Problem with reprojected images

[Stephen Woodbridge]

Dirk Jesko wrote:
> Ed,
> 
> I already thought of something like this already. However, this should 
> not happen. If the images are seamless (our images are seamless and do 
> not have any borders) in the original projection, they should also be 
> seamless after a reprojection. Otherwise, I would call it a bug.

May be, may be not! In a continuous math world this might be true, but 
pixels are not continuous, this might be caused by a rounding or 
numerical percision cause the pixel location to fall inside or outside 
of the patch.

> I just tested an older version of Mapserver (4.6.1). That one creates 
> seamless results, even if the images are reprojected. Only newer 
> versions (tested 4.8.1) produce non-seamless images. Can someone confirm 
> this?

Well a behavior change can certainly be considered a bug :) In which 
case you might want to submit a simple test case (like two tiles and a 
mapfile) the reproduct the results.

-Steve W.

> 
> Regards,
> Dirk
> 
> 
> 
> Ed McNierney wrote:
> 
>> Dirk -
>>
>> I assume your tiled source images do not overlap, and that the originals
>> do not have white border strips around them.
>>
>> If that's the case, this may be an inevitable artifact of the
>> reprojection.  Look at the situation along one north/south edge between
>> two tiles.
>>
>> If you render the tiles in the source projection you'll get two adjacent
>> vertical lines along the edges of each file, with no gap between them.
>>
>> If you reproject the tiles, then you will have two adjacent edges that
>> are two diagonal lines.  The distance between the edges may not be 1,
>> and may vary by a pixel along the course of those two diagonal lines.
>> When that happens, the images will sometimes overlap by one pixel (you
>> won't notice) and will sometimes leave a one-pixel gap (which is
>> obvious).  The obvious gaps will repeat in a regular pattern since the
>> phase difference between the two diagonal lines will be regular and
>> repeating.
>>
>>     - Ed
>>
>> Ed McNierney
>> President and Chief Mapmaker
>> TopoZone.com / Maps a la carte, Inc.
>> 73 Princeton Street, Suite 305
>> North Chelmsford, MA  01863
>> ed at topozone.com
>> (978) 251-4242
>> -----Original Message-----
>> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
>> Behalf Of Dirk Jesko
>> Sent: Tuesday, March 14, 2006 3:04 AM
>> To: MAPSERVER-USERS at LISTS.UMN.EDU
>> Subject: [UMN_MAPSERVER-USERS] Problem with reprojected images
>>
>> Hi,
>>
>> I set up a WMS for aerial images. The images are projected in
>> Gauss-Krueger 3 and have a size of one square kilometer. The WMS also
>> provides other projections, e.g. Gauss-Krueger 2-5, UTM, etc. If I query
>> the WMS with a projection other than the original one, I get little
>> white lines along the edges of the tiles (see the attached image). The
>> image format does not matter. How do I get rid of these? I can not find
>> any option that might be related to this. By the way, this also happens
>> if the data is accessed via mapscript or as normal cgi.
>>
>> Thanks for any help.
>>
>> Regards,
>> Dirk
>>
>>  
>>
>