I would also always recommend to use -co PREDICTOR=2 when compressing
with LZW (or DEFLATE). So far I made quite good experiences with LZW and
PREDICTOR=2, with typical compression rates close to gzip compression
(meaning when putting the tif file into a tgz archive file).
The only time that I discovered where LZW does not work well was when
using gdalwarp and directly writing the output as (untiled) LZW TIFF.
This could create LZW files up to 2-3 times bigger than the uncompressed
input TIFF. When using gdalwarp and one wants to save disk space one
should either create tiled LZW TIFF's, or warp to uncompressed TIFF and
then use gdal_translate with LZW to create the final LZW image,
PREDICTOR=2 is recommended for both methods.
armin
On 10/02/2014 01:13 AM, Even Rouault wrote:
Le jeudi 02 octobre 2014 01:06:57, David Strip a écrit :
On 10/1/2014 12:02 PM, Jukka Rahkonen wrote:
For comparison:
Tiff as zipped 347 MB
Tiff into png 263 MB
If I have understood right both zip and png are using deflate algorithm so
there might be some place for improving deflate compression in GDAL.
I was curious how png could achieve such better compression if it is
using the same deflate algorithm. I wouldn't think different
implementations would account for so much improvement. It turns out the
png compression uses a "filtering" step ahead of compression. This is
explained here. The filter is similar to a differential pulse code
modulation, in which the pixel is represented as the difference from the
pixels to the left, left upper diagonal, and above. This typically reduces
the magnitude of the value to something close to zero, making the encoding
more efficient.
True, a way to improve things might be to specify -co PREDICTOR=2. Should
apply to both LZW and DEFLATE.
This is one of the filter that might be used by PNG, except that PNG has
different filters, so it will eventually beat TIFF deflate.
David
_______________________________________________
gdal-dev mailing list
gdal-dev@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/gdal-dev
