This development was greeted with cheers by the BHL Technical Development Team, as it solves problems we’ve previously reported with the non-scalable, proprietary solutions used to serve JPEG 2000 images. Following a functional evaluation in December 2008, djatoka was integrated into the BHL staging site for performance testing and was promoted to production on Thursday, January 22, 2009.
To view djatoka in use on BHL materials, check out the following page from “Wild oxen, sheep & goats of all lands, living and extinct” by R. Lydekker published in 1898, selected in honor of the Chinese New Year, in this, the Year of the Ox:
JPEG 2000: An excellent format with poor support
Displaying JPEG 2000 images on the web can be a challenge because 1) the images are often too large to be downloaded quickly, 2) most web browsers don’t understand JPEG 2000 images without appropriate plug-ins, and 3) until the release of djatoka there were no active open source image servers; costly commercial solutions were the only option, aside from complete custom development.
JPEG 2000 images are different from traditional image formats like GIF, JPEG, and PNG in that they consist of several “layers”, one for each available resolution. Think of JPEG 2000 images as a pyramid - each layer of a JPEG 2000 file is a copy of the same image, each at progressively larger size as you step down the pyramid. The top layer may be relatively small but the bottom layer could be quite large. Consider a JPEG 2000 image whose bottom layer is 18,000 pixels square, such as this example. To view this image in its entirety using standard 1024 x 1280 monitors would require an array of 270 monitors stacked 15 high and running 18 wide! Cool, yes, but impractical.
So, to deliver JPEG 2000 images to web users, scalable software is needed to carve up the JPEG 2000 image into smaller chunks at a given resolution in a format natively understood by a users’ browser. And, since we are talking about this occurring during the load time of a web page, all of this has to happen very quickly.
BHL’s first approach to JPEG 2000 delivery
Though functional, the addition of BHL’s 1,500 users per day pushed ExpressServer beyond its available capacity, causing a significant delay in delivering page images. Since it is a commercial solution, and one that is licensed by processor, to increase capacity would require the purchase of an additional ExpressServer license, costing upwards of $15,000USD. An open source option was needed, but none was available until the release of djatoka.
For detailed information about djatoka, including source code for download, visit:
- djatoka info - http://african.lanl.gov/aDORe/projects/djatoka
- djatoka on SourceForge: http://sourceforge.net/projects/djatoka
djatoka in BHL
djatoka brought with it some improvements to our user interface:
- Preview thumbnail. This allows a user to see which portion of the overall image is being viewed. This also lets the user easily view different parts of the image by simply dragging the control to the desired part of the image.
- Simplified zoom controls. With our djatoka implementation we got rid of the radio buttons formerly used to zoom in and out and replaced them with plus and minus buttons.
- We needed to retain the Save and Print image features from the previous image viewer. Therefore, we rolled these into the djatoka viewer.
- BHL defaults to serving a single low resolution JPEG images, when available, and only escalates to more costly JPEG 2000 processing when instructed by the user. We made the djatoka viewer friendlier toward raw JPEGs. djatoka itself handles raw JPEGs with aplomb.
- We removed a text box that provides a way to embed a Region Of Interest (ROI), or "slice," of the image as tiled by djatoka. While this is a very useful feature, we felt the current implementation consumed too much screen space of the visible page image. We plan to reintroduce this in a less obtrusive manner.
Towards a scalable infrastructure
BHL developer Phil Cryer has written an excellent and detailed blog post about the technical infrastructure devised to provide scalability and fault tolerance to our djatoka implementation. It is available at http://www.fak3r.com/2009/01/27/howto-serve-jpeg2000-images-with-a-scalable-infrastructure/.
While we are happy with our initial implementation of djatoka, we have already planned some future enhancements and have begun discussions concerning priorities with the djatoka development community, including lead developers Chute and Van de Sompel. These enhancements are as follows:
- An embed image link. This will directly replace functionality we removed from our implementation of the djatoka viewer. Drawing from similar features elsewhere, we are leaning toward a clickable link icon which pops up a box which contains a URI for the image as the user is currently viewing it.
- The current djatoka viewer is based on an old version of MooTools. Newer versions of MooTools are not compatible with the djatoka viewer. We’d like to fix this so that it’s easier to drop into a website that uses a current or future version of MooTools. This will allow us to use a newer and more flexible version of this useful code library, and will allow others to reuse our work more easily.
As described above, djatoka has easily integrated into the production BHL infrastructure and user interface with minimal effort. It has nullified problems that existed with the previous, proprietary solution used to deliver JPEG 2000 images, and thus far has performed without significant error or delay since being promoted into production at www.biodiversitylibrary.org. Because this is an open source solution, BHL can easily scale up without significant expense should we require additional capacity. Our experience implementing djatoka has been overwhelmingly positive, and we would encourage any project currently serving JPEG 2000 images to evaluate its features and functionality within your infrastructure.
If you'd like to learn more about djatoka visit the main web site or get the code from SourceForge. Join the listservs to become an active member of the djatoka development community, as the lists are the best source of current information and have an active user base. Finally, please leave comments or feedback about our specific implementation of djatoka in BHL using the Comment form below.
Chris Freeland, BHL Technical Director
chris.freeland (at) mobot.org
Chris Moyers, BHL Developer
chris.moyers (at) mobot.org