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We have some jp2 or img satellite images that we compress to .ecw using ERDAS Imagine. We have realized that with some images we are losing too much information while doing that. What i wanted to ask is about preferences on ERDAS Imagine.

Target ECW Compression (Greyscale): Value

Target ECW Compression (RGB): Value

Target ECW Compression (Multispectral): Value

enter image description here

Do I need to adjust them each time I compress an image or is there an optimal setting that i can use?

(We are using mostly 16 bit satellite images as input)

Or where else could the problem be that the ecw files are so foggy and unsharp compared to the input files.

img:

enter image description here

ecw:

enter image description here

Thanks in advance,

Yasmin

  • Is the output still 16bit? last I looked ECW only supported 8 bit, but that was a few years ago. To get 16 bit to 8 bit we did a contrast stretch to 8bit img and when happy with the image contrast converted that to ECW. Monochrome should be good with 90% compression but colour images I don't go above 80% as the wavelets make capture from the result very difficult; however if you're not zooming in and just using it as a general background image (like for a map) then you could go higher if you need to save space. I can't comment on the multispectral as I've not done that before. – Michael Stimson Apr 14 '14 at 22:14
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ECW V3 version (since the 2015 version at least) preserves the 16-bitness too, you used V2 and it is more compatible but cannot preserve it. Raster --> Radiometric --> LUT Stretch is how to do that 16 to 8 bit, once the Contrast is as desired and the image layer in the Contents list has been right-clicked and Saved.

ECW is not compressed to certain size or % but certain quality or target compression ratio, e.g. 20:1 is a default for 3-band RGB images and 10:1 for others. The actual ratio depends on the image contents.

In any case, how some image generally looks like depends mostly on the settings: which bands are being displayed as R, B and G, have the histograms been calculated and 0 ignored, is the image opened with the Raster Options / No Stretch or not.

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