Krunoslav Hrnjak created GEOS-4969:
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Summary: dpi header in the output
Key: GEOS-4969
URL: https://jira.codehaus.org/browse/GEOS-4969
Project: GeoServer
Issue Type: Improvement
Components: WMS
Affects Versions: 2.1.3
Environment: General information about GeoServer
Build Information
Version 2.1.3
Subversion Revision 16668
Build Date 21-Dec-2011 11:55
GeoTools Version 2.7.4 (rev 38443)
Reporter: Krunoslav Hrnjak
Assignee: Andrea Aime
Priority: Minor
Please write value for dpi settings in output file metadata (header or
auxiliary file).
All WMS output files (.png, .jpeg, .gif, ...) are produced with DPI set to 96.
When displaying picture on the screen this doesn't have any influence (only
screen resolution will determine the size of picture). But when output is saved
to file system and printed out dpi settings will determine picture size on
paper. In most applications default dpi settings can be overridden by setting
some other custom dpi value but it can be hard to guess what is the right dpi
value that will provide right scale ratio.
This improvement proposal is very demanding but also very important.
To implement this change it is necessary to include dpi value as creation
option and this depends upond particular drivers used for writing particular
file type. For eg. GDAL can deal with the following baseline TIFF tags as
dataset-level metadata : TIFFTAG_XRESOLUTION, TIFFTAG_YRESOLUTION. In similar
manner support for writing dpi settings should be done on other formats or by
writing dpi settings in file structure according to the format specification.
Other solutions using various tools can be found on Internet like "setting dpi
for PNG files, you could use the imagemagick convert tool: convert -density 300
-units pixelsperinch infile.jpg outfile.png". I believe in this case it should
be done by GeoTools.
Writing dpi value is important to enable easy sharing and map management just
by saving map to files system and encapsulate all necessary information inside
the file header. If printing is done with wrong dpi settings map scale ratio
will not provide accurate information to end user.
Recommended readings: http://www.scantips.com/no72dpi.html
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