World Library  
Flag as Inappropriate
Email this Article

Jbig2

 

Jbig2

JBIG2
Developed by Joint Bi-level Image Experts Group
Latest release
2
Contained by Portable Document Format, FAX
Standard ITU T.88 & ISO/IEC 14492
Website .comjbig2

JBIG2 is an ISO/IEC 14492.[3]

Contents

  • Functionality 1
  • Technical details 2
    • Text image data 2.1
    • Halftones 2.2
    • Arithmetic entropy coding 2.3
  • Patents 3
  • Disadvantages 4
  • References 5
  • External links 6

Functionality

Ideally, a JBIG2 encoder will segment the input page into regions of text, regions of halftone images, and regions of other data. Regions which are neither text nor halftones are typically compressed using a context-dependent arithmetic coding algorithm called the QM coder. Textual regions are compressed as follows: the foreground pixels in the regions are grouped into symbols. A dictionary of symbols is then created and encoded, typically also using context-dependent arithmetic coding, and the regions are encoded by describing which symbols appear where. Typically, a symbol will correspond to a character of text, but this is not required by the compression method. For lossy compression the difference between similar symbols (e.g., slightly different impressions of the same letter) can be neglected; for lossless compression, this difference is taken into account by compressing one similar symbol using another as a template. Halftone images may be compressed by reconstructing the grayscale image used to generate the halftone and then sending this image together with a dictionary of halftone patterns.[4] Overall, the algorithm used by JBIG2 to compress text is very similar to the JB2 compression scheme used in the DjVu file format for coding binary images.

PDF files versions 1.4 and above may contain JBIG2 compressed data. Open source decoders for JBIG2 are jbig2dec,[5] the java-based jbig2-imageio[6] and the decoder found in versions 2.00 and above of xpdf. An open source encoder is jbig2enc.[7]

Technical details

Typically, a bi-level image consists mainly of a large amount of textual and halftone data in which the same shapes appear repeatedly and the bi-level image is segmented into three regions: text, halftone, and generic regions. Each region is coded differently and the coding methodologies are described in the following passage.

Text image data

Text coding is based on the nature of human visual interpretation. A human observer cannot tell the difference of two instances of the same characters in a bi-level image even though they may not exactly match pixel by pixel. Therefore, only the bitmap of one representative character instance needs to be coded instead of coding the bitmaps of each occurrence of the same character individually. For each character instance, the coded instance of the character is then stored into a "symbol dictionary".[8] There are two encoding methods for text image data: pattern matching and substitution (PM&S) and soft pattern matching (SPM). These methods are presented in the following subsections.[9]

Block diagrams of (Left) pattern matching and substitution method and (Right) soft pattern matching method.

Pattern matching and substitution: After performing image segmentation and match searching, and if a match exists, we code an index of the corresponding representative bitmap in the dictionary and the position of the character on the page. The position is usually relative to another previously coded character. If a match is not found, the segmented pixel block is coded directly and added into the dictionary. Typical procedures of pattern matching and substitution algorithm are displayed in the left block diagram of the figure below. Although the method of PM&S can achieve outstanding compression, substitution errors could be made during the process if the image resolution is low.

Soft pattern matching: In addition to a pointer to the dictionary and position information of the character, refinement data is also required because it is a crucial piece of information used to reconstruct the original character in the image. The deployment of refinement data can make the character-substitution error mentioned earlier highly unlikely. The refinement data contains the current desired character instance which is coded using the pixels of both the current character and the matching character in the dictionary. Since it is known that the current character instance is highly correlated with the matched character, the prediction of the current pixel is more accurate.

Halftones

Halftone images can be compressed using two methods. One of the methods is similar to the context-based arithmetic coding algorithm which adaptively positions the template pixels in order to obtain correlations between the adjacent pixels. In the second method, descreening is performed on the halftone image so that the image is converted back to grayscale. The converted grayscale values are then used as indexes of fixed-sized tiny bitmap patterns contained in a halftone bitmap dictionary. This allows decoder to successfully render a halftone image by presenting indexed dictionary bitmap patterns neighboring with each other.

Arithmetic entropy coding

All three region types including text, halftone, and generic regions may all use arithmetic coding. JBIG2 specifically uses the MQ coder.

Patents

Patents for JBIG2 are owned by IBM and Mitsubishi. Free licenses should be available after a request. JBIG and JBIG2 patents are not the same.[10][11][12]

Disadvantages

When used in lossy mode, JBIG2 compression can potentially alter text in a way that's not discernible as corruption. This is in contrast to some other algorithms, which simply degrade into a blur, making the compression artifacts obvious.[13] Since JBIG2 tries to match up similar-looking symbols, the numbers "6" and "8" may get replaced for example.

In 2013, various substitutions (including replacing “6” with “8”) were reported to happen on some Xerox Workcentre photocopier and printer machines, where numbers printed on scanned (but not OCRed) documents could have potentially been altered. This has been demonstrated on construction blueprints and some tables of numbers; the potential impact of such substitution errors in documents such as medical prescriptions was briefly mentioned. [14][15][16] David Kriesel and Xerox are investigating this.[17][18]

References

  1. ^ Press release from the Joint Bi-level Image experts Group
  2. ^
  3. ^
  4. ^ JBIG2-the ultimate bi-level image coding standard, by F. Ono, W. Rucklidge, R. Arps, and C. Constantinescu, in pp. 140–143, Proceedings, 2000 International Conference on Image Processing, (Vancouver, BC, Canada), vol. 1.
  5. ^ jbig2dec home page
  6. ^ open source jbig2 plugin for Java's ImageIO
  7. ^ jbig2enc home page
  8. ^ F. Ono, W. Rucklidge, R. Arps, and C. Constantinescu, "JBIG2-the ultimate bi-level image coding standard," Image Processing, 2000. Proceedings. 2000 International Conference on , vol.1, pp.140-143 vol.1, 2000.
  9. ^ P. Howard, F. Kossentini, B. Martins, S. Forchhammer, and W. Rucklidge, "The emerging JBIG2 standard," Circuits and Systems for Video Technology, IEEE Transactions on , vol.8, no.7, pp.838-848, Nov 1998.
  10. ^
  11. ^
  12. ^
  13. ^
  14. ^
  15. ^
  16. ^ http://fontfeed.com/archives/xerox-scanners%E2%80%8A%E2%80%8Aphotocopiers-randomly-alter-numbers/
  17. ^
  18. ^

External links

  • A JBIG2 Primer
  • The final draft of the specification
  • STDU Viewer - freeware viewer for JBIG2 files and some another formats
  • OpenSource Java JBIG2 image decoder from IDRsolutions
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.