5 ms·

This potential problem is overstated if the quality of the scans is decent. I have scanned literally hundreds of books, and encoded them in both djvu and pdf/j

This potential problem is overstated if the quality of the scans is decent. I have scanned literally hundreds of books, and encoded them in both djvu and pdf/jbig2 formats, and have never once found a bad character. (Yes, having heard of this issue, I did initially try to find examples) The free djvu tools have parameters you can adjust to make them more or less aggressive at combining similar characters, and above 200DPI it's hard to even force the issue to happen for something like an 8 versus a 6. That was my experience anyway. It's also telling that the example trotted out is always a decades-old photocopier implementation. Unless someone does some comparisons of recent scans and finds a bunch of altered characters, I just don't buy this concern. MRC+JBig2 is really great and the size savings have not been trivial for me personally. (I hesitate to add that the alternative would be, I assume, DCT/jpeg page images, which introduce lots of noisy artifacts of their own... destroying our history one DCT-block at a time!)

8n4vidtmkvmk 4y ago
I've been reading some books on my ereader and every now and then I've come across a word that has been misspelled with similar looking characters. now I'm wondering if it's this jbig2 issue
jimjimjim 4y ago
There are a number of scans I've come across on archive.org where the person uploading has used jbig2 and there are pages where letters like 'e' get swapped for an 's'. My general rule when creating pdfs with scans is: If it's mostly text or line drawings and it will be used in a professional setting use png/flate. If it's a photo use jpg (especially if the source is a jpg).
- richard_todd 4y ago
  I'd love to see some examples (especially if the scan appeared to be of good quality and not a grainy 100dpi affair). Do you have any idea what kinds of documents you were browsing when you found them?
  - deleted 4y ago
    [deleted]
  - jwilk 4y ago
    https://news.ycombinator.com/item?id=17435514 https://news.ycombinator.com/item?id=17435514 (found via https://en.wikipedia.org/wiki/DjVu https://en.wikipedia.org/wiki/DjVu) There's apparently 22 pages of fluff at the beggining, so you should add that the page numbers. Also "the third line" was meant to be the 3rd line from the bottom. Direct links (to be opened in DjView): https://ia800204.us.archive.org/28/items/cu31924026442156/cu31924026442156.djvu?djvuopts&page=80&highlight=960,277,173,39 https://ia800204.us.archive.org/28/items/cu31924026442156/cu... https://ia800204.us.archive.org/28/items/cu31924026442156/cu31924026442156.djvu?djvuopts&page=442&highlight=983,1544,146,41 https://ia800204.us.archive.org/28/items/cu31924026442156/cu...
    - richard_todd 4y ago
      Thanks! When I convert the jp2k file for that page 58 with ABBYY default ("balanced") jbig2 settings I don't get any of the errors we see in the examples. If I convert to DJVU and purposefully degrade the image to 150DPI, I still don't get the errors on display in this example. Besides the errors talked about in your linked discussion (germauic), the pdf clearly has many 'c' -> 'e' errors which aren't in the jp2k files. Again, I can't reproduce this with the software I use. I've also extensively used djvulibre and you just don't see errors like this from clean scans. So, I have to conclude that the original conversions were done with very aggressive settings (or very bad jbig2 implementations). If that's they way the internet archive did all its encodings, then I admit there must be several problem documents, and they should re-do the conversions from the original JP2k files they seem to have kept. Not sure if google books made similar mistakes.
- oynqr 4y ago
  There are both lossy and lossless jbig2 variants.
  - AlotOfReading 4y ago
    Unless something has changed, Google books has always used the lossy encoding. Quoting the original paper: The most obvious way to compress pages is to losslessly encode them as a single 1 bpp image (see Figure 2). However, we get much better compression by using symbol encoding and accepting some loss of image data. Although our compression is lossy it is not clear how much information is actually being lost - the letter forms on a page are obviously supposed to be uniform in shape, the variation comes from printing errors and lack of resolution in image capture. Probably only a small fraction of errors are JBIG2 substitutions though.
AreYouSirius 4y ago
superjan 4y ago
The problem lies therein that JBIG2 has a compression factor, that is controlled by people who have an incentive to increase it, where few people know what it actually does. And how much testing is really done when is changed? Will the test include text in other scripts, like Chinese or Japanse? It can go wrong in so many ways. Jpeg artifacts on text are ugly but they tend to stand out. You will see artifacts long before the text becomes unreadable or ambiguous, and that is a good thing.
ognyankulev 4y ago
My choice would be the recently standardized JPEG XL designed to replace all of: JPEG, JPEG 2000, PNG, GIF. Among other things, it is supposed to be the first choice for long-term storage. I guess mass adoption will begin after including it in the PDF standard. It is already experimentally in Chrome and Firefox.
- navjack27 4y ago
  2nded
- codedokode 4y ago
  JPEG is optimized for compressing photos, not text or high contrast images.
  - ognyankulev 4y ago
    This only applies to legacy JPEG (1992), not for JPEG XL (2022).

8n4vidtmkvmk 4y ago

I've been reading some books on my ereader and every now and then I've come across a word that has been misspelled with similar looking characters. now I'm wondering if it's this jbig2 issue

jimjimjim 4y ago

There are a number of scans I've come across on archive.org where the person uploading has used jbig2 and there are pages where letters like 'e' get swapped for an 's'. My general rule when creating pdfs with scans is: If it's mostly text or line drawings and it will be used in a professional setting use png/flate. If it's a photo use jpg (especially if the source is a jpg).

richard_todd 4y ago
I'd love to see some examples (especially if the scan appeared to be of good quality and not a grainy 100dpi affair). Do you have any idea what kinds of documents you were browsing when you found them?
- deleted 4y ago
  [deleted]
- jwilk 4y ago
  https://news.ycombinator.com/item?id=17435514 https://news.ycombinator.com/item?id=17435514 (found via https://en.wikipedia.org/wiki/DjVu https://en.wikipedia.org/wiki/DjVu) There's apparently 22 pages of fluff at the beggining, so you should add that the page numbers. Also "the third line" was meant to be the 3rd line from the bottom. Direct links (to be opened in DjView): https://ia800204.us.archive.org/28/items/cu31924026442156/cu31924026442156.djvu?djvuopts&page=80&highlight=960,277,173,39 https://ia800204.us.archive.org/28/items/cu31924026442156/cu... https://ia800204.us.archive.org/28/items/cu31924026442156/cu31924026442156.djvu?djvuopts&page=442&highlight=983,1544,146,41 https://ia800204.us.archive.org/28/items/cu31924026442156/cu...
  - richard_todd 4y ago
    Thanks! When I convert the jp2k file for that page 58 with ABBYY default ("balanced") jbig2 settings I don't get any of the errors we see in the examples. If I convert to DJVU and purposefully degrade the image to 150DPI, I still don't get the errors on display in this example. Besides the errors talked about in your linked discussion (germauic), the pdf clearly has many 'c' -> 'e' errors which aren't in the jp2k files. Again, I can't reproduce this with the software I use. I've also extensively used djvulibre and you just don't see errors like this from clean scans. So, I have to conclude that the original conversions were done with very aggressive settings (or very bad jbig2 implementations). If that's they way the internet archive did all its encodings, then I admit there must be several problem documents, and they should re-do the conversions from the original JP2k files they seem to have kept. Not sure if google books made similar mistakes.
oynqr 4y ago
There are both lossy and lossless jbig2 variants.
- AlotOfReading 4y ago
  Unless something has changed, Google books has always used the lossy encoding. Quoting the original paper: The most obvious way to compress pages is to losslessly encode them as a single 1 bpp image (see Figure 2). However, we get much better compression by using symbol encoding and accepting some loss of image data. Although our compression is lossy it is not clear how much information is actually being lost - the letter forms on a page are obviously supposed to be uniform in shape, the variation comes from printing errors and lack of resolution in image capture. Probably only a small fraction of errors are JBIG2 substitutions though.

AreYouSirius 4y ago

superjan 4y ago

The problem lies therein that JBIG2 has a compression factor, that is controlled by people who have an incentive to increase it, where few people know what it actually does. And how much testing is really done when is changed? Will the test include text in other scripts, like Chinese or Japanse? It can go wrong in so many ways. Jpeg artifacts on text are ugly but they tend to stand out. You will see artifacts long before the text becomes unreadable or ambiguous, and that is a good thing.

ognyankulev 4y ago

My choice would be the recently standardized JPEG XL designed to replace all of: JPEG, JPEG 2000, PNG, GIF. Among other things, it is supposed to be the first choice for long-term storage. I guess mass adoption will begin after including it in the PDF standard. It is already experimentally in Chrome and Firefox.

navjack27 4y ago
2nded
codedokode 4y ago
JPEG is optimized for compressing photos, not text or high contrast images.
- ognyankulev 4y ago
  This only applies to legacy JPEG (1992), not for JPEG XL (2022).