New answers tagged erdas-imagine
with ArcGIS, you can use the "copy raster" tools to convert the img file in a ENVI file. Just write ".dat" at the end of the name of the output file. By the way, you don't need to write code to use gdal. You can install it with OSGEO4W, then you type gdal_translate -of ENVI -co INTERLEAVE=BSQ myimage.img myoutputimage.dat and it runs the conversion.
Actually, I have had some good experience displaying images with QGIS. And you can play with transparency. But from what I know you cannot have two linked viewers. You have other free open source softwares (Monteverdi, BEAM ), but often those for raster image processing do not provide multi-image viewers. They are however quite fast to display your image ...
Idrisi - not free but cheap and there is a free trial version. Good for raster comparison and change direction. http://www.clarklabs.org Or saga! Not simple but very powerful and free! http://www.saga-gis.org/
I have not tried it but Opticks is worth a shot: http://opticks.org/confluence/display/opticks/Welcome+To+Opticks
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