressed when they are opened.
• How bad is JPEG?
• JPEG vs GIF

JAS

• The JAS format files were designed to create the smallest possible image files for 24 bits per pixel color images and 8 bits per pixel gray scaled images. JAS uses a discrete cosine transformation to alter the image data and then compress that data. This process results in a substantial reduction of the file size when the image is saved to the disk.
• This method is referred to as "lossy": saving and retrieving an image using the JAS file format will result in some loss of image data. The amount of loss is dependent on the compression level that you have selected in your application. By using the lowest possible value for the file compression you will have the least amount of loss.
• Tip: Whenever you save an image as a JAS file, you should immediately load the image that was just saved. Check to make sure the amount of data loss is acceptable before deleting your original image.
• Characteristics:
  • JAS (without color palette) format supports 24 bits per pixel.
  • JAS (gray scale) format supports 8 bits per pixel.

TIFF (Tagged-Image file Format)

• Tagged-Image File Format (TIFF) is used mainly for exchanging documents between different applications and different computer platforms.
• The Tagged Image File Format was primarily designed to become the standard format. In order to become the standard, the format was designed to handle just about any possibility. The result of this design provided the flexibility of an infinite number of possibilities of how a TIFF image can be saved.
• As a result, no application at all can claim to support all TIFF variations. Some professional applications support many TIFF variations, but there will always be an obscure variation that will cause a problem for some application.
• The TIFF format uses 6 different encoding routines:
  • No-compression
  • Huffman
  • Pack Bits
  • LZW
  • Fax Group 3
  • Fax Group 4
• In addition it differentiates between types of images in 3 different categories:
  • Black and white
  • Gray scaled
  • Colored
• The TIFF format supports LZW method compression for image types. (This is the same compression used by the GIF format for indexed color.)
• Tip: If an application is having a problem reading compm reading compressed TIFF files, try re-saving the file without compression.
• Characteristics:
  • TIFF - with no compression format supports 1, 4, 8, 24 bits per pixel.
  • TIFF - Huffman format supports 1 bit per pixel.
  • TIFF - Pack Bits format supports 1 bit per pixel..
  • TIFF - LZW format supports 4, 8, 24 bits per pixel.
  • TIFF - Fax Group 3 format supports 1 bit per pixel.
  • TIFF - Fax Group 4 format supports 1 bit per pixel.