I have done a bit more experimentation and have found a similar
rendering issue in a geoserver instance upgrading from v2.10.2 to
v2.11.5. This time an EPSG:4326 layer is affected while the polar
stereographic layers are not. An examination of the differences appears
to suggest once again that this is probably a difference in how
interpolation is performed between these versions.
I suppose this brings up the question of if the regression tests I am
performing should ever be expected to be consistent? Given that
requesting 'full resolution' images provides the same result (no
interpolation) between the versions I have tested, I could go that route
for our tests. This would be at the trade-off of storage space, but our
main concern is preventing a scenario in which a Geoserver upgrade
results in a layer not being rendered at all (we had some major issues
with our polar stereographic layers before, see
https://osgeo-org.atlassian.net/browse/GEOS-8542).
Does anyone have any thoughts on this? I'd appreciate any feedback!
Thanks again,
Trey
On 12/19/18 1:38 PM, Trey Stafford wrote:
Hello!
We are upgrading our Geoserver instance from version 2.11.5 to 2.14.1.
As part of that task, we have been running regression tests to ensure
that our data are still served and rendered as expected.
For the most part, our tests have shown no change, except for image
mosaic layers stored in northern hemisphere polar stereographic
projections (EPSGs 3411, 3413, 6931).
These layers show some differences in how WMS requests get rendered,
leading me to wonder if there was a Geoserver code change that may
have led to this observed behavior.
To be more specific:
We store thumbnail (low resolution, 300x300) images of all of our
layers. When we make configuration changes, we make wms requests that
we compare against those thumbnails. Any found differences get flagged
for review (in this case, all of our polar stereographic north
layers). When I discovered that there was a problem upgrading from
Geoserver v2.11.5 to v2.14.1, I did some additional testing.
* 'native' resolution wms requests are consistent. In other words,
if I request an image with a bounding box and size (in pixels) of
the matching source data file on disk, the resulting image is the
same in both v2.11.5 and v2.14.1
* Other WMS requests (e.g., 300x300 thumbnails, or double the native
resolution), result in differences in a grid-like pattern. See the
attachments vel_thumb_diff.png and conc_double_diff.png, which
show the grid-like pattern of differences for two of our layers.
These images were created using imagemagick's `compare` utility
(each red pixel represents a difference between wms responses in
v2.11.5 and v2.14.1).
All of our layers are image mosaics with the TIME dimension enabled. I
tried adding a single GeoTiff from one of the affected layers as a
GeoTiff store, and it did not experience this behavior. Appears as
though this observed behavior only affects image mosaics.
To reproduce this issue, see the attached 'steps_to_reproduce.txt'
Given that differences seem to only appear when an image is requested
that does not match the 'native' resolution of the source file on
disk, I suspect this might have something to do with differences in
how image interpolation occurs between these versions.
GEOS-5482 (https://osgeo-org.atlassian.net/browse/GEOS-8542), which
resolved an issue with polar stereographic projections may be related.
I have not seen anything else obvious in the changelogs for releases
since v2.11.5 which would suggest WMS rendering changes for polar
stereographic projections, but I may have missed something. Does
anyone know if what I am seeing is expected, or might this be a bug?
Thanks!
Trey Stafford
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