[mapserver-users] New Raster MapServer Tutorial

[Schepers, Benjamin]

Hi Seth,

nice work!

To your questions:

-          PNG24/32: lossless, so no quality-parameter, just compression efficiency (comparable to ZIP-Compression; size vs. needed time) [FORMATOPTION “ZLEVEL=n”]

-          PNG8: also lossless compression, but with color-depth-reduction (quantization), if you have data with more than 256 colors – this can/will  be lossy; also holds transparency-information (look-up-table for defined colors can have rgba-values)

-          JPEG: IMHO the optimum for base-layers because of it’s small size and good performance (at least, if the base layer is not optimized with reduced color – png8); optimum for aerial images and “noisy” data; quality can be controlled as show downwards…

-          GeoTIFF: good solution, if you’d like to use the wms as “download-service” – images then hold complete GeoTIFF-Header (was buggy with older mapserver/GDAL-versions, but seems OK for now)

Transparency can be controlled in different manners (in mapfile or in original data), at least these, please correct me:

Mapfile:

-          LAYER-MASK (polygon-layer, outer area is transparent)

-          LAYER-OFFSITE (mapfile-NoData-value)


Data-side:

-          NoData-value (e.g. GTIFF-Header)

-          Alpha/Transparency (e.g. GTIFF – per Pixel)

-          Mask-layer inside data (separate GDAL-special pixel-mask-layer inside GTIFF for full transparency; useful in combination with GTIFF inside JPEG-compression, because JPEG can’t hold alpha-channel and NoData doesn’t work calculable caused by lossy compression)

If compiling GDAL and mapserver by your own, then using libjpeg-turbo instead of libjpeg brings some extra pieces of performance, specially when dealing with large amounts of image-data.

You can define/modify default-formats and add special GDAL compression-options with ‘ FORMATOPTION “KEY=VALUE” ’, do not forget to add format to formatlist:

http://pastebin.com/1YztvEcw


Ben

Luftbild und Geoinformationssysteme
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Fon: +49 201 2069-232
Fax: +49 201 2069-500
schepers at rvr-online.de<mailto:schepers at rvr-online.de>

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