All images encoded to MrSID Generation 4 (MG4) have an alpha channel or alpha band. If one is present, GeoExpress uses the alpha band in the input image. If no alpha band is present in the input image, then GeoExpress creates one.
The alpha band is always encoded losslessly, even if the rest of the image undergoes lossy compression.
MG2 and MG3 do not support alpha channels, but the transparency information in the alpha band of your source image is still preserved when encoding to MG2 and MG3. The alpha band will first be applied to the source image and then discarded from the output file.
NOTE: For users of GeoExpress Standard Edition, the encoding of the alpha band does not additionally decrement your data cartridge. Your encode job will be metered the same with or without alpha bands.