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:
- The ECW technique uses a recursive algorithm pipileine technique and does not require immediate tiles to be stored to disk and then recalled during the DWT transformation and
- The ECW technique takes advantage of CPU , L1 and L2 levels of cache to do its linear and unidirectional data flow through the DWT process.
- Because the ER Mapper ECW method of DWT is much faster, this speed is used to provide more efficient compression in several ways.
Other advantages:
- No data royalties. There are no data royalties when using the ER Mapper Compression Wizard to produce compressed imagery.
Full geocoded map projection information stored. The ECW compressed image format uses the standard ER Mapper "ERS" image header file to record map projection information. This means that when an image is compressed, full image map projection information is retained.
The "ERS" header file is optional for ECW V2.0 compressed imagery. Georeference information is contained in the ECW compressed image file as well as in the .ERS file. This is useful when emailing. ECW file, because the recipient only has to be concerned with one file.
Evaluate using your own data. The free ER Mapper 6.1 (with Compression Enhancement Pack) evaluation CD ROM contains the complete ER Mapper 6.1 Software (including the .ECW V2.0 Compression Wizard and the free plugins for GIS and office software). You can trial run the full software, against your own data, for 14 days at no charge.
- Batch compression. You can use the command line version of ECW V2.0 to carry out compression in a batch operation. This allows you to compress a large number of images by entering a single command line.
You do not need ER Mapper 6.1 installed to enjoy the benefits of ECW compression. A free standalone utility is available for download at the ER Mapper web site. This free program offers a GUI-only compression interface to compress single files (No batch compression) up to 500MB in size.
- Free plugins. The free GIS and Office plugins can be distributed with your imagery to enable MS Office, ArcView, MapInfo, AutoCAD Map, ER viewer other users to directly access and use of your imagery. These plugins have been enhanced to support ECW V2.0 imagery. There is also a new plugins which enables you to both decompress and compress ECW V2.0 images from within Photoshop. This is the only large image-format compression plugin for photoshop that allows you to compress (as well as decompress) large images. The compressor is limited to compressing files of a maximum of 500MB in size. There is no cost for this plugin to Photoshop.
- Tightly Integrated. As the Compression wizard is tightly integrated with ER Mapper 6.1, you can use the Ortho Wizard, Mosaic Wizard and Balance Wizard to prepare mosaics prior to compression.
- Low cost. The entire ER Mapper 6.1 product, including Ortho rectification, mosaicing, balancing, compression and a wide range of other tightly integrated functionality, is about the same price as other companies charge for just the compression engines.
Free Compression library The ECW V2.0 Compression SDK is an open and freely available standard to compress imagery using wavelet technology. The ECW compression library is based on breakthrough patent-pending wavelet mathematics technique developed by ER Mapper, which enables the compression of very large images, very quickly, with high quality results.
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.
Reduced size. Because the images are compressed, less disk IO needs to be carried out in order to view images, resulting in a performance boost.
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.
Improved disk access. The ER Mapper Compressed Wavelet (ECW) format uses a technique known as clustering to locate related information in an image closely together. This reduces the number of seeks that need to be carried out - significantly improving performance when imagery is viewed directly from CD-ROM.