[QGIS-trac] [Quantum GIS] #3783: "Screen scaling" setting to control the screen size of things defined in points or mm

[Quantum GIS]

#3783: "Screen scaling" setting to control the screen size of things defined in
points or mm
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   Reporter:  Alister                           |              Owner:  nobody       
       Type:  enhancement                       |             Status:  new          
   Priority:  major: does not work as expected  |          Milestone:  Version 2.0.0
  Component:  MapCanvas                         |            Version:  Trunk        
   Keywords:                                    |   Platform_version:  XP           
   Platform:  Windows                           |           Must_fix:  No           
Status_info:  0                                 |  
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 With the new labelling engine (I have not tested with the old):
 If the label font size is set in points (instead of map units), then on a
 typical monitor the labels are displayed in the map canvas at
 approximately the same size as they are printed on paper.  If I remember
 correctly (I'll check tomorrow), the same is true for vector symbols if
 their size is set in mm instead of map units.

 Changing the DPI setting in the operation system does not affect the size
 (at least in Windows - perhaps someone could test in another operating
 system).  Presumably this means that QGIS (or maybe an underlying library
 such as QT) is using some "typical" value for screen DPI.  This is likely
 to be 72dpi or 91dpi or something.

 You might think it would be good for the size to change depending on the
 DPI setting in the operating system, so that if this was set correctly
 then the labels and symbols would be exactly the same size on paper as on
 the screen.  BUT in reality this would be bad, for the same reason that
 the current behaviour is bad: ''screen dpi is normally much lower than any
 printer output, so small labels and symbols which are easily legible on
 paper are not legible on the screen''.

 I think ideally QGIS should have its own setting to control the screen
 rendering size of labels and symbols (if they are not defined in map
 units).  I'm not sure what the best name for the setting would be - maybe
 something like "Screen scaling factor".  The units should be in DPI, so
 that if someone ''does'' want things to be exactly the same height on
 paper as on the screen, they can simply set it to the actual DPI of their
 monitor.

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Ticket URL: <http://trac.osgeo.org/qgis/ticket/3783>
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