You may compress your images "losslessly" (using lossless compression) or "lossily" (using lossy compression). Lossless compression preserves all data. Lossy compression discards some data in order to achieve smaller file sizes. For the purposes of real workflows, however, it is useful to think of GeoExpress as offering three categories of image quality: lossless, visually lossless and lossy.
Lossless compression retains all source image data. No data is discarded. This level of compression typically yields a 2:1 compression ratio, for a 50% reduction in the space required to store the image. Lossless compression should be used when it is critical that all bits of the original image be preserved. This is the case for archival storage, as well as for uncommon workflows where no possible loss of precision is ever acceptable. You may also wish to use lossless compression when you are generating a “master” image from which other derivative images will be made, as through the MrSID optimization process. From a lossless MrSID or JPEG 2000 file, the original TIFF or other raw file can be reassembled identically, bit-for-bit.
NOTE: When you use the lossless option to recompress an image that has already been compressed lossily, it means that no further loss of data will occur. It is helpful to remember that in lossless compression all image data from the source image is retained.
This is actually a form of lossy compression, and the name is not intended to mislead. Visually lossless represents the limit of data that can be discarded before the human eye can detect any degradation in image quality. This level of compression is typically 20:1 for RGB and 10:1 for grayscale imagery. This is the most common level of compression quality used, as it preserves the appearance of the imagery for most workflows, including use of your imagery as a background layer and for many forms of visual analysis and exploitation.
Beyond 20:1, image degradation and artifacts can appear, although often not too significantly until ratios of 40:1 or 50:1. Such lossy quality may be acceptable when the imagery is used only as a background layer for appearance or when the image quality is less important than the storage size or speed, such as for informal visual inspections.