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List: mapserver-users
Subject: [mapserver-users] Faster tileindex layer with reverse painter's algorithm
From: "Rahkonen Jukka (MML)" <jukka.rahkonen () maanmittauslaitos ! fi>
Date: 2018-11-22 11:25:29
Message-ID: 5dab876abdd64dbfbb353b3e1196f739 () C119S212VM042 ! msvyvi ! vaha ! local
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Hi,
Currently raster tileindex layers are rendered according to painter's algor=
ithm. First image that it found from the tileindex is rendered first, then =
pixels from the next image are added by method "b over a" which is the defa=
ult https://mapserver.org/uk/development/rfc/ms-rfc-113.html#rfc113 until t=
he last image is processed. The end result is fine and if user wants to get=
a result that is composed from the newest available images overall it can =
be done by reading the tileindex sorted by time "select * from tileindex or=
der by time".
Unfortunately the service gets pretty slow when the tileindex contains lots=
of overlapping images from different times because every image that gets s=
elected by the bounding box gets also rendered even if all the pixels from =
some image will get overwritten later. Setting a lower limit for time does=
not resolve our problem because the newest image can be quite old in some =
places.
I would like to see suggestions about how to implement a fast pre-selection=
that discards all the images which will not have any effect on the final r=
aster. Ideally Mapserver would do it natively if the default blend mode is =
used but SQL or function for PostGIS or Spatialite would suit me as well. I=
have been thinking that some sort of pre-selection with the reverse painte=
r's method might do for this purpose: test the footprints of the images fro=
m top to bottom, skip images which are covered by the growing union of foot=
prints and stop when the whole BBOX is full.
-Jukka Rahkonen-
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<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Currently raster tileindex layers are \
rendered according to painter’s algorithm. First image that it found from the \
tileindex is rendered first, then pixels from the next image are added by method \
“b over a” which is the default <a \
href="https://mapserver.org/uk/development/rfc/ms-rfc-113.html#rfc113"> \
https://mapserver.org/uk/development/rfc/ms-rfc-113.html#rfc113</a> until the last \
image is processed. The end result is fine and if user wants to get a result that is \
composed from the newest available images overall it can be done by reading the \
tileindex sorted by time “select * from tileindex order by \
time”.<o:p></o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US"><o:p> </o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US">Unfortunately the service gets pretty slow when the tileindex contains \
lots of overlapping images from different times because every image that gets \
selected by the bounding box gets also rendered even if all the pixels from some \
image will get overwritten later. Setting a lower limit for time does not \
resolve our problem because the newest image can be quite old in some \
places.<o:p></o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US"><o:p> </o:p></span></p> <p class="MsoNormal"><span lang="EN-US">I \
would like to see suggestions about how to implement a fast pre-selection that \
discards all the images which will not have any effect on the final raster. Ideally \
Mapserver would do it natively if the default blend mode is used but SQL or function \
for PostGIS or Spatialite would suit me as well. I have been thinking that some sort \
of pre-selection with the reverse painter’s method might do for this purpose: \
test the footprints of the images from top to bottom, skip images which are covered \
by the growing union of footprints and stop when the whole BBOX is \
full.<o:p></o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US"><o:p> </o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US">-Jukka Rahkonen-<o:p></o:p></span></p> <p class="MsoNormal"><span \
lang="EN-US"><o:p> </o:p></span></p> </div>
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