Directory layout for images

In digilib all images are identified by the fn and (optional) pn parameters. The value for fn can be a directory path or a directory path and a filename, separated by slashes, e.g. “fn=books/book1/page0002”.

If fn is a directory path without filename pn is the index number of the image files in this directory in alphabetical order, e.g. “fn=books/book1&pn=2”. The default for pn is 1 i.e. the first image.

If fn ends in a filename pn is ignored. File extensions are also ignored, i.e. “books/book1/page0002” and “books/book1/page0002.tif” identify the same image. It is recommended to omit the file extension.

The directory path in fn is relative to the base directory in the basedir-list parameter of the digilib-config.xml file, e.g. if

<parameter name="basedir-list" value="/docuserver/images" />

and

fn=books/book1/page0002

then digilib will try to load the file

/docuserver/images/books/book1/page0002.tif

(automatically finding the right file extension)

Prescaled images

You can provide any number of scaled-down versions of your images that digilib can use when a smaller version of an image is requested. Since less data has to be read and processed this can speed up digilib’s performance considerably.

The actual process is that the client requests a certain target size, digilib scans all available scaled-down versions of the same image, selects the smallest image that is larger than the requested size and scales it down to the requested size.

There is another optimization in digilib: if the requested image is exactly the same size and type as the pre-scaled image then the pre-scaled image is sent unmodified to the client which is a lot faster. So it makes sense to produce thumbnails of exactly 90 pixel width when they are used in an HTML page where all images are 90 pixel wide.

The scaled-down versions of the image have to have the same file name as the original hi-res file. They can have a different type and extension (e.g. img002.jpg for img002.TIFF)

The scaled down images have to have the same directory path (the part that shows up in digilib’s “fn” parameter) as the hi-res file wile the first part of each directory tree is configured by the basedir-list parameter in digilib-config.xml.

The sequence of directories in basedir-list is from high-res to low-res. Images must be present in the hires directory but they need not be present in all lower-res directories.

e.g. if digilib-config.xml contains

<parameter name="basedir-list" value="/images:/scaled:/thumb" />

and a user requests the image books/book1/page0002 digilib looks for

/thumb/books/book1/page0002.jpg
/scaled/books/book1/page002.jpg
/images/books/book1/page002.tif

(automatically finding the right file extension) and uses the first image that is bigger than or equal to the requested size.

For batch-prescaling our images we use a very old Perl script called “scale-o-mat” that uses a lot of freely available imaging libraries (ImageMagick, libtiff, netpbm) and is available in our public CVS. The script is given a hi-res base directory, a destination base directory, a destination size and a starting directory. It then processes all files in the starting directory and all its subdirectories and creates scaled images in corresponding directories under the destination base directory.

We currently use prescaled thumbnails of 100 pixels and images for browser display of 1500 pixels. Remember that the prescaled image has to be larger (or the same size) than the requested image size!