PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
In this article, LizardTech presents one solution to this
problem using LizardTech’s Express Server software,
which utilizes compressed MrSID and JPEG 2000 im-
agery to reduce storage costs and decrease image load-
LizardTech Express Server is an application for rapidly
distributing high-resolution satellite imagery, aerial pho-
tographs, and other large raster datasets. Imagery host-
ed on Express Server is stored in the MrSID, JPEG 2000,
and other compressed image formats that reduce storage
costs and minimize required network bandwidth.
MrSID is a patented compression technology perfected
by LizardTech over twenty years, and JPEG 2000 is an
ISO-standard file format (though it was not created spe-
cifically for geospatial applications). Both file formats
rely on similar advanced compression technologies, in-
cluding the wavelet transform and arithmetic encoding.
Express Server reads and serves imagery directly from
compressed source files to reduce storage costs. This is in
contrast to traditional image servers, also called tile serv-
ers, which generate and cache tiled images from source
images hosted on the server. By creating and storing
these tiled images, the tile server avoids reading the en-
tire source image every time that a small portion of the
image is requested. Although this increases the efficiency
of the server, it requires duplicating the source image data
at multiple resolutions, such that the cached tiles often
occupy several times the disk space of the source image.
The exceptional characteristic of Express Server is that
it can read directly from compressed images without
sacrificing responsiveness and efficiency. This special
quality is due to the wavelet-based nature of the MrSID
and JPEG 2000 formats, which can be decompressed
selectively and can incorporate multiple image resolu-
tions without duplicating image data. The MrSID Gen-
eration 4 format has the additional advantage of being
able to store either raster or lidar point cloud data.
With MrSID and JPEG 2000 encoding, images are
compressed such that multiple resolution levels of an
image, from an overview all the way up to the full reso-
lution, are the natural result of this compression. Addi-
tionally, because of the way the actual bits representing
the image are stored, any portion of an image can be
decompressed at any resolution level without the ne-
cessity of decoding the entire image. Express Server is
built specifically to take advantage of this selective de-
compression feature of MrSID and JPEG 2000, extract-
ing the requested scene quickly and converting it to an
image scene in one of several output formats.
Express Server runs on top of an existing web server
and provides access to compressed imagery stored on
the server’s file system or on the network. Clients can
then connect to the web server using multiple protocols,
including the OGC Web Map Service (WMS) standard,
the streaming JPEG 2000 Interactive Protocol (JPIP),
and an Express Server-specific web API.
Express Server imagery can be accessed from any WMS
client. WMS is a widely-supported standard that users
can access to view imagery from web browsers, from
desktop GIS applications, from the ExpressZip web ap-
plication included with Express Server, and more. If the
optional Express Server JPIP component is installed,
then users can also access imagery from JPIP clients.
Express Server works in conjunction with an existing
web server installation to distribute imagery. Express
Server is compatible with both the Microsoft IIS web
server on Windows and the Apache web server on Linux.
LizardTech services provide an interface between the
web server and the image server. The following list de-
scribes the LizardTech services created during the in-
stallation of Express Server:
Express Server Admin Backend Service.
Admin Backend service or the Express Server
Agent, processes requests from the Express Server
Manager to start or stop the web server and per-
form licensing tasks.