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We use ECW compressor shipped with ERDAS Imagine to export Orthoimagery for our different departments. Because we are facing problems with the Imagine license server, we had to choose GDAL (built with ECW SDK) as a fallback solution. Because the Orthotiles are all approximately 100MB large, thats possible.

As a compression ratio we use 1:20. With ERDAS ECW compressor (it does not matter if we use the licensed one or the free edition from some years ago) the images are all compressed to approximately 4 to 6 MB each.

With GDAL and the same compression ratio, the images got 10 to 20 (!) MB each. ALso some of the files cannot be opened afterwards. Also when I increase the compression ratio (to 1:30 for example) the imagefiles get smaller but face a lack of imagequality very quickly.

Why is GDALs compression significantly more worse than ECW Compressor, when they are both based on the same libraries from the ECW SDK?

Is it because GDALs implementation is so buggy or because ERDAS is keeping its Aces on their sleeve?

EDIT: Yeah, I know that this is truly a naive question ;)

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My guess is that you are actually using different versions of the ECW SDK. According to this website (gdal.org/frmt_ecw.html),it sounds like the 4.x SDK requires a key from ERDAS, while the 3.3 SDK runs for free on files less than 500MB. –  travis Jun 27 '13 at 8:54
    
I am able to use both (we have an erdas license key), what confuses me, is that if I compress a 150MB TIFF to ECW (1:20) it goes down to 7 MB with ERDAS ECW Compressor and down to 20 or more MB with GDAL. In fact both programs use the same libraries (even the modification date of the DLLs is the same). GDAL is linked to the free edition of the SDK. I am wondering if I face the same behaviour, if I link GDAL to a licensed version of the SDK or when I use the SDK in our own applications. Its not only the size of the ECWs, there are also disadvanteges in rendering performance. –  Jürgen Zornig Jun 27 '13 at 10:28
    
It sounds like the same ECD library can compress using both the 3.3 and 4.x methods. I would try linking GDAL to a licensed version and see what happens. GDAL also seems to have a number of runtime parameters that you can set, have you tried playing around with those? –  travis Jun 28 '13 at 8:56
    
If you can point where to get one sample image to play with I can do some testing with it. –  user30184 May 19 at 13:00

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