Smart Compression JBIG2, JPEG2000, and MPEG4
A simple definition of codec could be "any type of format used to compress a file to a smaller size." Clearly, the formats previously addressed in this Primer fit well into this definition. Compression protocols have been around for some time; development on codecs began decades ago.
But not all compression codecs qualify as "smart compression codecs." Earlier codecs, including JPEG, TIFF G3, and TIFF G4, employed fully-specified compression standards for both encoding and decoding. Recent developments in CPU power brought forth newer smart compression codecs, including JBIG2, JPEG 2000, and MPEG4. These protocols specify compression standards to decode a file, but do not specify precisely how to encode a file. This enables a sophisticated vendor to utilize a wide range of techniques to improve image quality, increase the compression ratio and print speed, and enhance other document properties.
In many ways the previous generations of compression codecs were similar in structure to uncompressed image formats. Raw image formats such as BMP store the value of every pixel in the image. The first image compression standards effectively did the same thing. In older compression standards such as TIFF, the decoder would traverse the entire image and "paint" each pixel based on the information in the compressed file. Even if a page was completely blank, the TIFF specs required each row of the compressed file to explicitly state that it was blank. Of course, this limited the compression advantage of the older compression codecs.
JBIG2 in Comparison
To allow a significant improvement of compression rates over these older standards, JBIG2 uses a technique of having a symbol dictionary and an addressing stream. The symbol dictionary contains the fonts. The addressing stream says where to place the fonts in the image. It is up to the encoder to choose and define the symbols in the symbol dictionary and to find the most efficient way to properly address them in the image.
The history of paperless document transmission and the development of compression standards to expedite transmission of those documents demonstrates that efficient encoding of captured bitonal and color documents is more compelling than ever. From fax transmission through the electronic transmission and storage of complex multilayer color image and text documents, JBIG2 is rapidly gaining acceptance as the new bitonal compression standard for the digital imaging and printing industry.