[mapserver-users] Resampling and image quality?

[Gregor Mosheh]

Hi, all. We're seeing some distortion in an image, a loss of viewing 
quality, and am hoping that y'all can provide some insight into what may 
be the issue.

Here are the URLs:
    http://dev.geowake.com/chart.test/index.php
    http://ms2.geowake.com/
Yes, it's Google Maps, but bear with me. :)
The "dev" one is using a PNG sliced for use in Google Maps.
The "ms2" one is Mapserver WMS, using the same PNG but with a worldfile 
and without slicing since the idea here is to use WMS and save a lot of 
work as well as being vendor-neutral.

You'll see that the "dev" one has a much "smoother" look to it. If you 
zoom in three notches, you can read the map's title clearly. The 
Mapserver WMS version, the text is very obviously blocky even if you 
zoom in 4 notches. As you keep zooming in, the quality difference 
becomes more obvious as you can read the PNG's smaller text but not the 
WMS's.


So, what to do? I have tried and eliminated several possibilities, and 
am stumped.

- Source image quality; the PNGs on both sides are visually identical in 
an image viewer. I have tried the source image in TIFF and PNG with 
various options.

- Bad world file; the spatial extent matches that of the other map, and 
I've checked the math repeatedly and found it A-OK

- Image format options; I have tried every image format supported, and 
the image always comes out looking the same, so it's not interlacing 
gone wrong, nor lossy JPEG compression, etc. The outputformat block is 
set to generate PNG-24 RGBA, and I have verified by downloading from the 
WMS that it is doing so.

Possibilities I've not yet exhausted:

- Resampling; using PROCESSING "RESAMPLE=AVERAGE" improved the image 
quality but only very slightly.

- Reprojection; the images are in WGS84 (EPSG 4326) and I know that 
Google uses their weird Mercator (54004). I added 54004 to our EPSG 
file, and have tried various approaches such as reprojecting the image 
to 54004 ahead of time, and using 54004's numbers in a worldfile so 
there'd be no reprojection at all, but the effect is always the same.


Any ideas what's up here?

-- 
Gregor Mosheh / Greg Allensworth, BS, A+
System Administrator
HostGIS cartographic development & hosting services
http://www.HostGIS.com/

"Remember that no one cares if you can back up,
  only if you can restore." - AMANDA