I need the ability to be able to display ECW and JPEG2000 format files within MapServer via a Community Mapbuilder client which sends WMS requests for the images to MapServer

Rahkonen Jukka Jukka.Rahkonen at MMMTIKE.FI
Fri Dec 1 02:24:36 EST 2006


Hi John,
 
ECW images are always compressed.  The first step what Mapserver does in your process is to decompress the original ECW image to some kind of uncompressed pixel map.  I haven't measured the times but I do not believe there would be any dramatic difference in how long it takes to perform the decompression if the original images are compressed to ratios 1:20 or 1:5.  So I do not believe you could improve the speed very much by adjusting the ECW compression ratio of the originals.
Most probably you could get some more speed by having the originals as uncompressed TIFF files because then MapServer could skip the ECW decompression step.  But I would like to encourage you to make a test by converting some of your ECW images to TIFF format and then compare the total time your whole process is taking with either TIFF or ECW files.
 
You can make TIFF images which are quite well optimised for Mapserver by the following procedure. First create tiled TIFFs from your ECW files with gdal_translate by using a command:
gdal_translate -of GTiff -co "TILED=YES" input.ecw output.tif 
and then create internal overviews to your new TIFFs with gdaladdo:
gdaladdo -r average output.tif 2 4 8 16
 
Then you ready to make some timing and gather information to help you in making decisions whether or not to have the originals compressed and if to compress the output or send it as uncompressed GeoTIFF instead.  I suppose you know your clients and the bandwidth they have because that matters, too.
 
Regards,
 
-Jukka Rahkonen-

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Lähettäjä: UMN MapServer Users List puolesta: John Mitchell
Lähetetty: pe 1.12.2006 5:17

Frank,

So if I have the ECW image compressed at a 2:1 ratio mapserver will process it faster than if I have the ECW image at 20:1 because mapserver has to decompress the image first.  Also the resulting image should be better since I have not compressed it as much. 

If I choose to have FORMATOPTION "TARGET=0" which means no compression then my image quality will be better, but it will be slower to download since it is not compressed?

John


On 11/30/06, Frank Warmerdam <warmerdam at pobox.com> wrote: 

	John Mitchell wrote:
	> Frank,
	>
	> The ECW is already in a compressed format, so this means that the image
	> is compressed another 95%?  This additional compression is for
	> performance purposes?; does it affect the image quality? 
	
	John,
	
	The imagery is decompressed within mapserver for rendering purposes, and
	then (if output is to ecw) re-compressed.  So the 95% is relative to
	uncompressed form, not cumulative.   Even if the original image was 
	at 95% compression the process of decompressing and recompressing can result
	in additional image damage.
	
	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    | President OSGeo, http://osgeo.org
	
	




-- 
John J. Mitchell 



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