Gnuplot allows input using a script file containing commands. You can write your program in any language and automate the creation of the script files. You might want to create a string for each section of the file and at the end send each string to an open file handler to write the file. For our labs we are only drawing lines and arrows so far. There are a few important things to add to every script file.

   set title "My Title"
   set xrange [-400.0: 400.0]
   set yrange [-400.0: 400.0]
   unset key
   set size square

set title is self explanatory. set xrange and set yrange are used to set the range of the displayed plot. unset key removes the key or legend to the plot as it is not necessary for our labs. “set size square” ensures the plot is square as normally an 800×800 plot displayed on most monitors will appear as a rectangle. Other parameters to this command include “set size ratio 2” where the the plot is set to be twice as high as wide. You will probably want to include this information in every plot file you produce.

There are at least two ways to draw lines. The first way to draw lines is using the following command:

   plot '-' with lines
   -400.000000 -400.000000 0.000000 0.000000
   ...
   e

You can plot as many lines as you want following the 'plot' command. You must end the plot command with 'e'. This works great for plotting static lines but can be problematic when animating. Once 'e' is reached if the 'plot' command is read in again, the previous set of lines disappears and the next set of points is read in. If you only want the last set of lines to be displayed in your animation, this is a good way to do it.

The second way to draw lines is like the way we created obstacles in the first lab:

   set arrow from 400.000000, 400.000000 to -400.000000, 400.000000 nohead lt 3

This command draws an arrow from the first pair of x,y coordinates to the second. The 'nohead' parameter removes the arrowhead while the 'lt #' parameter sets the line color to 3. Another option 'lw #' sets the line width as specified. Here is a list of available colors:

   -1 black
   1 red
   2 green
   3 blue
   4 magenta
   5 aqua
   6 brown
   7 yellow? (looks light orange)
   8 orange

Here is some sample code contributed by another student used to draw obstacles. Note that all of the obstacle data is put in a string that can later be written to a file. There may be a better way to do this, but you can at least consider this approach. You may want to write directly to the file:

   string PrintObstacleData() {
      char buff[100];
      string s;
      for (int i = 0; i < (int)obstacles->size(); i++) {
         double x1;
         double x2;
         double y1;
         double y2;
         obstacle_t o= obstacles->at(i);
         for (int j = 0; j < 4; j++) {
             x1 = o.o_corner[j][0];
             y1 = o.o_corner[j][1];
             x2 = o.o_corner[(j+1)%4][0];
             y2 = o.o_corner[(j+1)%4][1];
             sprintf(buff, "set arrow from %f, %f to %f, %f nohead lt 3\n", x1,y1,x2,y2);
             s+=buff;
         }
      }
      return s;	
   }

This function draws all the obstacles in blue lines cycling through vertices.

When you come to draw the next “frame” for your gnuplot animation, you will need to use the command “unset arrow”. If you don't, gnuplot will superimpose all previous arrows on top of each other with the rendering of each frame. This causes gnuplot to slow down significantly when you have a long search (such as breadth first search.)

Just in case you didn't know… To have anything show up in GNUPlot, you can't use only “set arrow” commands. You must also have at least one “plot”.

For animation you can use the 'pause' command within the script to tell gnuplot to wait before plotting the next line or lines. You may want the lines to remain on the screen, in this case use the 'set arrow' command mentioned previously. The 'set arrow' must be followed by a 'plot' command in order to appear in the plot. However the 'plot' command must contain a set of points to be valid. Here is a solution:

   set arrow from 370.000000, 70.000000 to 390.000000, 90.000000 nohead lt 6
   plot '-' with lines
   0 0 0 0
   e
   pause 0.010000

Some can not seem to get the second line (plot '-' with lines) to work. Or the e's. Alternatively, plotted a line on the border of the graph to get the plot going, set the arrows, then called replot to plot the arrows. For example:

   set yr [-20:20]
   set xr [-20:20]
   plot -20
   pause 1
   set arrow from 370.000000, 70.000000 to 390.000000, 90.000000 nohead lt 6
   replot
   ...
   

   ...
   string output;
   ...
   output+= PrintLine(currentNode, neighborNode, color);
   output+= PrintAniData(delay);
   ...

   //plots a line between the start node and end node
   string PrintLine(Node * start, Node * end, int color) {
       char buff[100];
       string s;
       double sx = start->getX();
       double sy = start->getY();
       double ex = end->getX();
       double ey = end->getY();
       sprintf(buff, "set arrow from %f, %f to %f, %f nohead lt %d\n", sx,sy,ex,ey, color);
       s+=buff;
       return s;
   }

   //Prints the rest of the command, including the delay for the animation
   string PrintAniData(double delay){
       string s;
       char buff[100];
       sprintf(buff, "\nplot '-' with lines\n0 0 0 0\ne\npause %f\n",delay);
       s+=buff;
       return s;
   }

   ...
   //within the search code, at the end you can push the path data to a file.
   PrintState(output, "AStar.gpi");
   ...

   void PrintState(string s, char * filename) {
       printf("Writing Plot Data to File: %s\n", filename);
       ofstream myfile;
       myfile.open(filename);
       myfile << PrintHeaderData();
       myfile << PrintObstacleData();
       myfile << s;
       myfile << PrintTankData();
       myfile << PrintNode(getGoalNode(), 1);
       myfile << PrintNode(getStartNode(), 2);
       myfile << PrintFooterData();
       myfile.close();
       printf("Finished writing plot data.\n");
   }

   gnuplot -persist filename.gpi