[gdal-dev] gdal_translate compress is larger??

[Frank Warmerdam]

On 11-06-15 01:43 PM, Matt Wilkie wrote:
> Hi Folks,
>
> I've run into a strange thing, "gdal_translate -co compress=lzw" is creating
> files that are larger than the source uncompressed image.
>
> Input:
>
> 04/04/2006 08:39 AM 959,236,993 A1.tif
>
> Results of "gdal_translate -co compress=[lzw,packbits,none]"
>
> 15/06/2011 10:35 AM 1,003,987,364 _lzw.tif
> 15/06/2011 10:31 AM 958,908,507 _none.tif
> 15/06/2011 10:30 AM 965,940,411 _packbits.tif
>
> gdalinfo report is attached.
> source file at http://files.environmentyukon.ca/matt/gdal-trans-lzw/ (in about
> 20 minutes).
>
> GDAL 1.8.0, released 2011/01/12

Matt,

LZW compression packs a little dictionary at the front of each strip/tile
with the table of patterns.  The size of an LZW compressed image can be
noticably larger than an uncompressed image if the chunk size (strip/tile)
is small so that this overhead becomes significant, and of course if the
imagery is essentially uncompressable with LZW.

Using larger strip/tile sizes can improve efficiency with LZW.

Also, using -co PREDICTOR=2 can help with imagery that is smoothly
varying as it compresses the differences from pixel to pixel instead of
the absolute values, and these will tend to be small and have more
patterns.

Best regards,
-- 
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