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       HTML display driver to create HTML image maps.


       The  HTML driver allows the generation of HTML image maps for area vector data. HTML image
       maps are used in conjunction with images to  provide  unique  URL  targets  for  different
       portions of an image. The HTML driver can create both client-side image maps embedded into
       HTML files, or server-side image maps used by web server software.

       Polygons can at most have 100 vertices (this limit imposed by HTML image map formats,  see
       GRASS_RENDER_HTMLMAXPOINTS below.) The driver will attempt to trim polygons that have more
       that 100 vertices by removing vertices with the least amount of angle to the next vertice.
       Also, any polygon that is entirely bounded by another polygon will be discarded.

       Text  written  to  the  driver before polygons are used as the HREF tag for all subsequent
       polygons written. All polygons that exist in a vector map will have the same HREF tag.

       The only GRASS display commands that should be used with this driver are:

           ·   d.text - pass href information for resulting image maps.

           ·   d.vect - draw polygons from a vector map.


   Environment variables
       The HTML driver can be enabled by setting GRASS_RENDER_IMMEDIATE variable, eg.
       export GRASS_RENDER_IMMEDIATE=html
       Several environment variables affect the operation of the HTML driver:

           ·   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_HTMLTYPE=type
               type of image mape to create (default is CLIENT):
               CLIENT    Netscape/IE client-side image map (NAME="map").
               APACHE    Apache/NCSA server-side image map.
               RAW Raw url and polygon vertices  (url   x1   y1   x2   y2  .....),  suitable  for
               conversion  to  CERN  server  format,  or  any  other  format  with  user supplied
               conversion program.

           ·   GRASS_RENDER_FILE=filename
               specifies the resulting file to store the html  image  map,  default  is  htmlmap.
               Files  without  absolute path names are written in the current directory where the
               driver was started.
               Any existing file of the same name is overwritten without warning.

               specifies the minimum distance in  pixels  that  a  point  must  change  from  the
               previous  point  to keep in the list of vertices for a polygon.  The default is 2,
               which means that a point’s x and y difference from the previous point must  change
               by  a number of pixels greater than this value.  This parameter helps to eliminate
               closely spaced points.

               specifies the minimum bounding box dimensions to record a polygon as  a  clickable
               area.   The  default  is 2, which means that a polygon with a bounding box of less
               than this value is not included. This parameter helps to eliminate  polygons  than
               are a point or line.

               specifies  the  maximum number of vertices included in a polygon’s clickable area.
               The default is 99. Some browsers can only accommodate polygons of 100 vertices  or
               less.  The HTMLMAP driver automatically ensures that a polygon is closed by making
               the last point the same as the first point.

       Start up the driver
       d.mon start=html
       Display text strings (HREF’s) and polygons
       echo "" | d.text
       d.vect map=area51
       echo "" | d.text
       d.vect map=roswell
       Stop the driver when all polygon have  been  displayed.  This  will  cause  a  file  named
       ’htmlmap’ to be created in your current directory.
       d.mon stop=html
       You  will  also  want  to create an image for your image map. Use the PNG driver and other
       utilities to create .gif or .jpg files.  The following example is somewhat out of date and
       refers to options available in GRASS 5.
       d.mon start=png
       d.rast map=terrain
       d.vect map=area51  fillcolor=white  linecolor=blue
       d.vect map=roswell fillcolor=yellow linecolor=blue
       d.vect map=states  color=green
       d.vect map=roads   color=black
       d.mon stop=png
       # make the region the same as the newly created cell for ppm export
       g.region save=saved.reg
       g.region raster=D_cell
       r.out.ppm -q input=D_cell output=alien.ppm
       # use the netpbm utilities to create a gif (quantize if needed)
       ppmquant 128 <alien.ppm |  ppmtogif >alien.gif
       # assemble some html with the image and the image map
       echo ’<html><body><img src="alien.gif" usemap="#map">’ >alien.html
       cat htmlmap                                           >>alien.html
       echo ’</body></html>’                                 >>alien.html
       # don’t forget to reset your region
       g.region region=saved.reg
       # take a look and test it out
       netscape file:`pwd`/alien.html &


       HTML was adapted from the CELL driver in GRASS 4.3.  Point-in-polygon test code was lifted
       from Randolph Franklin’s web page, see



       If you create an HTML file with two or more images and image maps, you will need  to  edit
       the  map  names. The HTML driver creates its map with the name map. A small sed script can
       easily change the map name:
       sed -e ’s/NAME="map"/NAME="foomap"/’ < htmlmap > foomap.html


        Cairo driver, PNG driver, HTML driver, variables
       d.rast, d.vect, d.mon, d.erase, d.redraw

       Last changed: $Date: 2014-12-09 18:23:56 +0100 (Tue, 09 Dec 2014) $

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