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ECW (Wavelet compression for Digital Imagery and Image Web server from Earth Resource Mapping pty. Ltd.)
Suresh Murthy DGM (Tech support) HOPE Technologies As imagery becomes more affordable and easier to use, GIS and CAD users are increasingly demanding more powerful imagery capabilities. The amalgamation of spatial information, be it CAD, GIS, database, imagery or whatever has resulted in much closer working relationships between imagery and GIS companies. With the easy availability of High resolution Imagery in the near future and the integration required for mosaicing of a large number of imagery, airphotos, scanned topographical maps and other types of imagery, for various applications the size of image files can become very large. For. eg. it would takes 1.5TB (1,500GB) of color imagery to cover all of California at a resolution of one meter. With ever more powerful computers, Data storage and Data retrieval systems capable of managing several thousand Terabytes of data it is possible to keep up with almost any amount of Imagery. The key issue is to easily use, manage and disseminate an always growing image database with limited equipment & funding. This is an important issue with most organisations. It is now feasible to work with such large images and also serve high-resolution orthophoto mosaics for entire cities, real-estate companies and other non-technical markets using a combination of Compression Technologies like Wavelet compression and Image Web server (IWS) from ER Mapper. What we are seeing is a definite trend where the user base for imagery is expanding rapidly. This is why companies like ER Mapper has expanded their product offerings into a whole solution range; ER Mapper (to preparing imagery), ECW (to use imagery inside any application), and the Image Web Server (to serve imagery into those applications using the Internet or Intranet). What is a compressed image? A compressed image is one that has been compressed to reduce the image file size, while still providing an almost perfect version of the original. With some compressed formats, such as the ER Mapper Compressed Wavelet (ECW) imagery format created using the ER Mapper Compression Wizard, you can use a compressed image almost. This is achieved by automatically selectively decompressing only the portion of the image, at the level of detail currently being viewed or printed. There are various compression techniques. The most effective technique is wavelet based, which is a method of processing, quantizing and then encoding the image in a way that result in very high levels of compression, with an almost perfect reconstruction of the original image. ER Mapper 6.1 offers wavelet based compression technology to compress images, resulting in very high quality results at very high compression rates. Typically a color image such as an airphoto can be compressed to less than 2% to 5% of its original size (50:1 to 20:1 compression ratio). This means that, at 20:1 compression, 10GB (10,000MB) of color imagery will compress down to 500MB, which is small enough to fit on to a single CD_ROM. If your source image has less information content, such as scanned topographic maps or airphotos over areas with less land change, higher compression rates may actually be achieved - 100:1 or more is not uncommon for compressed scanned topographic maps. Wavelet based compression: performance gains In addition to the advantages of smaller files, (which requires lower seek time to access data from CD_ROM, DVD_ROM or Hard disk drive) compression of digital imagery offers higher performance, because compressed imagery trades slower disk speeds against faster CPU speeds. Compression is "compute bound" rather than "disk bound", meaning that decompression is limited by CPU speed rather tan by disk IO or seek speed. A 10GB image that has been compressed down to a 500MB ECW compressed image file can be read as quickly from a CD-ROM as the uncompressed imagery can be read from hard disk. This makes distribution and use of imagery directly from CD-ROM a practical reality. Working with large images The following table demonstrates the reduction in file sizes when compressing imagery using the new ER Mapper wavelet based compression technology, which can compress very large images, at high compression rates, with high quality results.
Usage of Compression Compression is very useful for the following requirements.
Interactive image decompression An important consideration when dealing with large images is the ability to:
ER Mapper ECW V2.0 compressed images maintain the full geocoding and locational information about the file. This means that compressed images will be automatically registered with other imagery and vector data, just like an uncompressed file. Full real time roaming, zooming and panning is supported in ER Mapper for compressed images, and all the normal algorithm capabilities, such as selecting imagery within a vector polygon, or using statistics, all operate fully. RAM requirements during interactive viewing/decompression The ER Mapper Compressed Wavelet (ECW) format has a very low memory footprint during imagery decompression. About 2MB of RAM will be used during interactive decompression of imagery, thus compressed imagery can be used even on quite small computers with limited amounts of RAM - the ER Viewer will view compressed imagery on a machine with a total of 16MB of RAM. Comparing different compression techniques Wavelet versus older compression techniques Over the years, considerable effort has been spent in compressing information in order to make it easy to distribute, and to reduce storage requirements. Techniques used by common compression program such as ZIP, or by older compression formats such as JPEG or TIFF, suffer from sever disadvantages when trying to compress large digital images. These problems include:
Wavelet compression involves a way of analyzing an uncompressed image in a recursive fashion, resulting in a series of higher resolution images, each "adding to" the information content in lower resolution images. The primary step in wavelet compression are performing a Discrete Wavelet Transformation (DWT), quantization of the wavelet-sace image subbands, and then encoding these sub bands. Wavelet images by and of themselves are not compressed images, rather it is the quantization and encoding stages that do the image compression. Image decompression, or reconstruction, is achieved by carrying out the above steps in reverse and inverse order. Thus, to restore the original image, the compressed image is decoded, dequantized, and then an inverse-DWT is performed. Because wavelet compression inherently results in a set of multi-resolution images, it is well suited to work with large imagery which needs to be selectively viewed at different resolutions, as only the levels containing the required level of detail need to be decompressed. Wavelet mathematics embraces an entire range of methods each offering different properties and advantages. For example, it is possible to compress 3 or more dimensional imagery using wavelets. Wavelet compression has not been widely used because the DWT operation takes a lot of compute power, and because historical techniques perform the DWT operation in memory or by storing intermediate results on hard disk. This limits either the size of the image that can be compressed, or the speed at which it can be compressed. The ER Mapper 6.1 ECW V2.0 wavelet compression uses a breakthrough new technique for performing the DWT and inverse-DWT operations (patent pending), which makes the use of wavelet based compression a practical reality. Compression, Storage then selective decompression You can include compressed imagery in GIS applications using free imagery plugins for GIS and office application with your compressed data. These plugins read the compressed imagery in a wide range of software applications such as ArcView, AutoCAD MAP, MapInfo, ER Mapper, ER Viewer, Photoshop, Microsoft Office and Excell, and other software applications. There is no data royalty fee when using the ER Mapper Compression Wizard to compress images. You can also use the free ECW compression and decompression SDKs to create your own applications that can open, read and write compressed images. Because the compressed imagery is constructed of multi-resolution wavelet levels, you also enjoy fast roaming and zooming on the imagery-even from slow media such as CD ROM. Advantage of ER Mapper Compressed Wavelet (ECW) versus other Wavelet compression techniques The ER Mapper ECW compression by Earth Resource Mapping uses advanced wavelet compression techniques. The ER Mapper Compression Wizard has been designed as part of a complete end to end solution to prepare, compress and distribute imagery. There are a number of benefits that result from using the ER Mapper Compressed Wavelet (ECW) imagery format: Historically, the methods for doing Decrete Wavelet Transformations (DWT) compression, have been memory based. This has limited the size of the file that can be compressed using wavelet compression. Most existing wavelet techniques enable compression of very large images - there is no limitation to image size in the compression technology. The primary advantages inherent in the ER Mapper ECW technique are that it is much faster, for several reasons as listed below:
Any companies (including competitors to ER Mapper) who wish to compress large images can now do so using the ECW Compression SDK free of costs and royalty fees. Link the library to your application to enable it to directly perform wavelet compression to the ECW V2.0 file format. The library is provided in binary form, and can be linked to C and C++ code, and is small (less than 100K in size). It features a very clean library interface. It makes no assumptions as to the input data - you simply fee it lines of imagery to compress, and it does the rest. The library enables wavelet images of upto 500MB in size to be compressed. You still need to use ER Mapper for images beyond this size, (or to contact ER Mapper to discuss access to an unlimited size compression library). Compressing Mosaics of input images to a single compressed image file The ECW Decompression and Compression SDK's are to the only libraries on the market that are provided on an open basis and enable you to both compress and decompress and is tightly linked with the Image processing software through the ER Mapper 6.1 Compression Wizard. This means that you can use all the other ER Mapper tools, such as the Ortho Wizard ,m the Mosaic Wizard, and the Balance Wizard, to create a seamless mosaic which can then be compressed out to a single compressed ECW file. When using the Compression Wizard, you specify the input image to compress. Here, you may specify an ER Mapper smart data algorithm as the input to be compressed. Using this technique one can use the ER Mapper Mosaic Wizard and Balance Wizard to create an algorithm that creates seamless and balanced mosaic of input airphoto, which can then be compressed out to a single compressed image file. CD ROM/DVD-ROM versus Internet distribution Internet distribution is likely to increase as Internet access rates increase. Currently, imagery distributed over the Internet is best suited for imagery under 5MB compressed in size, which means that the original image size would e 100MB in size assuming color imagery compressed at 25:1 CD ROM and DVD ROM are ideally suited for distribution of large amounts of imagery, as 15GB of imagery can be stored on a single DVD-ROM when compressed at 25:1 Reducing storage requirements The following table demonstrates typical compression rates that can be achieved for imagery. If your imagery contains less detail- for example the imagery covers forested areas, desert or other areas with limited features, the actual compression rate achieved may be higher. Wavelet compressed imagery offer several advantages that enable the effective use of imagery directly from CD-ROM and DVD-ROM media. Multi resolution level of detail. The nature of wavelet compression is such that it processed imagery into multiple levels of detail. This means that when a user is viewing imagery at different resolutions, only information needed to display at that resolution is retrieved. This reduces disk IO and disk seek requirements, resulting in faster image access. Using Compressed Imagery with GIS/CAD/Office application When disturbing imagery, an important consideration is the ultimate ability of users to access and effectively use the imagery. Studies have continually shown that the ability to be able to directly access imagery, without having to import it first, and which geographic information preserved, consistently ranks as the highest requirement by imagery users in the GIS, CAD and office software environments. There is a range of free plugins for common GIS/CAD/MS office applications, which directly read compressed ECW image files. Plugins are available than enable products to directly read compressed image files, for products including:
AutoCAD and AutoCAD MAP 3
Arc View 3.1 MapInfo 4.0/4.5/5.0 Microsoft Office (Word, Excel, etc) ER Mapper Photoshop Standalone image viewing When distributing imagery, these plugins can be distributed with your imagery, free of charge. Each plugin is typically 2MB to 3MB in size prior to installation, and is available as a single .EXE file that uses InstallShield to install the plugin. The Image Web Server (IWS) The Image Web Server has been designed by Earth Resource Mapping for the following:
 The need for the Image Web Server Currently the large size of image data and network bandwidth limitations preclude making imagery available over a network, in particular the Internet. Typically, sites with imagery would post thumbnail JPEG compressed images for viewing on-line. Distribution of larger images is done via CD. Apart from the time taken and inconvenience, other problems are
Various organizations like Image Data Sales and Distribution, Real Estate Agencies, local Govt. Bodies, Defense forces, Govt. regulatory bodies etc. could benefit from usage of ECW and IWS. The Internet Web Server makes it possible to supply up to data imagery to many different types of recipients over diverse media. Also, since the imagery is encapsulated, it can be encrypted and sent over secure links with no degradation or corruption. Solutions using Image Web Server Technology The image Web Server solves the problems of serving large image on the Internet or Intranet by combining the powerful ECW compression with image sub setting. It not only compresses the images, but also only transfers the portion of the image currently being displayed by the client. Compression Products You can use any of the following products to compress imagery for the Image Web Server.
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