Hi John, In my experiments with compression algorithms, it looks like ECW loses power in the mid-frequency range which may be more important in geological applications (I can send you some sample images if you like, not certain if this list takes attachments). Of course you also lose the highest frequencies, with a resulting loss of image "texture".
ERM don't tell you what wavelet shape they are using for compression (eg. Daubechies, Symmlet, Wavelet, Haar etc). What I think is happening is the wavelet shape is inappropriate for the type of data, resulting in unacceptable loss of power at certain frequencies. For example, with potential field data (magnetics, gravity), a step wavelet (Haar) would distort the shape of the potential field which would fundamentally change the data. What would be really interesting would be to analyse an image for the ideal wavelet shape, then compress with this optimum wavelet maintaining the fidelity of the information you are looking for. So in answer to your question: No, it is probably not possible to design a high-compression algorithm which is lossless, but it is possible to choose a custom wavelet shape to better suit your data or the component frequencies you are interested in. If anyone would like to see the sample images I generated to demonstrate the loss of information, please drop me a line and I will send to you directly. Cheers, 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:[EMAIL PROTECTED]]On Behalf Of John Berry Sent: 09 October 2002 11:25 PM To: [EMAIL PROTECTED] Subject: Re: Importing TIF files 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? 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. 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:[EMAIL PROTECTED]]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
