[GRASS-dev] [GRASS GIS] #3033: Cairo and PS drivers display only one raster or vector for SVG and PS

[GRASS GIS]

#3033: Cairo and PS drivers display only one raster or vector for SVG and PS
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  Reporter:  wenzeslaus  |      Owner:  grass-dev@…
      Type:  defect      |     Status:  new
  Priority:  major       |  Milestone:  7.2.0
 Component:  Display     |    Version:  svn-trunk
Resolution:              |   Keywords:  d.mon, cairo, ps, SVG, vector
       CPU:              |  graphics
  Unspecified            |   Platform:  Unspecified
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Comment (by glynn):

 Replying to [ticket:3033 wenzeslaus]:

 > However, using Cairo driver with SVG output, you get only the vector
 which is rendered last:

 This is a limitation of cairo's SVG (and PostScript) "surface" and the
 fact that d.* commands run as distinct processes. It isn't possible to
 initialise a surface from an existing SVG file so that subsequent drawing
 operations will be appended to the document structure, and you can't
 simply concatenate the files.

 In other words, GRASS_RENDER_FILE_READ=TRUE doesn't work for vector
 formats. You need to generate multiple output files then combine them with
 external tools.

 > Same applies for the PS driver:

 The PS driver does support combining multiple commands into a single
 stream, via the environment variables GRASS_RENDER_PS_HEADER and
 GRASS_RENDER_PS_TRAILER (although I have no idea whether these can be
 controlled via d.mon).

 With GRASS_RENDER_PS_HEADER=FALSE, the output file is opened for append
 rather than write (i.e. the existing contents are retained) and no header
 (prologue) is written.

 With GRASS_RENDER_PS_TRAILER=FALSE, no trailer is written.

 Both variables default to TRUE, meaning that each command will replace any
 previous file and a header and trailer will be included. To combine the
 output from multiple commands into a single file, all commands but the
 first should have GRASS_RENDER_PS_HEADER=FALSE, while all commands but the
 last should have GRASS_RENDER_PS_TRAILER=FALSE.

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3033#comment:1>
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