[mapserver-users] ECW Image Quality

[Andrew Hallam]

Hi Rich,

--On Monday, January 21, 2002 20:23 -0700 Richard Greenwood 
<Rich at GreenwoodMap.com> wrote:

> I am displaying ecw color images in mapserver, but the color quality is
> very poor. It looks like a palette problem.

Possibly a palette issue. From memory GDAL converts from 24bit RGB to an 
8bit palette (Frank?). I'm not sure how it does it (median cut, whatever) 
but I've not had great success with displaying good looking aerial or 
satellite imagery in MapServer (compared to the 24 bit originals).

The ECW compression process introduces a lot of additional colour values to 
the image. Even though these colour values are close to the originals it 
may be causing problems in the "palettisation" process.

> Also, when I zoom in below I screen pixel == 1 image pixel, the ecw image
> goes to black.

Another (more educated) guess: The ECW bitmap renderer cannot super-sample 
an image so if you ask for anything below 1:1 (screen pixel:ECW image 
pixel) it will complain.

I'm in the process of writing an OpenGIS WMS interface to Image Web Server 
(and ECW file data). If anyone is interested I'd appreciate an email.

Regards
Andrew Hallam

Digital Earth Pty Ltd
http://www.digitalearth.com.au