[gdal-dev] MapServer's image quality stinks serving up these NOAA NRCs

[Gregor Mosheh]

I asked the GDAL list about this some weeks back, but we only explored a 
few ideas and then forgot about it for a few days.

URL   http://ms2.geowake.com/
These were KAP files downloaded from NOAA's Raster Nautical Charts 
(NRCs), converted to TIFF with gdalwarp.

The image quality there stiiiinks! A second map (no longer available) 
made of the raw images sliced into PNGs and not run via MapServer, looks 
very nice. You can make out text in 1 zoom, the high-level maps are less 
grainy, the quality once you zoom in is better...

Any thoughts on why it'd be so ugly? The images almost look as if 
they've been downsized, don't they?

Factors which we've tried and seem to have eliminated:

* The KAPs themselves aren't damaged, and GDAL isn't creating crummy 
TIFFs. I can gdalwarp them to TIFF or PNG, and back and forth, and the 
output image looks great -- if I download it and view it on my PC.

* Reprojection: If I request a WMS image in a browser, using plain 
latlong (the KAPs are georef'd in latlong) the image quality doesn't 
improve. Also, if I project the KAP to EPSG:54004 and request there, it 
makes no difference. That seems to eliminate reprojection as a likely cause.

* Output image format. Requesting PNG, JPEG, or GIF from MapServer WMS 
makes no difference. I can share the IMAGEFORMAT blocks if requested.

Items still not entirely eliminated:

* Resampling: If I add PROCESSING "RESAMPLE=BILINEAR" it makes a small 
difference in the quality, but not a lot. Same goes for bicubic as well: 
a visible difference, but not significant. The other two didn't make any 
difference at all.

* Color depth: gdalwarp on the KAPs generates 8-bit palleted TIFFs. That 
may not be the best for quality.


Any thoughts?

-- 
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