new faq entry: outputting values of 2d grids

[Paul Johnson]

Matt Aylward gave me a few tips on outputting values from grid2d objects
(value or object grids).  I've cut them down to what I see as the bar
minimum, here is what I propose to add:

How can I: Save data values from grid2d objects?

You can take a pixmap of a ZoomRaster display, but sometimes you want to
save the underlying data.  You can output the data in integer format if
you use Swarm's Int2dFiler protocol.  Here is a concrete example. 
Suppose you have a class that is subclassed from Discrete2d.  Suppose
you have objects in that grid, and each object is able to answer a
message "getOutputValue".  This must return an int!  Then in your 
subclass of the Discrete2d, you can add a method like this: 

-writeValuesToFile
{
   id gridfiler;
   char * filename;
   asprintf(&filename, "%05dWrappedGrid.txt", getCurrentTime());
   gridfiler=[Int2dFiler createBegin: [self getZone]];
   [gridfiler setDiscrete2dToFile: self];
   [gridfiler setValueMessage: M(getOutputValue)];
   gridfiler=[gridfiler createEnd];
    
   [gridfiler fileTo: filename];
   return self;
}

In your schedule, then you can add a message that causes the object to
write its values to a file.

Then, in Matt's words:

"Once you have the data there are many ways of analysiing it. One of the
easiest is to
turn it into a pgm portablr grey map:
add an ascii header to it as follows:

P2
x_size y_size
max_value
.........int ascii data here.....

e.g., in my case:

P2
200 200
10

Then give it a .pgm extension and open it in xv, gimp ..etc.

I also import the raw data into apps like GRI for contouring and
postscript output for
LaTeX."

Cool!  
Gimp gives a (somewhat gross, IMHO) contour plot.  I plan to use this in
a 3d graphics program to make 3d bar charts as in the Nowak articles on
diffusion of opinion.

If I get an more stuff from Matt on this, I'll make it publicly
available.


-- 
Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ukans.edu/~pauljohn
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66045                FAX: (785) 864-5700

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