16-bit to 8-bit stretch options

[Vincent J. Troisi]

We have 16-bit geotiff images we are serving to google earth. Google earth
is making requests for 8-bit images (png). However, when the conversion
is done from 16-bit to 8-bit, some pixel values which are not "0" in the
16-bit geotiff get set to "0" during the conversion process. Is there 
anyway
to control the options for this conversion in the map file? Is there a 
document
available that describes the conversion process performed by mapserver?

Thanks.