Mapserver/GDAL/ECW Zoom beyond 1:1 problem

[dgraham@…]

Frank:

Just set up a Mapserver using the GDAL/ECW on Win32.  We had set this up once
before and noticed it, but now we are trying to do ECW as a production server. 
The problem we noticed is that when you zome in on the ECW in the native
projection much beyond 1:1 pixels the picture breaks down (blank or small
lines).  The odd thing about the problem is that when you zoom in while
reprojection is on, the problem goes away.

I have two samples to show you.  They are of the same location in North West
Alabama DOQQ.  Both images are from the similar map files.  The only difference
between the two is that one is in the data's native projection NAD83 UTM Zone 16
(26913) and the other is reprojected into NAD83 Geodetic.  Both images are made
from a tileindex that references both Tiff imagery and ECW imagery.

1) The first example shows the problem.  This is the intersection of 4 tiles. 
The upper two tiles are GeoTiff, the lower two tiles are ECW.  You will notice
that  you cannot see the data from the ECW tiles, unless you zoom out.

http://www3.i3.com/cgi-bin/mapserv.exe?imgxy=299.5+299.5&imgext=402009.989229+3859464.791483+403057.200011+3860512.930155&map=c%3A%5CData%5Cmapfiles%5Cecw_no_repro.map&savequery=true&mapext=shapes&imgsize=600+600&img.x=291&img.y=297&zoomdir=1&zoomsize=2

2) The second example shows how there is no problem if you reproject.  It is the
same area with both the GeoTiff and ECW data, but you can zoom in on the ECW
side and see the data.

http://www3.i3.com/cgi-bin/mapserv.exe?imgxy=299.5+299.5&imgext=-88.073925+34.870313+-88.059277+34.884961&map=c%3A%5CData%5Cmapfiles%5Cecw_repro.map&savequery=true&mapext=shapes&imgsize=600+600&img.x=297&img.y=298&zoomdir=1&zoomsize=2

The build I am using is right off the mapserver website
(http://mapserver.gis.umn.edu/win32binaries.html).  It is the
ms36png_gdal_pg_wms.zip with the ms35_libs.zip.

Now I checked the bug lists for both the Mapserver and GDAL and did not find
anything about this.  My question to you is:

Do you know if this problem is (1) in Mapserver, or (2) in GDAL, or (3) a
function of the build/install?

Dave

P.S. The data at www3.i3.com will only be up for a few days.  When our testing
is done it will come back down.

-- 
David W. Graham
Director of Geospatial Applications Development
information integration and imaging, LLC
201 Linden St, Third Floor
Fort Collins, CO 80524
(970) 482-4400
dgraham@i3.com
http://www.i3.com

[fwarmerdam]

David, 

This is a problem with the use of ECW in GDAL.  In particular, GDAL will 
sometimes request data from the ECW SDK at greater than full res but this
fails, and so GDAL returns nothing. 

I will fix this by tomorrow and update the bug report. 

[fwarmerdam]

Dave, 

I have committed a fix for ecwdataset.cpp in GDAL's CVS tree that should avoid
problems with supersampling.  Note that performance when rendering maps from
ECW via GDAL may not be ideal as my fallback for supersampling issues is to
go through the regular block oriented API which means that full image scanlines
will be loaded for the area of interest, not just the minimum data needed to 
satisfy the request.  

Let me know if you encounter any further bugs with the ECW support.