[FWTools] IMG file compression

Frank Warmerdam warmerdam at pobox.com
Tue Dec 30 15:56:12 EST 2008

David Fawcett wrote:
> Please forgive my raster naivete...
> I have a few questions about compression in .img files.  Essentially,
> I want to use an .img file as an intermediate storage step for a
> 1-band, 8-bit unsigned raster.  I am unsure about the nature of the
> compression used in/with img files.
> 1.  Are .img files always lossless, or is there potential that a
> lossey compression has been applied?


To the best of my knowledge there is no lossy compression mechanism
available for .img (HFA) files.

> 2.  From gdalinfo, I see that run length compression has been applied,
> but is this referring to the overviews, or the base raster?  I guess
> that if it is the base image, or the overviews, RLC is lossless, so it
> doesn't really matter.

The base raster.  I'm not sure if the overviews are RLC compressed in
this case or not.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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