Use this dialog box to
specify the color that should appear transparent in the image.
Click Select and
pick a color on the image. When transparency is turned on, the selected color
color is available only if the image is loaded and the Raster Extension
is loaded. To load the image, select the image, then right-click
the image frame Image Show
Image. To load the Raster Extension, choose a Raster
transparency color is stored in the drawing as an AutoCAD Map 3D custom
object. If you send the drawing to other users, they can see the
transparency color only by opening the drawing in AutoCAD Map 3D or
Autodesk Raster Design. If they open the drawing in AutoCAD, they
see a message that AutoCAD cannot reference the custom object and
will not display the transparent color.