Hi,
Your comparison about file sizes is not good because you do not even try to control the compression rate. You can adjust the compression rate with GDAL by using -co TARGET=[percent] for ECW and with -co JPEG_QUALITY=[1-100] for JPEG compression.
http://www.gdal.org/frmt_ecw.html
http://www.gdal.org/frmt_gtiff.html
Generally speaking, it is usually possible to compress more with wavelet based compression methods than with the JPEG method while the quality is the same when measured as Peak signal to noise ratio (PSNR). Unfortunately there are lots of quasi-scientific comparisons between JPEG and JPEG2000 which are mostly nonsense and many of those are high up in the search engine hits. This one was on the first page but seems to be still scientifically sound
http://cdn.intechopen.com/pdfs/196/InTech-Image_compression_effects_in_face_recognition_systems.pdf
I have the same experience from aerial images than the table 1 in this article: The difference between JPEG and JPEG2000 (and ECW as well) is not at all dramatic when compression rate is moderate, that is, something up to 1:20. However, in geospatial use there must be overviews/pyramid layers and because ECW and JPEG2000 have overviews by nature in the progressive bit stream it means 25% save in disk space
Selection between the formats is more a matter of taste:
- ECW and JPEG2000 utilities are usually using good defaults for compression and the one who is running the machine does not need to understand so much what happens.
- Tiff files with JPEG/LZW/DEFLATE compression can perform as well as ECW, but especially GDAL user must know what to do and which compression to use. JPEG is usually good for aerial and satellite images, LZW and deflate for raster maps. Paletted raster maps may compress better with LZW and deflate than with ECW or JPEG2000 if the quality must remain high.
- It is a bad idea to use tiffs without tiles and overviews for anything real.
- Between the TIFF variants, uncompressed tiffs can be fastest but not necessarily if disk is slow. LZW, deflate and JPEG are simple methods and fast to uncompress.
- ECW and good JPEG2000 libraries are not free.
There is only one ECW library and also GDAL is using it for accessing the images. However, GDAL has sort of an additional processing level which tends to make is a bit slower with ECW than direct use of the library. For a real comparison you should make sure that GDAL is using the same version of ECW SDK than ArcGIS does. GDAL is often compiled with version 3.x of the SDK because that comes with an easier license. No version of ECW SDK can be used on a server without paying a license so remember that with your Mapserver and Geoserver experiments.
Regerds,
-Jukka Rahkonen-
Mark Volz wrote:
Hello,
I converted a ECW file to a compressed GeoTiff file. I am happy with the
performance, but I noticed that the resulting file is significantly larger. The
original ecw file is 204MB, while the new Geotiff file is 981MB. Is there any
reason why the new file is almost five times as large? What can I do to minimize
the file size of an image while still being fast in GeoServer, Mapserver, and
ArcGIS?These are the commands that I ran, which in most part are adopted from the
"geoserver on steroids" article:
gdal_translate -CO TILED=YES -CO TFW=YES -CO COMPRESS=JPEG -CO
BIGTIFF=YES MNLACQ024021.ecw MNLACQ024021.tif gdaladdo -r average --
config compress_overview JPEG --config photometric_overview YCBCR
MNLACQ024021.tif 2 4 8 16 32 64 128 256 512 1024NOTE: The original ecw file does contain tiles. The file seems fast in ArcGIS, but
it is slower in GeoServer and Mapserver.Thank You
Mark Volz, GISP
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