.TH htmldriver 1grass "" "GRASS 7.8.5" "GRASS GIS User's Manual"
.SH HTML DISPLAY DRIVER
\fIHTML display driver\fR to create HTML image maps.
.SH DESCRIPTION
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.
.PP
Polygons can at most have 100 vertices (this limit imposed by HTML
image map formats, see \fBGRASS_RENDER_HTMLMAXPOINTS\fR 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.
.PP
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.
.PP
The only GRASS display commands that should be used with this driver
are:
.RS 4n
.IP \(bu 4n
\fId.text\fR \- pass href
information for resulting image maps.
.IP \(bu 4n
\fId.vect\fR \- draw polygons from a
vector map.
.RE
.SH USAGE
.SS Environment variables
The HTML driver can be enabled by
setting \fBGRASS_RENDER_IMMEDIATE\fR variable, eg.
.br
.nf
\fC
export GRASS_RENDER_IMMEDIATE=html
\fR
.fi
Several environment variables affect the operation of the HTML driver:
.RS 4n
.IP \(bu 4n
\fBGRASS_RENDER_WIDTH=xxx\fR
.br
the width of the image map (default is 640).
.IP \(bu 4n
\fBGRASS_RENDER_HEIGHT=yyy\fR
.br
the height of the image map (default is 480).
.IP \(bu 4n
\fBGRASS_RENDER_HTMLTYPE=type\fR
.br
type of image mape to create
(default is CLIENT):
.br
\fBCLIENT\fR Netscape/IE client\-side
image map (NAME=\(dqmap\(dq).
.br
\fBAPACHE\fR Apache/NCSA server\-side image
map.
.br
\fBRAW\fR
Raw url and polygon
vertices (\fIurl x1 y1 x2 y2
\&.....\fR), suitable for conversion to CERN server format, or
any other format with user supplied conversion program.
.IP \(bu 4n
\fBGRASS_RENDER_FILE=filename\fR
.br
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.
.br
\fIAny existing file of the same name is overwritten without
warning.\fR
.IP \(bu 4n
\fBGRASS_RENDER_HTMLMINDIST=n\fR
.br
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\(cqs 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.
.IP \(bu 4n
\fBGRASS_RENDER_HTMLMINBBOX=n\fR
.br
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.
.IP \(bu 4n
\fBGRASS_RENDER_HTMLMAXPOINTS=n\fR
.br
specifies the maximum number of vertices included in a polygon\(cqs
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.
.RE
.SS Example
Start up the driver
.br
.nf
\fC
g.region vector=zipcodes_wake
d.mon start=html
\fR
.fi
Display text strings (HREF\(cqs) and polygons
.br
.nf
\fC
echo \(dqhttps://en.wikipedia.org/wiki/Raleigh,_North_Carolina\(dq | d.text
d.vect map=zipcodes_wake where=\(dqZIPNAME = \(cqRALEIGH\(cq\(dq
echo \(dqhttps://en.wikipedia.org/wiki/Cary,_North_Carolina\(dq | d.text
d.vect map=zipcodes_wake where=\(dqZIPNAME = \(cqCARY\(cq\(dq fill_color=180:200:210
\fR
.fi
Stop the driver once all polygon have been displayed. This will create
a file named \(cqhtmlmap\(cq in your current directory:
.br
.nf
\fC
d.mon stop=html
\fR
.fi
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.
\fIThe following example is somewhat out of date and refers to options
available in GRASS 5.\fR
.br
.nf
\fC
# 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.gif
# assemble some html with the image and the image map
echo \(cq
\(cq >alien.html
cat htmlmap >>alien.html
echo \(cq\(cq >>alien.html
# don\(cqt forget to reset your region
g.region region=saved.reg
# take a look and test it out
netscape file:\(gapwd\(ga/alien.html &
\fR
.fi
.SH NOTES
HTML was adapted from the CELL driver in GRASS 4.3.
Point\-in\-polygon test code was lifted from Randolph Franklin\(cqs web
page, see
.RS 4n
.IP \(bu 4n
http://www.ecse.rpi.edu/Homepages/wrf/
.IP \(bu 4n
http://www.ecse.rpi.edu/Homepages/wrf/research/geom/pnpoly.html
.RE
.PP
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:
.br
.nf
\fC
sed \-e \(cqs/NAME=\(dqmap\(dq/NAME=\(dqfoomap\(dq/\(cq < htmlmap > foomap.html
\fR
.fi
.SH SEE ALSO
\fI
Cairo driver,
PNG driver,
HTML driver,
variables
.br
.br
d.rast,
d.vect,
d.mon,
d.erase,
d.redraw
\fR
.PP
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.PP
© 2003\-2020
GRASS Development Team,
GRASS GIS 7.8.5 Reference Manual