If you need to make images with higher compression ratios, use the Despeckle tool to minimize compression artifacts.
A side effect of lossy encoding schemes is that subtle compression artifacts are introduced which alter the pixel values of the source image. While these changes are usually invisible to the human eye, they can damage the integrity of any collar regions in the image, which makes it difficult to seamlessly mosaic images together. The best way to avoid this "speckling" in the collar areas is to compress to MrSID Generation 4 (MG4), or to losslessly encode source images to MrSID Generation 3 or JPEG 2000.
Despeckling is a computationally expensive operation that increases the time it takes to encode an image. Also, despeckled JPEG 2000 files employ an alpha band to mask the transparent region; most, but not all, applications can view JPEG 2000 files with alpha bands.
Despeckling is only supported for MG3, MG4, and JPEG 2000 output. If you select JPEG 2000 as the format of your despeckled output, that output will be a GMLJP2 file and thus a JPEG 2000 Part II file, even if your input was a JPEG 2000 Part I file. Source images with alpha bands cannot be despeckled when outputting to JPEG 2000. Also, the area of valid image data must be contained within a single polygon (for example, an image of a series of islands cannot be despeckled).
By default, images are not despeckled upon encoding. Further, despeckling is not recommended for images without a collar. To despeckle images by default for the MrSID or JPEG 2000 formats, set the despeckling option in the preferences. For more information on preferences, see Preferences.
NOTE: Some existing JPEG 2000 images are encoded in such a way that certain decoders may fail to display them when despeckled with GeoExpress' default settings. To accommodate these images, GeoExpress handles several JPEG 2000 parameters in a special way. If you are despeckling images using the Despeckling dialog and wish to change parameters for tile length markers, packet length markers, or tile parts, you must change them on the JPEG 2000 tab of the Preferences dialog. (For more information see JPEG 2000 Preference Options and Advanced JPEG 2000 Options.)