Hi everyone.
We've tried to add some more raster-handling features to OpenJUMP, in
detail:
- Overview handling (read only) for TIFF files. External (.ovr file) and
internal overviews are now handled to speed up reading/displaying time.
The .ovr file can be created using GDAL or ArcMap. In addition, when
neither external nor internal overviews are found, decimation
(subsampling) is used. When only a portion of the image is needed for
display, only that part is loaded. Note that when only a portion of an
image is loaded, there are two sets of image metadata (columns, rows,
envelope...), one for the image actually loaded, the other for the whole
image. You therefore will find two sets of getters in RasterImageLayer,
one for the actual image, one for the whole image.
- Raster statistics for TIFF files: they are calculated just once, and
then written in an .aux.xml file. For display purposes the min and max
cell values need to be known, and clearly it's better to look for them
just once. For big rasters (> 500 MB) calculating statistics takes a few
seconds, but I still haven't added a Pane with a message: "Please wait
while calculating statistics", so be patient).
Now memory consumption when loading large TIFF files is always
reasonable, and when the .aux.xml and .ovr files are present, reading
even a big (1 GB) raster is almost immediate.
Again, we had to modify several classes, and we didn't feel like pushing
them in SVN without your previous consent. Therefore, just to test you
can download this OJ temporary build
<https://www.dropbox.com/s/lqmlkfprnoopfi1/OpenJUMP-1.7.1-r4004-CORE.7z?dl=0>
(built against OJ 1.7) and you can download the modified sources here
<https://www.dropbox.com/s/sxqo96rh8jtan0k/Modified.7z?dl=0>.
Let us know
Alberto
On 26/11/2014 9:52 AM, Rahkonen Jukka (Tike) wrote:
Hi,
http://docs.geoserver.org/stable/en/user/production/data.html
I believe that most Geoserver users are creating overviews with gdaladdo but the document
mentions that there is also "the OverviewsEmbedded command included in
Geotools".
I use external overviews myself with Mapserver but there are pros and cons
+ External overviews can be created for all formats
+ When image goes to archive external overviews can be deleted for saving
space. Removing internal overviews from tiff file does make the file any
smaller without complete rewrite.
+ It is easier to experiment with different overview resampling methods because
not satisfying result can just be deleted.
- Tiff + external overviews take a bit more disk space than tiff with internal
overviews.
- Some programs support internal but not external overviews. Image mosaic in
Geoserver is one such example.
In server use overviews are a must. About 30 % more disk space is needed but
that's nothing compared to improved speed. Overviews can also be compressed
(LZW or deflate for topographic maps, --config COMPRESS_OVERVIEW JPEG --config
PHOTOMETRIC_OVERVIEW YCBCR for aerial images) which is reducing the size very
much.
For OpenJUMP usage my wish list is
- Ability to utilize existing overviews, ideally both external and internal,
and also compressed
- Create overviews with java from OJ. Some programs ask user "Image does not
have overviews. Do you want to create them?
As you can see from the gdaladdo page there are lots of options for creating
overviews. For OJ perhaps the most simple recipe would do: external,
uncompressed overviews with so many levels that at the lowest resolution the
image fits into about 200x200 pixels.
It is also possible improve the speed in small scales at least with some type
of tiffs even without overviews. If the tiff file is written as stripes it is
possible to read for example only every 10th or 50th stripe, depending on the
required output resolution. Probably that is not possible or at least not so
efficient with tiled tiffs.
-Jukka Rahkonen-
alberto deluca wrote:
Hi everyone.
overviews sound interesting to me. I don't know either if there exists a
standard,
but the GDAL way to go sure is the more widely accepted/supported. I've had a
look at the GDALADDO tool: overviews can be internal to the GeoTIFF or
external (.ovr file). The latter is the option used by esri's ArcMap.
Personally I
don't like either the idea of modifying the original tif file.
There might be some GeoServer or GeoTools code that creates overviews, I'll try
to find out something more. I hope reading/writing the overviews won't imply
that we will need to dump the JAI image drivers for something totally new...
Alberto
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