[gdal-dev] Re: Artifacts when presenting WMS based on a number of adjoining vrt files with overviews

[Jukka Rahkonen]

jonas nielsen <jonasln <at> gmail.com> writes:

> 
> 
> The original images that the overviews are built on are 10000 by 
10000 pixels.
> 
> And yes, the artifacts appear without reprojection.
> 
> -Jonas

Hi,

I do also see such artifacts with Mapserver tileindex layers when 
zoomed far away so that the overviews are used. I have a slight 
feeling that it may come from subsampling when the resolution and 
image extents do not math exactly.  With your images I would guess 
that the first overview levels up to 16 might work fine because 
the image extents can be filled exactly with the new subsampled pixels. 

samling theoretical
level   image width
1       10000
2        5000
4        2500
8        1250
16        625
32        312.5
64        156.25
128       78.125
256       39.0625
512       19.53125
1024       9.765625
2048       4.8828125

I cannot imagine what GDAL is really doing at, let's say, redused 
level 128. Your whole image width could be covered with 78.125 pixels,
each 12.8 meters wide. Obviously this is not possible. Either the pixel
size must be a bit smaller or this overview level contains 79 pixels 
and thus it is a bit wider than the original image. If the latter is 
the case, what colour does the border pixels, which have the majority 
of their area off-site, get?

I may be totally on the wrong track but this is what I have been 
wondering. Same looking but warp process related artifacts can be 
seen in figure 3 in article 
http://www.scangis.org/scangis2007/papers/r3_rahkonen.pdf.

Figures 4 and 5 show how we managed to remove them.


-Jukka Rahkonen-