htmldriver(1grass) GRASS GIS User's Manual htmldriver(1grass)
HTML DISPLAY DRIVER
HTML display driver to create HTML image maps.
DESCRIPTION
The HTML driver allows the generation of HTML image maps for area vec-
tor data. HTML image maps are used in conjunction with images to pro-
vide 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 im-
age 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.
USAGE
Environment variables
The HTML driver can be enabled by setting GRASS_RENDER_IMMEDIATE vari-
able, 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, de-
fault 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 warn-
ing.
• GRASS_RENDER_HTMLMINDIST=n
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 num-
ber of pixels greater than this value. This parameter helps to
eliminate closely spaced points.
• GRASS_RENDER_HTMLMINBBOX=n
specifies the minimum bounding box dimensions to record a poly-
gon as a clickable area. The default is 2, which means that a
polygon with a bounding box of less than this value is not in-
cluded. This parameter helps to eliminate polygons than are a
point or line.
• GRASS_RENDER_HTMLMAXPOINTS=n
specifies the maximum number of vertices included in a poly-
gon’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.
Example
Start up the driver
g.region vector=zipcodes_wake
d.mon start=html
Display text strings (HREF’s) and polygons
echo "https://en.wikipedia.org/wiki/Raleigh,_North_Carolina" | d.text
d.vect map=zipcodes_wake where="ZIPNAME = ’RALEIGH’"
echo "https://en.wikipedia.org/wiki/Cary,_North_Carolina" | d.text
d.vect map=zipcodes_wake where="ZIPNAME = ’CARY’" fill_color=180:200:210
Stop the driver once all polygon have been displayed. This will create
a file named ’htmlmap’ 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.
# using previous GRASS_RENDER_WIDTH & GRASS_RENDER_HEIGHT
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 &
NOTES
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
• http://www.ecse.rpi.edu/Homepages/wrf/
• http://www.ecse.rpi.edu/Homepages/wrf/research/geom/pnpoly.html
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
SEE ALSO
Cairo driver, PNG driver, HTML driver, variables
d.rast, d.vect, d.mon, d.erase, d.redraw
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© 2003-2022 GRASS Development Team, GRASS GIS 7.8.7 Reference Manual
GRASS 7.8.7 htmldriver(1grass)
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