PNG display driver to create PNG, PPM, or BMP images.
DESCRIPTION
The PNG driver generates PNG, PPM, or BMP images from GRASS display
commands. Per default PNG files are written with this driver. This
driver is used by default if Cairo
driver is not available.
USAGE
Environment variables
The PNG driver can be enabled by setting GRASS_RENDER_IMMEDIATE
variable, eg.
export GRASS_RENDER_IMMEDIATE=png
  - GRASS_RENDER_WIDTH=xxx
 the width of the image map (default is 640).
- GRASS_RENDER_HEIGHT=yyy
 the height of the image map (default is 480).
- GRASS_RENDER_BACKGROUNDCOLOR=RRGGBB
 specifies the background color to use in RGB notation (hex or
    R:G:B values). Named colors are also supported. Default
    is FFFFFF (white).
- GRASS_RENDER_TRANSPARENT=[TRUE|FALSE]
 sets transparent background on (TRUE) or off (FALSE, default).
- GRASS_RENDER_TRUECOLOR=[TRUE|FALSE]
 sets true-color support. Default is TRUE.
- GRASS_RENDER_FILE=filename
 the filename to put the resulting image in, default ismap.png.
     If you set GRASS_RENDER_FILE to a filename which ends in ".ppm", a PPM
     file will be created (with alpha channel stored in a PGM image, if applicable).
     If you set GRASS_RENDER_FILE to a filename which ends in ".bmp", a 32-bpp
     BMP file will be created (these are not readable by some older viewers).
- GRASS_RENDER_FILE_COMPRESSION=[0|1|9]
 compression level of PNG files (0 = none, 1 = fastest, 9 = best, default is 6)
- GRASS_RENDER_FILE_READ
 ifTRUE, the PNG driver will initialize the image from
     the contents of GRASS_RENDER_FILE.
- GRASS_RENDER_FILE_MAPPED
 ifTRUE, the PNG driver
    will map GRASS_RENDER_FILE as its framebuffer, rather than using
    memory. This only works with BMP files.
Example
export GRASS_RENDER_IMMEDIATE=png
export GRASS_RENDER_TRUECOLOR=TRUE
g.region raster=elevation
d.rast elevation
d.vect roadsmajor color=red
map.png in your current directory.
NOTES
The PNG driver uses the libpng (see
the libpng home page) and
zlib (see the
zlib home page), all which needs to
be installed for the PNG driver to work (it's worth it).
The resolution of the output images is defined by current region
extents. Use g.region -p to get the number of rows and cols
and use the environment variables to set the image size. If you would
like a larger image, multiply both rows and cols by the same whole
number to preserve the aspect ratio.
Further PNG file processing (e.g. quantization to 1 bit for monochrome
images) can be done with pnmquant of
the netpbm tools.
SEE ALSO
  Cairo driver,
  PS driver,
  HTML driver,
  variables
  d.rast,
  d.vect,
  d.mon,
  d.erase,
  d.redraw
AUTHORS
Original version: Per Henrik Johansen <phj (at)
norgit.no> 
Rewritten by: Glynn Clements, 2003