Zoomable Image Information
As the DAC Digital Imaging Initiative moved to higher-resolution image capture in 2001, we experimented with various means of making detailed images available in pedagogically useful ways on the museum website. This page offers some technical details.
Here's some information about this testbed as it was developed in 2001. The master source image began as a 5000x3750-pixel raw capture file made with a Schneider Makro-Symmar lens and a Better Light scanning back. Capture-stage color balance was adjusted with reference to a GretagMacbeth ColorChecker card.
After slight cropping of the master image as needed, square views were made at five resolution levels that zoomed in by a factor of about 1.8 each. Each of these views then was scaled to 400-pixel dimensions. This number of levels and these dimensions struck a workable balance by:
Because it yields increasingly large numbers of component images overlapping by varying numbers of pixels at higher resolutions, this process has the added benefit of making the reconstruction of a complete image at full resolution too much work to be practical. This difficulty of reassembly can pose a useful disincentive to unauthorized use of images. One tradeoff in taking this multiple-file approach is the need for increased storage space. For example, after removing duplicate component images at deeper zoom levels, the Dürer image requires 262 JPEGs occupying about 14MB; a flashpix file made from the same master image came in at about 8MB.
All views are exactly 400 x 400 pixels. The full view was padded to square dimensions, views at the second level were cropped from each image corner, and views at each deeper level were taken from each successive corner, always with about 10 percent overlap at each level.
This procedure resulted in varying amounts of overlap between some adjacent views at higher resolutions, but its systematicity enabled consistent image production and reasonably straightforward navigation functions (with offset compensation for the trickle-down effects of cutting square second-level quadrants out of non-square full views). The resulting images were treated with Photoshop's Unsharp Mask (with parameters optimized for each level) and exported with the HVS JPEG plug-in, which at the time afforded optimal control over file parameters.
The consistency of this production workflow was designed to make semi-automated processing of source images possible. Having been developed by means of one-off manual work on the Dürer image used here to test this workflow (as well as naming scheme, scripts, user interface, and so on), these processes were designed to support automation with Photoshop actions and AppleScript.
The second key standard is JPEG 2000. After years of seemingly unrealized promise, this has begun to be a format supported by an expanding range of image-server software and image-viewer clients. As support for navigating within JPEG 2000 files becomes more widespread, moving towards that standard will be more sustainable than maintaining an in-house application--since an underlying collection of JPEG 2000 files should be usable down the road with various delivery tools. This will neatly address the need for such resources' digital longevity, something ill-served by reliance on proprietary file formats and less than assured by in-house systems of tiled files. Future navigable images from the DAC are quite likely to use JPEG 2000 files, probably delivered as part of a campus-wide implementation of an image server and related software.
Return to the zoomable image page.