PNG driver

## NAME

PNG driver - driver to create PNG and PPM images.
(drivers)

## DESCRIPTION

The PNG driver generates PNG and PPM images from GRASS display commands. Per default PNG files are written with this driver. It is started as 'pseudo' monitor (image file instead) and when stopped, all output from previously used display commands are written to the PNG image file.

## USAGE

### Environment variables

Several environment variables affect the operation of the PNG driver.

- GRASS_WIDTH=xxx
  the width of the image map (default is 640).
- GRASS_HEIGHT=yyy
  the height of the image map (default is 480).
- GRASS_PNGFILE=filename
  the filename to put the resulting image in, default is `map.png`. If you set GRASS_PNGFILE 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_PNGFILE to a filename which ends in ".bmp", a 32-bpp BMP file will be created (these are not readable by some older viewers).
- GRASS_BACKGROUNDCOLOR=RRGGBB
  specifies the background color to use in RGB notation (hex values). Default is 000000 (black).
- GRASS_TRANSPARENT=[TRUE|FALSE]
  sets transparent background on (TRUE) or off (FALSE, default).
- GRASS_TRUECOLOR=[TRUE|FALSE]
  sets true-color support
- GRASS_PNG_COMPRESSION=[0|1|9]
  compression level of PNG files (0 = none, 1 = fastest, 9 = best, default is 6)
- GRASS_PNG_AUTO_WRITE=[TRUE|FALSE]
  if set to `TRUE`, the image file will be written after each operation (i.e. whenever a client disconnects), rather than only being written out when the driver terminates.
- GRASS_PNG_READ
  if `TRUE`, the PNG driver will initialize the image from the contents of GRASS_PNGFILE.
- GRASS_PNG_MAPPED
  if `TRUE`, the PNG driver will map GRASS_PNGFILE as its framebuffer, rather than using memory. This only works with BMP files.
- GRASS_RENDER_IMMEDIATE=[TRUE|FALSE]
  tells the raster library to use its built-in PNG driver rather than connecting to an external monitor process using sockets. If `TRUE`, there is no need to run `d.mon start=PNG`.

### Example

1. Define driver settings (here: bash shell syntax)

       export GRASS_RENDER_IMMEDIATE=TRUE
       export GRASS_TRUECOLOR=TRUE

2. Start up the driver

       d.mon start=PNG

3. Display raster map and vector polygons

       d.rast somerastermap
       d.vect somevectormap color=red

4. Stop the driver subsequently. This will write a file named `map.png` to be created in your current directory:

       d.mon stop=PNG

## 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 PNG raster map is defined by the map extents. Use g.region -p to get the number of rows and cols and use the environment variables to set the PNG 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

Display drivers
HTMLMAP driver, PostScript driver, XDRIVER
d.frame, d.mon, d.rast, d.vect

## AUTHOR

Original version: Per Henrik Johansen <phj (at) norgit.no> NORGIT AS
Rewritten by: Glynn Clements, 2003

Last changed: $Date: 2012-03-02 21:22:34 -0800 (Fri, 02 Mar 2012) $