Hello John, > Theoretical question to follow up on Steven's comment: > > Is it theoretically possible to devise a good compression > algorithm which does > NOT lose high-frequency information?
Yes, depending on your definitiion of "good". It's certainly possible to have high-quality compression for high-frequency information. But it becomes very difficult to do generic high-ratio compression without losing high-frequency information - usually around 4-6:1 is the limit for "lossless" wavelet-based compression (ie, maintain the exact data). > I have always had the same problem that Steven has obviously had > - I need the > high frequencies. > > It seems likely to me that it would be almost impossible to > achieve any high > degree of compression and keep the high frequencies. Generally yes, but it depends a lot on the data. With something like seismic/mag/rm/dem data it might be possible as the data is fairly "smooth" and contains a lot of redundant data. 1-2bit data (eg, scanned topo maps) is a snap for something simple like Run Length Encoding and you might get 50-100:1 in some cases. ECW is primarily aimed at compressing Airphotos where it's generally acceptable to loose a lot of high-frequency data without noticable visual artifacts, however it's usefulness for other types of data really depends on your application. regards -- Simon ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^ Simon Cope, : e-mail: [EMAIL PROTECTED] Chief Software Architect : WWW: http://www.ermapper.com : Int'l Phone: +61 8 9388 2900 Earth Resource Mapping Ltd, : Int'l Fax: +61 8 9388 2901 Level 2, 87 Colin St, West Perth. : Australia Phone: (08) 9388 2900 Western Australia 6005 : Australia Fax: (08) 9388 2901 Q: 1.2TB (1,200GB) Image over the Internet? A: Image Web Server! http://www.earthetc.com > Are there any expert opinions out there? > > John > > John Berry Assocs - Remote Sensing Services > 5000 Beverly Hills Drive, AUSTIN, TX 78731 > Ph: +1-512-452-8068 Fx: +1-512-452-8068 > Mo: +1-512-293-8068 > e-mail: [EMAIL PROTECTED] > > > ----- Original Message ----- > From: SR McMullan <[EMAIL PROTECTED]> > To: <[EMAIL PROTECTED]> > Sent: Wednesday, October 09, 2002 8:20 AM > Subject: RE: Importing TIF files > > > > Hi Bartley, > > > > It sounds like the files you are importing are compressed > (probably with LZW > > compression). ER Mapper only stores the full image matrix, > which may be much > > larger as you have discovered. Try saving the resulting image using ECW > > compression, which is a bit lossy but much more manageable file sizes. > > > > To see how much fidelity is lost with ECW compression, try > compressing then > > uncompressing the image, then subtract the compressed/uncompressed image > > from the original. I did some experimentation with this when > ECW was first > > released and I was losing some valuable high frequency > information which is > > not acceptable in our applications, so I still use the full fidelity > > uncompressed images. > > > > Good luck, and happy imaging! > > > > SRM. > > > > Steven R. McMullan, P.Geo. > > Principal Geophysicist > > > > Poseidon Geophysics (Pty) Limited > > Plot 3423 Matima Crescent Extention 12 > > Private Bag X018 > > Gaborone, Botswana > > > > Tel +267 351 612 > > Cell +267 71 300 820 > > Fax +267 300 787 > > E-mail [EMAIL PROTECTED] > > Personal E-mail [EMAIL PROTECTED] > > > > > > > > > > -----Original Message----- > > From: [EMAIL PROTECTED] > > [mailto:ermapper-l-owner@;ermapper.com]On Behalf Of Bartley Doyle > > Sent: 09 October 2002 01:44 PM > > To: '[EMAIL PROTECTED]' > > Subject: Importing TIF files > > > > > > Hi all, > > I have imported some TIF files (1Mb each in size - B&W). The > resulting image > > file sizes are 200Mb! > > Can you tell me if there is a way of (batch preferrably) > importing these TIF > > to a much smaller file size (say 1Mb each)? > > Regards, > > Bartley > > > > > > > > ********************************************************************** > > Bartley R. Doyle > > Compass Informatics > > Tel: +353-1-6705761 / 087-2902343 > > Fax: +353-1-6703037 > > Eml: [EMAIL PROTECTED] > > Pst: 19 Nassau Street, Dublin 2, Ireland > > > > Visit www.compass.ie ! > > And enter the Compass GeoShop at www.compass.ie/geoshop/. > > > > *********************************************************************** > > > > This email and any files transmitted with it are confidential and > > may contain commercially sensitive material. They are intended > > solely for the use of the individual(s) to whom they are addressed > > and should not be released to a third party without the consent > > of Compass Informatics. > > > > *********************************************************************** > > > > ----------------------------------------------------------- > > > > To make changes to your subscription, please visit our website, > > http://www.ermapper.com/technicl/ermapperl/index.htm > > > > ----------------------------------------------------------- > > > > To make changes to your subscription, please visit our website, > http://www.ermapper.com/technicl/ermapperl/index.htm > > ----------------------------------------------------------- > > To make changes to your subscription, please visit our website, > http://www.ermapper.com/technicl/ermapperl/index.htm > > ----------------------------------------------------------- To make changes to your subscription, please visit our website, http://www.ermapper.com/technicl/ermapperl/index.htm
