[mapserver-dev] Better resolution handling for printing

[Brent Fraser]

  Daniel says "it is not sufficient to update the map->resolution setting since all the label and symbol sizes as well as line widths also need updating if you want to maintain the same map appearance."

  Perhaps we just need to allow a SIZEUNIT of POINTS (in addition to the current PIXELS, FT, etc), as in "Font Points" (e.g 72 per inch).  That way you could set the map (output image) resolution to whatever you want and the text, symbols, and line widths will always be the same readable size on the printed output.


  And actually I want a resolution setting per [raster] layer.  My intent is to use "vector" (+raster) style PDF (or SVG) output.  If I've got a 125 dpi scanned topo map layer and a 300 dpi satellite image layer I'd want the two layers as two images in my PDF, each with their different resolutions.

Brent Fraser



Tamas Szekeres wrote:
> Alan,
> 
> I think this issue is more about a theoretical than a technical problem. 
> It depends on how do we define the concept of the resolution related to 
> the size and sizeunit settings.
> For example when we set sizeunit=MS_PIXELS and setting the symbolsize to 
> 8, I consider the every would expect to draw the symbol at exactly 8 
> pixels in width regardless of the actual resolution setting.
> 
> On the other hand we could declare alternative concepts as well. For 
> example we declare that this is only the case when the resolution is set 
> exactly to 72.0 dpi and at any other setting the actual drawing width 
> will increase/decrease proportionately. If this concept is acceptable 
> then you could safely multiply the scalefactor with resolution/72.0 when 
> drawing the symbols/labels in the code.
> 
> However you might want to allow the user to set this magnification 
> factor manually either by using this new parameter or the using 
> resolution/default_resolution concept mentioned by Daniel before.
> 
> Best regards,
> 
> Tamas
> 
> 
> 
> 2009/2/18 Alan Boudreault <aboudreault at mapgears.com 
> <mailto:aboudreault at mapgears.com>>
> 
>     Tamas,
> 
>     I would like to add the resolution handling for printing
>     transparently without
>     the necessity to use another keyword. And if we decide to add the
>     "magnification" keyword ulteriorly, we will probably be able to use
>     the same
>     code/functions.
> 
>     Any other thoughts about how could we achieve the better resolution
>     handling?
>     Else, i'll begin to check in code for the best way to
>     increase/decrease all
>     values dependently of the resolution value.
> 
>     Alan
> 
>     On February 11, 2009 05:19:10 pm Tamas Szekeres wrote:
>     >  Daniel,
>     >
>     >  I would introduce a new parameter of the map called
>     'magnification' which
>     >  would be multiplied with the resolution when calculation the scale. In
>     >  addition the scalefactor should also multiplied with this in
>     msPrepareImage
>     >  to achieve the desired effect.
>     >  I guess it wouldn't be annoying for the user that by setting
>      magnification
>     >  = 2.0 would increase the width of the map elements 2 times.
>     >  magnification = 1.0 would be the default value.
>     >
>     >
>     >  Best regards,
>     >
>     >  Tamas
>     >
>     >
>     >
>     >
>     >
>     >  2009/2/11 Daniel Morissette <dmorissette at mapgears.com
>     <mailto:dmorissette at mapgears.com>>
>     >
>     >  > Dev's,
>     >  >
>     >  > When using MapServer to generate a printable map at higher
>     resolution
>     >  > than the default screen resolution, say 300dpi instead of 72dpi,
>     it is
>     >  > not sufficient to update the map->resolution setting since all
>     the label
>     >  > and symbol sizes as well as line widths also need updating if
>     you want to
>     >  > maintain the same map appearance.
>     >  >
>     >  > This issue has been discussed a few times before and I even
>     think some
>     >  > have worked on this at their end. We have a client who needs a
>     fix for
>     >  > this so I'd like to open a discussion on this before we go ahead and
>     >  > prepare a RFC.
>     >  >
>     >  > The approach I was thinking of using is to add a new
>     map->defresolution
>     >  > (better names welcome) that would be the default resolution of a
>     given
>     >  > mapfile, that is the reference resolution for which all sizes in
>     labels,
>     >  > classes and styles are set.
>     >  >
>     >  > In normal web browsing operation, map->defresolution should be
>     set to the
>     >  > same value as map->resolution (e.g. 72dpi). When you want to print a
>     >  > 300dpi map, then you'd set map->resolution=300, and MapServer would
>     >  > automagically apply a factor of
>     map->resolution/map->defresolution to all
>     >  > size values in the mapfile before rendering. For instance, if your
>     >  > mapfile is set for a defresolution of 72dpi and you request a output
>     >  > resolution of 300dpi, all size values would be multiplied by
>     300/72=4.16
>     >  > and labels, lines and symbols on the resulting map would have the
>     >  > same/expected relative sizes.
>     >  >
>     >  > I think I've read suggestions of setting resolution as an
>     outputformat
>     >  > option before, but this may not work well with remote (e.g. WMS)
>     >  > services, we need something better integrated and with the proposed
>     >  > approach just passing a new vendor-specific resolution param in
>     the WMS
>     >  > GetMap URL could do the trick.
>     >  >
>     >  > Does my explanation make sense? Has anyone worked on this problem
>     >  > already? Any thoughts on this before we start working on a RFC?
>     >  >
>     >  > Daniel
>     >  > --
>     >  > Daniel Morissette
>     >  > http://www.mapgears.com/
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