[GRASS-dev] Re: [GRASS GIS] #967: r.out.png: respect TRANSPARENT and COMPRESSION enviro vars

[GRASS GIS]

#967: r.out.png: respect TRANSPARENT and COMPRESSION enviro vars
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  Reporter:  hamish       |       Owner:  grass-dev at lists.osgeo.org
      Type:  enhancement  |      Status:  new                      
  Priority:  normal       |   Milestone:  7.0.0                    
 Component:  Raster       |     Version:  svn-trunk                
Resolution:               |    Keywords:  r.out.png                
  Platform:  All          |         Cpu:  All                      
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Comment (by glynn):

 Replying to [comment:2 hamish]:
 > patch attached to add transparency, compression, and background color
 options to r.out.png. Patch is vs 6.5svn.
 >
 >  * I'm not sure how useful bgcolor= is, as I'm not sure how often "set"
 is likely to return no-color from G_lookup_raster_colors(), or really what
 triggers that to be set==0. Or should bgcolor= override the color table's
 "nv" color for NULL cells when there's no alpha channel? Is "set" like the
 default("*") lesser used rule in the color tables?

 r.out.png shouldn't write the background colour into the image data; it
 should just pass it to png_set_background(). If the image has an alpha
 channel or transparent colour index but the reader wants an opaque image,
 the PNG library overlays the image on the background colour.

 The "set" value is 0 if the default colour was used, 1 if a colour was
 found for the cell's value (including null; the null value colour is
 always defined).

 If you want to write an RGB image (no alpha), ignore the "set" value and
 the bgcolor= option, and just write exactly what G_lookup_raster_colors()
 returns.

 If you want to also write an alpha channel, there are two independent
 options: whether to make null cells transparent and whether to write cells
 with no defined colour (set == 0) as transparent. In all cases, the RGB
 channels should still contain exactly what G_lookup_raster_colors()
 returns.

 >  * the pointer for G_is_null_value() is specified in a rather raw way.
 Ideas for improvement are welcome.

 Arithmetic on a void pointer is undefined (gcc supports it as an
 extension). Use:
 {{{
         size_t rsize = G_raster_size(rtype);
         ...
         G_incr_void_ptr(voidc, col * rsize);
 }}}

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Ticket URL: <https://trac.osgeo.org/grass/ticket/967#comment:3>
GRASS GIS <http://grass.osgeo.org>