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-
________________________________
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 <[EMAIL PROTECTED]> 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, [EMAIL
PROTECTED]
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
