Images are stored as TIFF files for archival purposes, however as most Web clients currently only support GIF format for in-line images, that is the format we deliver. This decision restricts us to 8-bit colour images.
Readers viewing a folio first see an image of the complete page and can then zoom in on the image.
The overall image is scaled down by a factor of eight to fit within a rectangle of 290 by 384 pixels. Repeatedly zooming-in returns partial images that scaled down by a factor of four and then by two, before part of the full image is returned unscaled. The captured images typically contain about 10,000-20,000 distinct colours and scaling the images tends to increase this number as pixel values are interpolated. Reducing the number of colours down to 256, as required for GIF images, is a slow process. It was therefore decided to reduce the number of colours in the image ahead of time.