[FWTools] IMG file compression

[David Fawcett]

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?

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.

Thanks,

David.


Driver: HFA/Erdas Imagine Images (.img)
Files: dor17064.img
       dor17064.img.rrd
Size is 6193, 8151
Coordinate System is:
LOCAL_CS["Unknown",
    UNIT["other",1]]
Origin = (229851.427841041940000,5246544.006513102000000)
Pixel Size = (1.000000000000000,-1.000000000000000)
Corner Coordinates:
Upper Left  (  229851.428, 5246544.007)
Lower Left  (  229851.428, 5238393.007)
Upper Right (  236044.428, 5246544.007)
Lower Right (  236044.428, 5238393.007)
Center      (  232947.928, 5242468.507)
Band 1 Block=64x64 Type=Byte, ColorInterp=Undefined
  Description = Band_1
  Min=1.000 Max=255.000   Computed Min/Max=1.000,255.000
  Minimum=1.000, Maximum=255.000, Mean=129.067, StdDev=46.454
  NoData Value=0
  Overviews: 1549x2038, 775x1019, 388x510, 194x255, 97x128, 49x64
  Metadata:
    STATISTICS_MINIMUM=1
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=129.06651980747
    STATISTICS_MEDIAN=130
    STATISTICS_MODE=137
    STATISTICS_STDDEV=46.453529041224
    LAYER_TYPE=thematic
    STATISTICS_HISTOBINVALUES=11184|4467|5138|5777|6566|7290|8189|9133|9959|11091|12216|13865|14677|16260|18047|19647|21381|23149|25237|27314|29508|31688|34192|36492|39198|41535|43791|46772|49273|52032|54577|57069|59660|61648|65081|68041|69996|73471|75995|78669|81135|84191|86766|89596|91737|94746|97940|100687|102735|105144|108065|111042|113940|115884|118456|121575|123935|127971|129986|133561|136793|139209|141859|145022|149123|152280|155886|157836|162531|165434|168865|172854|176389|180631|183907|187283|191908|194928|198912|201973|206606|210317|214889|217722|222190|226473|230465|234621|238777|242460|246560|250857|254438|257189|262666|266955|270215|274172|277995|282391|286664|290064|292857|296861|301953|304412|308920|312565|315625|319145|323766|326789|330788|333632|337022|341572|344230|347753|351921|354086|358727|361825|363940|367940|369913|371375|376224|376171|378440|379602|381482|384014|385585|386132|386811|386858|386976|386011|383595|382555|381462|378435|377537|373845|370866|366691|362907|360270|354108|350204|345182|340820|334495|329224|324260|317932|313236|307414|302721|295697|291828|285538|279826|273970|269023|263464|257552|252809|246459|241828|237708|232342|228594|221756|217406|213313|208349|203856|199654|195870|191912|188067|183859|180241|176375|172429|168904|165910|161732|158978|155263|152347|149181|147772|143327|141238|138019|136250|132735|130146|127371|123913|120944|117723|114728|111358|106478|103355|100088|96491|92706|89018|86192|82789|79344|77618|74277|71469|68949|66370|64425|61224|58978|56062|53422|50417|47254|44482|41723|38800|36873|34867|32860|30917|29189|27469|25956|23649|22698|21528|20408|19511|18399|17446|16385|14982|13764|12085|9832|8170|6174|4663|3263|1988|1281|
  Image Structure Metadata:
    COMPRESSION=RLC