A portable OpenSlide viewer for Windows: Smart Zoom Viewer

[jcupitt at gmail.com]

On 12 November 2015 at 14:54, Yves Sucaet via openslide-users
<openslide-users at lists.andrew.cmu.edu> wrote:
> * Is SZI compatible with any of the other "open" formats like OME TIFF or
> DICOM?
> * Is the format documented somewhere?
> * Do you have actual benchmark information about how many tiles can be served
> from SZI in contrast to e.g. SVS or MRXS?

You're right Yves, it needs to be written up, Martin and I were
thinking of composing a small note about it.

It's just deepzoom in an uncompressed zip container. The idea is that
deepzoom is great, because it can be served extremely quickly, but
also very annoying, since each 256x256 tile is a separate JPEG file.
If your slide is 100,000 x 100,000 pixels, your highest-resolution
directory will contain 150,000 files.

This isn't so bad on Linux hosts, but Windows really struggles with
large directories. Very large numbers of small files can also be
rather inefficient in disk usage: many filesystems will allocate a
separate 4kb page for each file, so for 1kb JPEGs, 3kb will be wasted per tile.
Huge directory trees are also rather slow to copy about between hosts,
especially on Windows.

To serve images in this format, we've written a small FastCGI program
which can be added to apache or iis hosts. Combined with mod_rewrite,
you can make it respond to URLs which are compatible with any deepzoom
viewer. The server component keeps an index of the files inside the
zip, so to serve a tile, all it does is a single seek() and a single
read(), it's as fast or faster than the filesystem.

Summary: easy to manage, especially on Windows, fast to build, fast to
serve, minimal CPU use, less disk usage than deepzoom.

John