Image file formats
Information on some image file formats.
Contents
Primary image formats for OLPC
Whatever format you use, check your file sizes! Even SVG and PNG images can end up being bloated if the right options are not used. Use applications which trim down image file size for the web.
There are four primary image formats that will probably see much use on the OLPC:
- SVG
- This is an object-oriented drawing format ideally used for drawings created with tools like Inkscape, CorelDraw, CAD software, etc.
- JPEG
- This is a compression format that is ideally suited to photographs whether they are scanned or photographed on a digital camera. This is a very, very bad choice for scanned text. You can tell when someone has made this mistake because the text is blurry.
- PNG
- This lossless compression format is the best choice when you have to scan material like line drawings, cartoons, or text. The common factor in all these source materials is that they have a few different shades of color, perhaps only black and white. Even when the scanned original is stained or has shadows on it, you can usually tell your image editor to convert it to a 2-color black and white image (or increase the contrast to maximum) and sharpen the image
- The same applies to JPEGs sourced from a digital camera. For instance, if you photograph some text then take the JPEG from the camera and convert it to PNG. This will sharpen the edges of the letters and make it easier to read. In addition you should reduce the size of the image by taking into consideration the number of bits available on the screen. The first systems will be 1200 x 900 at 200dpi.
- Important note: PNG can waste a lot of space if you try to keep the full range of 16777216 colors or even 256 shades of gray - and if it's text or diagram all this space will be wasted anyway. Use PNG with color palette instead.
- GIF
- This format is only useful for small animations (big ones will chew space on flash-drive too fast). Properly compressed PNG is almost always smaller then GIF. Just use PNG with index (not true color) and don't forget about Pngcrush!
Other image file formats
Some other formats can be useful as well but they have only limited support in applications and it's not clear if their advantages are worth the pain of non-standard format support.
- wikipedia:APNG
- this animated extension to of PNG format is supported by XULRunner and thus works in Browse
- wikipedia:MNG
- is not supported by XULRunner and thus does not work in Browse
More on DjVu
- DJVU
- This is not primarily an image format although it is used to encode compressed page images from books. DJVU is the best choice when you are scanning a multipage book intended to be used in an Ebook reader like Evince. However it should not be used for images in your applications or in HTML documents. In this usage scenario, it is possible to add OCR results behind to page images, which makes the text searchable, without the labor intensive proofreading a text-only version would require.
The main site for information on DjVu compression format for ebooks is here http://www.djvuzone.org/
Recently a good overview article was published on News Forge.
In a nutshell, DjVu was invented to solve this problem:
- Conventional web formats such as JPEG, GIF, and PNG produce prohibitively large image files at decent resolution. As a result, Web site content developers have been largely unable to leverage existing printed materials.
DjVu is intended to be used with scanned images of book pages, either black & white or full color. It then compresses those scanned pages to produce very highly compressed files.
Given that the target countries for the OLPC have poorly developed computing infrastructures, scanning of existing printed documents into DjVu format may be the fastest way of making a wide variety of educational material and Ebooks available to the kids.
Status of DjVu
DjVu is supported by the Evince document viewer which is used by the Read activity. But, it requires the DjVuLibre backend and/or evince-djvu support which is not packaged in Update.1 or 8.1.1 (see #6223).
If you want to contribute to the DjVu project in any way, here is the site: http://djvulibre.djvuzone.org/
Why is DjVu important?
In regions where computers are scarce and there is little support for native scripts, DjVu allows existing paper books to be scanned and distributed as ebooks. Even handwritten books can be distributed this way. Tie this together with the OLPC chat application's support for SVG input and the GECKO support for displaying SVG graphics and it is conceivable to distribute a computer with no font support and NO TEXT AT ALL in its user interface. Icons would substitute for text in the UI and handwriting would be a primary mode of input.
Of course, this is a bootstrap scenario. Once the OLPC is deployed in this way, native language speakers will begin to work on fonts, and a keyboard layout to enable text use on the OLPC. This could take months or years to sort out, but in the meantime, the kids have an educational tool to use.
Producing DjVu Documents
A simple guide to create DjVu files (from JPEG and PDF) and to bind multiple DjVu files into one, is available at howtoforge.
Workflow Planning
First, you need to think of this in terms of setting up a workflow. There are several steps, some of which require technical expertise and some which do not. In addition, the expertise required to set up and maintain the workflow is different from that required to make encoding decisions and check the quality of scans.
Scanning
Some scanners can handle bound books but they cost a lot more money. However, if you can spare a copy, then you can take it apart and scan the pages on a flatbed scanner. Save the files in an uncompressed TIFF format because they will be processed further. Pages should be scanned in color because the DjVu compression software produces a better result that way.
If your original scans are not perfect, you may need to use software such as Book Restorer or Unpaper to clean them up. This is especially important when you are scanning old, rare books that have been damaged in some way, for instance stains on the pages. In addition, when a book is rare you cannot cut out the pages to do perfectly flat scans. This means that the scans will be curved but software can repair these curves.
Check this wiki for additional scanning advice.
Encoding the Pages
Next, you need to process the individual page scans with various tools to encode the pages. Different encoding tools may be used for different pages depending on the presence of illustrations, photos, colored text, etc. Pages can be segmented into a black and white layer and a color layer so that different encoders can be used on each. In addition, if you have an OCR program for the script that the book is written in, you can run the black and white segment through it. DjVu readers are capable of using the OCR to do text searches and then highlighting the words in the actual scanned text image.
Bundling and Postprocessing
After this you have various pieces which you need to bundle together into a multipage book file. Then, you may wish to further process the book to add text annotations, precompute thumbnail images of pages, etc. Perhaps the book is written in an archaic form of the language and you wish to annotate it with a glossary similar to what we do with Shakespeare's plays.
Testing:
Don't forget to test your book thoroughly using the Read activity to make sure that there are no problems with using it on the OLPC.
Tools
If you would rather have the scanning done by a company with expertise in the field, that is possible. Once the first pilot country is deployed, there will likely be other companies who can offer this service. But the tools needed are all open source so you can also set up your own production line for scanning books.
Articles and Papers
- this article from Professional Surveyor magazine explains how the National Land Survey of Sweden went about converting their historical archive to DjVu format.
- this article on howtoforge details the entire workflow for producing DjVu books on Linux. It also includes some scripts useful in making the process run more smoothly.
GIF
GIF is old, proprietary but widely supported format. Today it's mostly surpassed by PNG in all regards except one: support for simple animations. If you need an animated icon or something like that, then use GIF (all patents related to GIF are expired by now), but for anything else... use PNG and JPEG.
There is a page on choosing image formats that will help you to understand the differences and how to know which format will be best for the intended use.
More info can be found on Wikipedia.
JPEG 2000
JPEG 2000 is the most recent work of the JPEG standards committee. It is intended to improve upon several areas of the original JPEG 2000 standard such as lossy encoding, blocky appearance, and inefficient compression.
Although this standard is intended to be implemented on a no-charge basis, the JPEG committee does allow for inclusion of patented or otherwise non-free technology as per ITU and ISO policy on reasonable and non-descriminatory terms. It is problematic or impossible for open source/free software to implement such standards.
