[mapserver-users] MapServer's image quality stinks serving up theseNOAA NRCs

[Rahkonen Jukka]

Gregor Mosheh wrote:

> 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?

Aren't they inevidently downsized if you are looking at the whole North American coastline with just something like 800 by 600 pixels on screen?  I can see very similar ugly view when I fit on the screen a large OpenStreetMap map (12000 by 12000 pixels)  rendered with a lot of details. The map begins to look nice once I zoom in such much that the labels come readable. I have been thinking that it is because the street names and other details every now and then makes some downsampled image pixel to appear black. For the algorithm it might be OK but for the one who is looking at the map it makes just noise. In your case there are whole lot of contour lines and depth values causing the noise. I guess that if you mosaic lots of image tiles together and open it with Photoshop or whatever else viewer your experience would not be much better.
I don't believe you can improve the far-away zoom quality much with Mapserver. You may try to filter out digitally some depth values squeesed nos to one pixel only or try some other tricks, but I think that best you could do is to find out another map that is originally made to be be used at nationwide scale.

-Jukka Rahkonen-