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

828 November 2016 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING