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Kursprogramm Frühjahr 2013

Sourcepole bietet Grundlagen- und Aufbau-Kurse für den Betrieb von Geodaten-Infrastrukturen auf der Basis von PostgreSQL/PostGIS und Quantum GIS an. Detaillierte Informationen zu den Kursen, die im Frühjahr 2013 stattfinden, entnehmen Sie bitte http://www.sourcepole.ch/kurse Die Anmeldung ist ab sofort online möglich. Wir freuen uns darauf Sie in Zürich begüssen zu können.

The state of QGIS Globe

The Region of Umbria, Italy, sponsored 4 days of work to update QGIS Globe for current QGIS versions. Most of the functionality is working again and the globe is now compatible with osgEarth 1.0 up to 1.3. The bad news is, that the globe plugin is not working on Windows with OSGeo4W. It seems that one of the OSGeo4W libraries (GDAL?) is compiled with an incompatible MS compiler version. Christmas holidays are coming…

At least it gives Linux users the possibility to play with the globe using the current development version and do exciting stuff like Oslandia does:

PostGIS 3D demo PostGIS 3D demo

Imagine someone would sponsor four weeks of QGIS Globe work!

New configuration options in QGIS server

In QGIS server, it is now possible to selectively exclude layers from WMS publication. These layers will be available only on the desktop and hidden from WMS clients. Similarly, print layouts can be excluded from WMS publication. Of course, these settings are conveniently accessible from the project properties dialog of QGIS (but you need to have a nightly build or a recent compile):

screenshot

Additionally, attributes per layer can be excluded from WMS or WFS publication in the vector properties dialog:

screenshot

There is also a new request type called ‘GetProjectSettings’. The output of this request is similar to the GetCapabilities output, but with more details and more specific to QGIS:

  • Initial visibility of layers
  • Information about vector attributes and their edit types
  • Information about layer order and drawing order
  • List of layers published in WFS

These new features have been developed in collaboration and with funding from the city of Uster. More details are available on the QGIS server / webclient wiki page

Creating png8 images with QGIS server

For providing maps via WMS over the internet, it is important to generate image files with a small size. Because normally, most of the perceived WMS delay comes from transfering large images files over the internet (and not from map rendering itself). Therefore, QGIS server supports the conversion of png24 and png32 images into png8, therefore generating a file with only 1/3 resp. 1/4 of the original size (but with lower quality). However, until recently, QGIS server was using the default algorithm from the Qt library for conversion to png8, resulting in very ugly png8 images if used with a transparent background and sometimes with shifted colors. Therefore, I decided to implement a better conversion algorithm: the median cut algorithm, which was first described by Heckbert in 1982 ( Paul S. Heckbert, Color Image Quantization for Frame Buffer Display. ACM SIGGRAPH ‘82 Proceedings). This algorithm creates color boxes in 4-dimensional colour space and subdivides at the mean of the largest dimension. It stops if the number of boxes equals the requested number of colors (256 in the case of png8 conversion). First tests show the quality of the converted images are quite good for vector maps. So a performant configuration for a webmap means probably to fetch background rasters as jpg and to overlay vectors as png8 with transparent background.

Let’s see with an example what the conversion from png24 to png8 means in terms of image quality. The first file is a png24 image, 590 KB size: screenshot

And this one is the conversion to png8, only 190 KB size. screenshot

To test the png8 conversion, you need to have the latest QGIS version from git (or a nightly build tomorrow). Then simply replace the FORMAT paramter in the url with ‘&FORMAT=image/png; mode=8bit’.