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

[jonas nielsen]

Hi List,

Help would be sincerely appreciated:



We are seeing artifacts when presenting WMS images based on a number of
adjoining .vrt files with overviews.



Background.

We are processing 47.000 files containing aerial photos of 1 km by 1 km
each, at a resolution of 10 cm by 10 cm. These are to be distributed as WMS
through MapServer. Originally we planned to create one or more vrt files
with overviews for this.

We observed the processing efficiency of GDAL (gdaladdo) deteriorate as the
number of image files in the vrt increased. Therefore we create vrt files
for every 100 photos (10 km by 10 km area), build an external,
jpeg-compressed overview for each, and then present these 470 files together
through MapServer. We build the jpeg-compressed overviews with gdaladdo.* *

* *

The bug.

Viewing the output for a 2000 km2 test area, we see vertical black or white
lines in parts of the output. Normally, these lines only extend partially
from top to bottom in the image. The lines do not appear in all zoom levels.
Changing width or height by a pixel or two may make them go away. Panning in
the image also may make them go away. We never see the lines when zoomed in
far enough to be viewing the raw image files. The lines do not allways
appear along the ’seamlines’ between the 10 by 10 km blocks – but mostly do
though.



The artifacts appear both when viewing the output from MapServer with Gaia
and through a browser.





Any thoughts on what might be causing this?





For reference, the gdaladdo command:

gdaladdo /data/file.vrt -r average -ro --config GDAL_CACHEMAX 1000 --config
COMPRESS_OVERVIEW JPEG --config JPEG_QUALITY_OVERVIEW 75 --config
PHOTOMETRIC_OVERVIEW YCBCR --config INTERLEAVE_OVERVIEW PIXEL --config
BIGTIFF_OVERVIEW YES 2 4 8 16 32 64 128 256 512 1024 2048
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