Tuesday, 17 November 2009

Djvu vs. Pdf

Long blog again, so here is the executive summary: Djvu files are typically smaller than Pdf files. Why? Can we further compress pdf files? Yes, we can, but the current best solution has limitations. And you can forget all "advanced" commercial solutions. They are not as good as a free solution.


DJVU is a proprietary file format by LizardTech. Incidentally, it was invented by some machine learning researchers, Yann LeCun, Léon Bottou, Patrick Haffner and the image compression researcher Paul G. Howard at AT&T back in 1996. The DJVULibre library provides a free implementation, but is GPLd and hence is not suitable for certain commercial softwares, like Papers, which I am using to organize my electronic paper collection. Hence, Papers, might not support djvu in the near future (the authors of Papers do not want to make it free, and, well, this is their software, their call).
Djvu files can converted to Pdf files using ddjvu, a command line tool which is part of DJVULibre (djvu2pdf is a script that calls this tool). Djvu can also be converted into PS files using djvups (then use ps2pdf). However, all these leave us with pretty big files compared to the originals and, on the top of it, if there was an OCR layer in the Djvu file, it gets lost, but this is another story. How much bigger? Here is an illustration:

Original djvu file: 9.9MB
djvu2pdf file: 427.6MB(!)
djvu2ps file: 1.0GB
djvu2ps, ps2pdf file: 162.6MB

Note that I have turned on compression in the conversion process (-quality=50). (The quality degradation was not really noticeable at this level.) So, at best, I got more than 16 times the original file size. Going mad about it, I started to search the internet for better solutions. I have spent almost a day on this (don't do this, especially if you are a student!)..

JBig2 and the tale of commercial solutions

First, I figured, the difference is that these use general image compression techniques (like jpeg), while djvu is specialized to text and black&white images. Thus, for example, it can recognize if the same character appears multiple times on the page, store a template and a reference to the template. This is clever. I then figured that PDF files support the so-called jbig2 encoding standard, which is built around this idea. Hence, the quest for software that would support encoding a document using a jbig2 encoder and put the result into a pdf format. The easiest would be, if such a software just existed out there. A few commercial packages indeed mention jbig2. I felt lucky (especially, seeing that there are a few cheap ones). So, I started to download trial versions. Here are the results:

PDFJB2: 34.1MB
CVision PdfCompressor: 48MB
CVision PdfCompressor with OCR: 49MB
A-PDF: 106.8MB
A-PDF + PDFCompress: 106.8MB
djvu2pdf + PDFCompress: conversion failed

Hmm, interesting. 34MB is much better than 160MB, but it is still a long way from 9.9MB. (After a superficial look at the resulting files I concluded that only the A-PDF compressed file lost quality. What happened with this file is that on some page in some line containing a mathematical formula, the top of the letters got chopped.)

Free, open source solutions

Becoming desperate, I continued hunting for better solutions. Searching around, I have found iText, which is an open source, free Java library supporting all kinds of manipulations of Pdf files. I have figured that it "uses" Jbig2, but it was not clear if it uses it for compression or just knows how to handle the encoding. So, here I go, I wrote a java program opening a pdf file and then writing it out in "compressed" mode. Hmm, this few lines of coding allowed me to create a file of size 26MB, smaller than what I could ever get previously. Exciting! Unfortunately, opening the file revealed the `secret': Quality was gone. The file looked to be seriously downsampled (i.e., the resolution was decreased). Not good.

Then I have found pdfsizeopt on google code, which aims exactly at compressing the size of pdf files! The Holy Grail? Well, installing pdfsizeopt on my mac was far from easy (I use a Mac, which also runs Windows; quite handy as some of the above software runs only under Windows..). However, finally, I was able to run pdfsizeopt. Unfortunately, it seems to crash, without even looking at my pdf file (I hope the bug will be corrected soon and then I can report results using it). Along the way, I had to install jbig2enc. For this, I just had to install leptonica (version 1.62, not the latest one), which is really the part that is doing the image processing part of the process. JBig2Enc expects a tif file and produces "pdf" ready output (every page is put in a separate file), which can be concatenated into a single pdf file by a python script provided. Having jbig2enc on my system, I gave it a shot. I first used ddjvu to transform the input to a tif file (using the command line option, "-quality=75", resulting in a file of size 1GB). Then I used the jbig2 encoded with the command line arguments "-p -s". The result is this:

jbig2enc: 3.8MB

Wow!! Opening the file revealed a dirty little secret: Color images are gone, as well as the quality of some halftoned gray-scale images got degraded. However, line drawings were kept nicely and, in general, the quality was good (comparable to the original djvu file). Conversion to tif took 5 minutes, conversion from tif to jbig2 took ca. 4 minutes, altogether making the whole process take close to 10 minutes. (Other solutions were not faster at all either. And the tests were run on a resourceful MacBook Pro.)


jbig2enc seems to work, but you will lose colors. If you are happy with this, jbig2enc is the solution, though the process should be streamlined a bit (a small script good do this). Oh yes, I did not mention that these processes are not fast. I did not attempt to measure the speed, but conversion takes a lot of time. Jbig2Enc is maybe on the faster end of the spectrum.

Future work
  1. pdfsizeopt is a good idea. It should be made work.
  2. It would be nice to create a jbig2enc wrapper
  3. ddjvu is open source: Maybe it can be rewritten to support jbig2 directly. The added benefit could be that one could also keep the OCR layer in the original djvu file if one existed
  4. Along the way, I have found a cool google code project, Tesseract, which is an open source OCR engine. How cool would it be if we had an OCR engine that helps the compression algorithm and eventually also puts an OCR layer on the top of documents which lack text information (think of scanned documents, or documents converted from an old postscript file). Currently, I am using Nuance's Pdf Converter Professional (yes, I paid for it..), which I am generally very satisfied with apart from its speed. However, this could be the subject of another post.
PS: I have tested the capabilities of Nuance's Pdf Converter Professional and Abbyy's in terms of their compression capabilities:
Nuance: 132MB
Abbyy: 129MB
Yes, I tried their advance "MRC" compression, in Nuance I have explicitly selected jbig2. No luck.

Saturday, 14 November 2009

Keynote vs. Powerpoint vs. Beamer

A few days ago I decided to give Keynote, Apple's presentation software, a try (part of iWork '09). Beforehand I used MS Powerpoint 2003, Impress from NeoOffice 3.0 (OpenOffice's native Mac version) and LaTeX with beamer. Here is a comparison of the ups and downs of these software, mainly to remind myself when I will reconsider my choice in half a year and also to help people decide what to use for their presentation. Comments, suggestions, critics are absolutely welcome, as usual. Btw, while preparing this note I have learned that go-oo.org has a native Mac Aqua version of OpenOffice. Maybe I will try it some day and update the post. It would also be good to include a recent version of Powerpoint in the comparison.


  • Keynote: Excellent
    After a few days of usage, so take this statement with a grain of salt..
  • MS Powerpoint 2003: Excellent
  • Impress: Poor
    Save your work very often
  • Beamer: Excellent

Creating visually appealing slides, graphics on slides

  • Keynote: Excellent
    Positioning rulers help a lot. The process is really smooth. Keynote forces you to use less text. Built in templates are professional looking. Adding presentation graphics (tables, basic charts) is very easy. Cooler (technical drawing) better done with OmniGraffle. You can also easily animate the graphics, tables. Overall, very impressive.
  • MS Powerpoint 2003: Good
    Aligning to other objects is more cumbersome than in Keynote. The quality of fonts, color palettes, templates is not as good in Keynote.
  • Impress: Good
    Same as MS Powerpoint, maybe somewhat below (but the difference is not big).
  • Beamer: Poor
    The fonts and styles (templates) are great. However, creating slides with lively graphic is a nightmare (due to the lack of a GUI): You will end up with a few standard layouts, you will in general not use graphics, let alone animated graphics (or you will spend days on creating your slides). Also, departing from the styles is difficult and I am just bored of some of these styles that everyone seems to use.

LaTeX (math) support

  • Keynote: Poor
    Supported through LatexIt (free), but overall a cumbersome process. Details below.
  • MS Powerpoint 2003: Medium
    Supported through TexPoint (commercial, USD30) process is roughly same as with LatexIt and Keynote, slightly better integration.
  • Impress: Medium
    Supported through OOoLatex (free), same as MSPowerPoint + TexPoint, the integration is slightly better.
  • Beamer: Excellent
    Beamer is built for this!


  • Keynote: Near perfect
    Magic slide transition helps a lot with continuity across slides. What does this do? If you have the same object on two consecutive slides, Keynote will create an animation, keeping the object on screen and flying it to its new position. Works with multiple objects, too. I have found this very helpful for presenting a multi-slide argument. In general, Keynote animations are slick, polished, the flexibility is great. I lack some features of Beamer, such as animated highlighting, in-place replacement of some text (these can all be simulated with the existing tools, but with difficulty only).
  • MS Powerpoint 2003: Basic
    I miss Keynote's magic transitions. In general, Keynote is richer in animations. Again, some features of Beamer would be nice to have.
  • Impress: Weak
    Impress is inferior in terms of its animation caps to MS Powerpoint
  • Beamer: Good
    If only someone added support for magic transitions between slides. Some other cool effects would also come handy.

Dual screen presentation support

The idea is to show notes, time left in addition to the current and next slide on your screen, while showing the current slide on the big screen.
  • Keynote: Excellent
    Keynote supports double screen presentations natively. If you need to swap displays, go on the notes screen in the options menu. This will be on the big screen, obviously, if you need to swap the the screens.
  • MS Powerpoint 2003: Not available
    I have no experience with this feature of MS Powerpoint. Maybe you can use and add-on or something, but the basic software does not support it. I am pretty sure newer versions of Powerpoint must support this.
  • Impress: Excellent(?)
    The "Sun Presenter Console" extension supposedly supports dual screen presentations just like Keynote, but I have never had the chance to test it. Hence, the question mark. Some posts on the internet indicate that the extension might leak memory.
  • Beamer: Basic support
    Use Splitshow for this purpose. However, as far as I know, you cannot show the current time or the time remaining on the notes screen.


I want to put my presentations on the web so that people can look at them no matter what (major) operating system they use, without loosing animations or any other features. Another desired feature is the ability to create a compact, printable version of the slides: That is, if you have animations spanning multiple slides, somehow they should get handled intelligently. There is a tradeoff here: The more animation rich your slides are, the more bloated/complicated your printout will be.
  • Keynote: OK
    Proprietary file format. This is my biggest complaint. A keynote presentation is a keynote presentation. Apple likes to lock you in. Export to PDF and PPT works relatively well, but will lose some features of the presentation, like the cool animations. Exporting to PDF without animations to create printable versions seems to work well.
  • MS Powerpoint 2003: Good
    Free powerpoint viewers exist that can play any PPT file. Export to PDF will again lose some features.
  • Impress: Good
    Same as powerpoint.
  • Beamer: Excellent
    Produces PDF outputs: The presentations can be viewed on any computer! Also, the source is later, beamer is available on all systems. Add [handout] to the style and beamer will create an animation free version of your slides that works almost all the cases.

More about using formulae in Keynote (and why it sucks)

I used LatexIt which produces a PDF that can be embedded into the presentation. Style is not matched automatically. The PDF contains the latex source for the formulae, copy paste it back to LatexIt to edit it. When done with the edit, you need to drag and drop the formula back into Keynote. This sucks, since you need to delete the original that you have edited, reposition the new formula and reapply animations if you had any. Horrible.

Another issue is that the source saved with the formula by default does not have the preamble, thus using a command set specific to a presentation is difficult to achieve (you have to set this up manually). Another major headache is that you will not be able to use inline formula (a text is either in LaTeX, or in Keynote, the fonts in general do not match and mix well, alignment is a nightmare), nor will you be able to animate easily formulae (e.g., displaying a multiline formula line by line requires you to split the formula into multiple PDFs and use Keynote animations to show them one by one; this is problematic because formula alignment by hand is time consuming).