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# External Modules

An external module is a program that interacts with Geomview. A module communicates with Geomview through gcl and can control any apsect of Geomview that you can control through Geomview's user interface.

In many cases an external module is a specialized program that implements some mathematical algorithm that creates a geometric object that changes shape as the algorithm progresses. The module informs Geomview of the new object shape at each step, so the object appears to evolve with time in the Geomview window. In this way Geomview serves as a display engine for the module.

An external module may be interactive. It can respond to mouse and keyboard events that take place in a Geomview window, thus extending the capability of Geomview itself.

## How External Modules Interface with Geomview

External modules appear in the Modules browser in Geomview's Main panel. To run a module, click the left mouse button on the module's entry in the browser. While the module is running, an additional line for that module will appear in red in the browser. This line begins with a number in brackets, which indicates the instace number of the module. (For some modules it makes sense to have more than one instance of the module running at the same time.) You can kill an external module by clicking on its red instance entry.

By default when Geomview starts, it displays all the modules that have been installed on your system.

For instructions on installing a module on your system so that it will appear in the Modules browser every time Geomview is run by anyone on your system, See section Module Installation.

When Geomview invokes an external module, it creates pipes connected to the module's standard input and output. (Pipes are like files except they are used for communication between programs rather than for storing things on a disk.) Geomview interprets anything that the module writes to its standard output as a gcl command. Likewise, if the exernal module requests any data from Geomview, Geomview writes that data to the module's standard input. Thus all a module has to do in order to communicate with Geomview is write commands to standard output and (optionally) receive data on standard input. Note that this means that the module cannot use standard input and output for communicating with the user. If a module needs to communicate with the user it can do so either through a control panel of its own or else by responding to certain events that it finds out about from Geomview.

## Example 1: Simple External Module

This section gives a very simple external module which displays an oscillating mesh. To try out this example, make a copy of the file ``example1.c'` (it is distributed with Geomview in the ``doc'` subdirectory) in your directory and compile it with the command

    cc -o example1 example1.c -lm

Then put the line

    (emodule-define "Example 1" "./example1")

in a file called ``.geomview'` in your current directory. Then invoke Geomview; it is important that you compile the example program, create the ``.geomview'` file, and invoke Geomview all in the same directory. You should see "Example 1" in the Modules browser of Geomview's Main panel; click on this entry in the browser to start the module. A surface should appear in your camera window and should begin oscillating. You can stop the module by clicking on the red "[1] Example 1" line in the Modules browser.

    /*
     * example1.c: oscillating mesh
     *
     * This example module is distributed with the Geomview manual.
     * If you are not reading this in the manual, see the "External
     * Modules" chapter of the manual for more details.
     *
     * This module creates an oscillating mesh.
     */

    #include <math.h>
    #include <stdio.h>

    /* F is the function that we plot
     */
    float F(x,y,t)
         float x,y,t;
    {
      float r = sqrt(x*x+y*y);
      return(sin(r + t)*sqrt(r));
    }

    main(argc, argv)
         char **argv;
    {
      int xdim, ydim;
      float xmin, xmax, ymin, ymax, dx, dy, t, dt;

      xmin = ymin = -5;             /* Set x and y            */
      xmax = ymax = 5;              /*    plot ranges         */
      xdim = ydim = 24;             /* Set x and y resolution */
      dt = 0.1;                     /* Time increment is 0.1  */

      /* Geomview setup.  We begin by sending the command
       *            (geometry example { : foo})
       * to Geomview.  This tells Geomview to create a geom called
       * "example" which is an instance of the handle "foo".
       */
      printf("(geometry example { : foo })\n");
      fflush(stdout);

      /* Loop until killed.
       */
      for (t=0; ; t+=dt) {
        UpdateMesh(xmin, xmax, ymin, ymax, xdim, ydim, t);
      }
    }

    /* UpdateMesh sends one mesh iteration to Geomview.  This consists of
     * a command of the form
     *    (read geometry { define foo
     *       MESH
     *       ...
     *    })
     * where ... is the actual data of the mesh.  This command tells
     * Geomview to make the value of the handle "foo" be the specified
     * mesh.
     */
    UpdateMesh(xmin, xmax, ymin, ymax, xdim, ydim, t)
         float xmin, xmax, ymin, ymax, t;
         int xdim, ydim;
    {
      int i,j;
      float x,y, dx,dy;

      dx = (xmax-xmin)/(xdim-1);
      dy = (ymax-ymin)/(ydim-1);

      printf("(read geometry { define foo \n");
      printf("MESH\n");
      printf("%1d %1d\n", xdim, ydim);
      for (j=0, y = ymin; j<ydim; ++j, y += dy) {
        for (i=0, x = xmin; i<xdim; ++i, x += dx) {
          printf("%f %f %f\t", x, y, F(x,y,t));
        }
        printf("\n");
      }
      printf("})\n");
      fflush(stdout);
    }

The module begins by defining a function `F(x,y,t)` that specifies a time-varying surface. The purpose of the module is to animate this surface over time.

The main program begins by defining some variables that specify the parameters with which the function is to be plotted.

The next bit of code in the main program prints the following line to standard output

    (geometry example { : foo })

This tells Geomview to create a geom called `example` which is an instance of the handle `foo`. Handles are a part of the OOGL file format which allow you to name a piece of geometry whose value can be specified elsewhere (and in this case updated many times); for more information on handles, See section OOGL File Formats In this case, `example` is the title by which the user will see the object in Geomview's object browser, and `foo` is the internal name of the handle that the object is a reference to.

We then do `fflush(stdout)` to ensure that Geomview receives this command immediately. In general, since pipes may be buffered, an external module should do this whenever it wants to be sure Geomview has actually received everything it has printed out.

The last thing in the main program is an infinite loop that cycles through calls to the procedure `UpdateMesh` with increasing values of `t`. `UpdateMesh` sends Geomview a command of the form

    (read geometry { define foo
    MESH
    24 24
    ...
    })

where `...` is a long list of numbers. This command tells Geomview to make the value of the handle `foo` be the specified mesh. As soon as Geomview receives this command, the geom being displayed changes to reflect the new geometry.

The mesh is given in the format of an OOGL MESH. This begins with the keyword `MESH`. Next come two numbers that give the x and y dimensions of the mesh; in this case they are both 24. This line is followed by 24 lines, each containing 24 triples of numbers. Each of these triples is a point on the surface. Then finally there is a line with "`})`" on it that ends the "`{`" which began the `define` statement and the "`(`" that began the command. For more details on the format of MESH data, see section MESH: rectangularly-connected mesh.

This module could be written without the use of handles by having it write out commands of the form

    (geometry example {
    MESH
    24 24
    ...
    })

This first time Geomview receives a command of this form it would create a geom called `example` with the given `MESH` data. Subsequent `(geometry example ...)` commands would cause Geomview to replace the geometry of the geom `example` with the new `MESH` data. If done in this way there would be no need to send the initial `(geometry example { : foo })` command as above. The handle technique is useful, however, because it can be used in more general situations where a handle represents only part of a complex geom, allowing an external module to replace only that part without having to retransmit the entire geom. For more information on handles, See section gcl: the Geomview Command Language.

The module loops through calls to `UpdateMesh` which print out commands of the above form one after the other as fast as possible. The loop continues indefinitely; the module will terminate when the user kills it by clicking on its instance line in the Modules browser, or else when Geomview exits.

Sometimes when you terminate this module by clicking on its instance entry the Modules browser, Geomview will kill it while it is in the middle of sending a command to Geomview. Geomview will then receive only a piece of a command and will print out a cryptic but harmless error message about this. When a module has a user interface panel it can use a "Quit" button to provide a more graceful way for the user to terminate the module. See the next example.

You can run this module in a shell window without Geomview to see the commands it prints out. You will have to kill it with `ctrl-C` to get it to stop.

## Example 2: Simple External Module with FORMS Control Panel

This section gives a new version of the above module -- one that includes a user interface panel for controlling the velocity of the oscillation. We use the FORMS library by Mark Overmars for the control panel. The FORMS library is a public domain user interface toolkit for IRISes; for more information See section The FORMS Library.

To try out this example, make a copy of the file ``example2.c'` (distributed with Geomview in the ``doc'` subdirectory) in your directory and compile it with the command

    cc -I/u/gcg/ngrap/include -o example2 example2.c \
      -L/u/gcg/ngrap/lib/sgi -lforms -lfm_s -lgl_s -lm

If you are not using the Geometry Center's computer system you should replace the string ``/u/gcg/ngrap'` above with the pathname of the Geomview distribution directory on your system. (The forms library is distributed with Geomview and the `-I` and `-L` options above tell the compiler where to find it.)

Then put the line

    (emodule-define "Example 2" "./example2")

in a file called ``.geomview'` in the current directory and invoke Geomview from that directory. Click on the "Example 2" entry in the Modules browser to invoke the module. A small control panel should appear. You can then control the velocity of the mesh oscillation by moving the slider.

    /*
     * example2.c: oscillating mesh with FORMS control panel
     *
     * This example module is distributed with the Geomview manual.
     * If you are not reading this in the manual, see the "External
     * Modules" chapter of the manual for an explanation.
     *
     * This module creates an oscillating mesh and has a FORMS control
     * panel that lets you change the speed of the oscillation with a
     * slider.
     */

    #include <math.h>
    #include <stdio.h>
    #include <sys/time.h>           /* for struct timeval below */

    #include "forms.h"              /* for FORMS library */

    FL@_FORM *OurForm;
    FL@_OBJECT *VelocitySlider;
    float dt;

    /* F is the function that we plot
     */
    float F(x,y,t)
         float x,y,t;
    {
      float r = sqrt(x*x+y*y);
      return(sin(r + t)*sqrt(r));
    }

    /* SetVelocity is the slider callback procedure; FORMS calls this
     * when the user moves the slider bar.
     */
    void SetVelocity(FL@_OBJECT *obj, long val)
    {
      dt = fl@_get@_slider@_value(VelocitySlider);
    }

    /* Quit is the "Quit" button callback procedure; FORMS calls this
     * when the user clicks the "Quit" button.
     */
    void Quit(FL@_OBJECT *obj, long val)
    {
      exit(0);
    }

    /* create@_form@_OurForm() creates the FORMS panel by calling a bunch of
     * procedures in the FORMS library.  This code was generated
     * automatically by the FORMS designer program; normally this code
     * would be in a separate file which you would not edit by hand.  For
     * simplicity of this example, however, we include this code here.
     */
    create@_form@_OurForm()
    {
      FL@_OBJECT *obj;
      FL@_FORM *form;
      OurForm = form = fl@_bgn@_form(FL@_NO@_BOX,380.0,120.0);
      obj = fl@_add@_box(FL@_UP@_BOX,0.0,0.0,380.0,120.0,"");
      VelocitySlider = obj = fl@_add@_valslider(FL@_HOR@_SLIDER,20.0,30.0,
                                              340.0,40.0,"Velocity");
        fl@_set@_object@_lsize(obj,FL@_LARGE@_FONT);
        fl@_set@_object@_align(obj,FL@_ALIGN@_TOP);
        fl@_set@_call@_back(obj,SetVelocity,0);
      obj = fl@_add@_button(FL@_NORMAL@_BUTTON,290.0,75.0,70.0,35.0,"Quit");
        fl@_set@_object@_lsize(obj,FL@_LARGE@_FONT);
        fl@_set@_call@_back(obj,Quit,0);
      fl@_end@_form();
    }

    main(argc, argv)
         char **argv;
    {
      int xdim, ydim;
      float xmin, xmax, ymin, ymax, dx, dy, t;
      int fdmask;
      static struct timeval timeout = {0, 200000};

      xmin = ymin = -5;             /* Set x and y            */
      xmax = ymax = 5;              /*    plot ranges         */
      xdim = ydim = 24;             /* Set x and y resolution */
      dt = 0.1;                     /* Time increment is 0.1  */

      /* Forms panel setup.
       */
      foreground();
      create@_form@_OurForm();
      fl@_set@_slider@_bounds(VelocitySlider, 0.0, 1.0);
      fl@_set@_slider@_value(VelocitySlider, dt);
      fl@_show@_form(OurForm, FL@_PLACE@_SIZE, TRUE, "Example 2");

      /* Geomview setup.
       */
      printf("(geometry example { : foo })\n");
      fflush(stdout);

      /* Loop until killed.
       */
      for (t=0; ; t+=dt) {
        fdmask = (1 << fileno(stdin)) | (1 << qgetfd());
        select(qgetfd()+1, &fdmask, NULL, NULL, &timeout);
        fl@_check@_forms();
        UpdateMesh(xmin, xmax, ymin, ymax, xdim, ydim, t);
      }
    }

    /* UpdateMesh sends one mesh iteration to Geomview
     */
    UpdateMesh(xmin, xmax, ymin, ymax, xdim, ydim, t)
         float xmin, xmax, ymin, ymax, t;
         int xdim, ydim;
    {
      int i,j;
      float x,y, dx,dy;

      dx = (xmax-xmin)/(xdim-1);
      dy = (ymax-ymin)/(ydim-1);

      printf("(read geometry { define foo \n");
      printf("MESH\n");
      printf("%1d %1d\n", xdim, ydim);
      for (j=0, y = ymin; j<ydim; ++j, y += dy) {
        for (i=0, x = xmin; i<xdim; ++i, x += dx) {
          printf("%f %f %f\t", x, y, F(x,y,t));
        }
        printf("\n");
      }
      printf("})\n");
      fflush(stdout);
    }

The code begins by including some header files needed for the event loop and the FORMS library. It then declares global variables for holding a pointer to the slider FORMS object and the velocity `dt`. These are global because they are needed in the slider callback procedure `SetVelocity`, which forms calls every time the user moves the slider bar. `SetVelocity` sets `dt` to be the new value of the slider.

`Quit` is the callback procedure for the Quit button; it provides a graceful way for the user to terminate the program.

The procedure `create_panel` calls a bunch of FORMS library procedures to set up the control panel with slider and button. For more information on using FORMS to create interface panels see the FORMS documentation. In particular, FORMS comes with a graphical panel designer that lets you design your panels interactively and generates code like that in `create_panel`.

This example's main program is similar to the previous example, but includes extra code to deal with setting up and managing the FORMS panel.

To set up the panel we call the GL procedure `foreground` to cause the process to run in the foreground. By default GL programs run in the background, and for various reasons external modules that use FORMS (which is based on GL) need to run in the foreground. We then call `create_panel` to create the panel and `fl_set_slider_value` to set the initial value of the slider. The call to `fl_show_form` causes the panel to appear on the screen.

The first three lines of the main loop, starting with

    fdmask = (1 << fileno(stdin)) | (1 << qgetfd());

check for and deal with events in the panel. The call to `select` imposes a delay on each pass through the main loop. This call returns either after a delay of 1/5 second or when the next GL event occurs, or when data appears on standard input, whichever comes first. The `timeout` variable specifies the amount of time to wait on this call; the first member (0 in this example) gives the number of seconds, and the second member (200000 in this example) gives the number of microseconds. Finally, `fl_check_forms()` checks for and processes any FORMS events that have happened; in this case this means calling `SetVelocity` if the user has moved the slider or calling `Quit` if the user has clicked on the Quit button.

The purpose of the delay in the loop is to keep the program from using excessive amounts of CPU time running around its main loop when there are no events to be processed. This is not so crucial in this example, and in fact may actually slow down the animation somewhat, but in general with external modules that have event loops it is important to do something like this because otherwise the module will needlessly take CPU cycles away from other running programs (such as Geomview!) even when it isn't doing anything.

The last line of the main loop in this example, the call to `UpdateMesh`, is the same as in the previous example.

## The FORMS Library

Geomview itself is written using Mark Overmar's public domain FORMS library. FORMS is a handy and relatively simple user interface toolkit for IRISes. Many Geomview external modules, including the examples in this manual, use FORMS to create and manage control panels.

We distribute a version of the FORMS library with Geomview because it is necessary in order to compile Geomview and many of our modules. If you use FORMS to write Geomview modules (or anything else, for that matter) you may use this copy. The header file ``forms.h'` is in the ``include'` subdirectory, and the library file ``libforms.a'` is in the ``lib/sgi'` subdirectory (these are subdirectories of the Geomview distribution directory, ``/u/gcg/ngrap'` on the Geometry Center's system). In particular, you can link the example modules in this manual using this copy.

FORMS is available via ftp on the Internet from a variety of sites, including `cs.ruu.nl` or `glaurung.physics.mcgill.ca`. It comes with source code and extensive documentation.

If you wish you may use any other interface toolkit instead of FORMS in an external module. We chose FORMS because it is free and relatively simple.

## Example 3: External Module with Bi-Directional Communication

The previous two example modules simply send commands to Geomview and do not receive anything from Geomview. This section describes a module that communicates in both directions. There are two types of communication that can go from Geomview to an external module. This example shows asynchronous communication -- the module needs to be able to respond at any moment to expressions that Geomview may emit which inform the module of some change of state within Geomview.

(The other type of communication is synchronous, where a module sends a request to Geomview for some piece of information and waits for a response to come back before doing anything else. The main gcl command for requesting information of this type is `write`. This module does not do any synchronous communication.)

In ansynchronous communication, Geomview sends expressions that are essentially echoes of gcl commands. The external module sends Geomview a command expressing interest in a certain command, and then every time Geomview executes that command, the module receives a copy of it. This happens regardless of who sent the command to Geomview; it can be the result of the user doing something with a Geomview panel, or it may have come from another module or from a file that Geomview reads. This is how a module can find out about and act on things that happen in Geomview.

This example uses the OOGL lisp library to parse and act on the expressions that Geomview writes to the module's standard input. This library is actually part of Geomview itself -- we wrote the library in the process of implementing gcl. It is also convenient to use it in external modules that must understand a of subset of gcl --- specifically, those commands that the module has expressed interest in.

This example shows how a module can receive user pick events, i.e. when the user clicks the right mouse button with the cursor over a geom in a Geomview camera window. When this happens Geomview generates an internal call to a procedure called `pick`; the arguments to the procedure give information about the pick, such as what object was picked, the coordinates of the picked point, etc. If an external module has expressed interest in calls to `pick`, then whenever `pick` is called Geomview will echo the call to the module's standard input. The module can then do whatever it wants with the pick information.

This module is the same as the Nose module that comes with Geomview. Its purpose is to illustrate picking. Whenever you pick on a geom by clicking the right mouse button on it, the module draws a little box at the spot where you clicked. Usually the box is yellow. If you pick a vertex, the box is colored magenta. If you pick a point on an edge of an object, the module will also highlight the edge by drawing cyan boxes at its endpoints and drawing a yellow line along the edge.

Note that in order for this module to actually do anything you must have a geom loaded into Geomview and you must click the right mouse button with the cursor over a part of the geom.

    /*
     * example3.c: external module with bi-directional communication
     *
     * This example module is distributed with the Geomview manual.
     * If you are not reading this in the manual, see the "External
     * Modules" chapter of the manual for an explanation.
     *
     * This module is the same as the "Nose" program that is distributed
     * with Geomview.  It illustrates how a module can find out about
     * and respond to user pick events in Geomview.  It draws a little box
     * at the point where a pick occurrs.  The box is yellow if it is not
     * at a vertex, and magenta if it is on a vertex.  If it is on an edge,
     * the program also marks the edge.
     *
     * To compile:
     *
     *   cc -I/u/gcg/ngrap/include -g -o example3 example3.c \
     *      -L/u/gcg/ngrap/lib/sgi -loogl -lm
     *
     * If you are not on the Geometry Center's system you should replace
     * "/u/gcg/ngrap" above with the pathname of the Geomview distribution
     * directory on your system.
     */

    #include <stdio.h>
    #include "lisp.h"               /* We use the OOGL lisp library */
    #include "pickfunc.h"           /* for PICKFUNC below */
    #include "3d.h"                 /* for 3d geometry library */

    /* boxstring gives the OOGL data to define the little box that
     * we draw at the pick point.  NOTE:  It is very important to
     * have a newline at the end of the OFF object in this string.
     */
    char boxstring[] = "\
    INST\n\
    transform\n\
    .04 0 0 0\n\
    0 .04 0 0\n\
    0 0 .04 0\n\
    0 0 0 1\n\
    geom\n\
    OFF\n\
    8 6 12\n\
    \n\
    -.5 -.5 -.5     # 0   \n\
    .5 -.5 -.5      # 1   \n\
    .5  .5 -.5      # 2   \n\
    -.5  .5 -.5     # 3   \n\
    -.5 -.5  .5     # 4   \n\
    .5 -.5  .5      # 5   \n\
    .5  .5  .5      # 6   \n\
    -.5  .5  .5     # 7   \n\
    \n\
    4 0 1 2 3\n\
    4 4 5 6 7\n\
    4 2 3 7 6\n\
    4 0 1 5 4\n\
    4 0 4 7 3\n\
    4 1 2 6 5\n";

    progn()
    {
      printf("(progn\n");
    }

    endprogn()
    {
      printf(")\n");
      fflush(stdout);
    }

    Initialize()
    {
      extern LObject *Lpick();  /* This is defined by PICKFUNC below but must */
      			    /* be used in the following LDefun() call */
      LInit();
      LDefun("pick", Lpick, NULL);

      progn(); {
        /* Define handle "littlebox" for use later
         */
        printf("(read geometry { define littlebox { %s }})\n", boxstring);

        /* Express interest in pick events; see Geomview manual for explanation.
         */
        printf("(interest (pick world * * * * nil nil nil nil nil))\n");

        /* Define "pick" object, initially the empty list (= null object).
         * We replace this later upon receiving a pick event.
         */
        printf("(geometry \"pick\" { LIST } )\n");

        /* Make the "pick" object be non-pickable.
         */
        printf("(pickable \"pick\" no)\n");

        /* Turn off normalization, so that our pick object will appear in the
         * right place.
         */
        printf("(normalization \"pick\" none)\n");

        /* Don't draw the pick object's bounding box.
         */
        printf("(bbox-draw \"pick\" off)\n");

      } endprogn();
    }

    /* The following is a macro call that defines a procedure called
     * Lpick().  The reason for doing this in a macro is that that macro
     * encapsulates a lot of necessary stuff that would be the same for
     * this procedure in any program.  If you write a Geomview module that
     * wants to know about user pick events you can just copy this macro
     * call and change the body to suit your needs; the body is the last
     * argument to the macro and is delimited by curly braces.
     *
     * The first argument to the macro is the name of the procedure to
     * be defined, "Lpick".
     *
     * The next two arguments are numbers which specify the sizes that
     * certain arrays inside the body of the procedure should have.
     * These arrays are used for storing the face and path information
     * of the picked object.  In this module we don't care about this
     * information so we declare them to have length 1, the minimum
     * allowed.
     *
     * The last argument is a block of code to be executed when the module
     * receives a pick event.  In this body you can refer to certain local
     * variables that hold information about the pick.  For details see
     * Example 3 in the Extenal Modules chapter of the Geomview manual.
     */
    PICKFUNC(Lpick, 1, 1,
    {
      handle@_pick(pn>0, &point, vn>0, &vertex, en>0, edge);
    })

    handle@_pick(picked, p, vert, v, edge, e)
         int picked;                /* was something actually picked?     */
         int vert;                  /* was the pick near a vertex?        */
         int edge;                  /* was the pick near an edge?         */
         HPoint3 *p;                /* coords of pick point               */
         HPoint3 *v;                /* coords of picked vertex            */
         HPoint3 e[2];              /* coords of endpoints of picked edge */
    {
      Normalize(&e[0]);             /* Normalize makes 4th coord 1.0 */
      Normalize(&e[1]);
      Normalize(p);
      progn(); {
        if (!picked) {
          printf("(geometry \"pick\" { LIST } )\n");
        } else {
          /*
           * Put the box in place, and color it magenta if it's on a vertex,
           * yellow if not.
           */
          printf("(xform-set pick { 1 0 0 0  0 1 0 0  0 0 1 0  %g %g %g 1 })\n",
                 p->x, p->y, p->z);
          printf("(geometry \"pick\"\n");
          if (vert) printf("{ appearance { material { diffuse 1 0 1 } }\n");
          else printf("{ appearance { material { diffuse 1 1 0 } }\n");
          printf("  { LIST { :littlebox }\n");

          /*
           * If it's on an edge and not a vertex, mark the edge
           * with cyan boxes at the endpoins and a black line
           * along the edge.
           */
          if (edge && !vert) {
            e[0].x -= p->x; e[0].y -= p->y; e[0].z -= p->z;
            e[1].x -= p->x; e[1].y -= p->y; e[1].z -= p->z;
            printf("{ appearance { material { diffuse 0 1 1 } }\n\
      LIST\n\
       { INST transform 1 0 0 0 0 1 0 0 0 0 1 0 %f %f %f 1 geom :littlebox }\n\
       { INST transform 1 0 0 0 0 1 0 0 0 0 1 0 %f %f %f 1 geom :littlebox }\n\
       { VECT\n\
              1 2 1\n\
              2\n\
              1\n\
              %f %f %f\n\
              %f %f %f\n\
              1 1 0 1\n\
       }\n\
      }\n",
                   e[0].x, e[0].y, e[0].z,
                   e[1].x, e[1].y, e[1].z,
                   e[0].x, e[0].y, e[0].z,
                   e[1].x, e[1].y, e[1].z);
          }
          printf("    }\n  }\n)\n");
        }

      } endprogn();

    }

    Normalize(HPoint3 *p)
    {
      if (p->w != 0) {
        p->x /= p->w;
        p->y /= p->w;
        p->z /= p->w;
        p->w = 1;
      }
    }

    main()
    {
      Lake *lake;
      LObject *lit, *val;
      extern char *getenv();

      Initialize();

      lake = LakeDefine(stdin, stdout, NULL);
      while (!feof(stdin)) {

        /* Parse next lisp expression from stdin.
         */
        lit = LSexpr(lake);

        /* Evaluate that expression; this is where Lpick() gets called.
         */
        val = LEval(lit);

        /* Free the two expressions from above.
         */
        LFree(lit);
        LFree(val);
      }
    }

The code begins by defining procedures `progn()` and `endprogn()` which begin and end a Geomview `progn` group. The purpose of the Geomview `progn` command is to group commands together and cause Geomview to execute them all at once, without refreshing any graphics windows until the end. It is a good idea to group blocks of commands that a module sends to Geomview like this so that the user sees their cumulative effect all at once.

Procedure `Initialize()` does various things needed at program startup time. It initializes the lisp library by calling `LInit()`. Any program that uses the lisp library should call this once before calling any other lisp library functions. It then calls `LDefun` to tell the library about our `pick` procedure, which is defined further down with a call to the `DEFPICKFUNC` macro. Then it sends a bunch of setup commands to Geomview, grouped in a `progn` block. This includes defining a handle called `littlebox` that stores the geometry of the little box. Next it sends the command

    (interest (pick world * * * * nil nil nil nil nil))

which tells Geomview to notify us when a pick event happens.

The syntax of this `interest` statement merits some explanation. In general `interest` takes one argument which is a (parenthesized) expression representing a Geomview function call. It specifies a type of call that the module is interested in knowing about. The arguments can be any particular argument values, or the special symbols `*` or `nil`. For example, the first argument in the `pick` expression above is `world`. This means that the module is interested in calls to `pick` where the first argument, which specifies the coordinate system, is `world`. A `*` is like a wild-card; it means that the module is interested in calls where the corresponding argument has any value. The word `nil` is like `*`, except that the argument's value is not reported to the module. This is useful for cutting down on the amount of data that must be transmitted in cases where there are arguments that the module doesn't care about.

The second, third, fourth, and fifth arguments to the `pick` command give the name, pick point coordinates, vertex coordinates, and edge coordinates of a pick event. We specify these by `*`'s above. The remaining five arguments to the `pick` command give other information about the pick event that we do not care about in this module, so we specify these with `nil`'s. For the details of the arguments to `pick`, See section gcl: the Geomview Command Language.

The `geometry` statement defines a geom called `pick` that is initially an empty list, specified as `{ LIST }` ; this is the best way of specifying a null geom. The module will replace this with something useful by sending Geomview another `geometry` command when the user picks something. Next we arrange for the `pick` object to be non-pickable, and turn normalization off for it so that Geomview will display it in the size and location where we put it, rather than resizing and relocating it to fit into the unit cube.

The next function in the file, `Lpick`, is defined with a strange looking call to a macro called `PICKFUNC`, defined in the header file ``pickfunc.h'`. This is the function for handling pick events. The reason we provide a macro for this is that that macro encapsulates a lot of necessary stuff that would be the same for the pick-handling function in any program. If you write a Geomview module that wants to know about user pick events you can just copy this macro call and change it to suit yours needs.

In general the syntax for `PICKFUNC` is

    PICKFUNC(name, maxfaceverts, maxpathlen, block)

where name is the name of the procedure to be defined, in this case `Lpick`. The next two arguments, maxfaceverts and maxpathlen, give the sizes to be used for declaring two local variable arrays in the body of the procedure. These arrays are for storing information about the picked face and the picked primitive's path. In this module we don't care about this information (it corresponds to some of the things masked out by the `nil`'s in the `interest` call above) so we specify 1, the minimum allowable, for both of these. The last argument, block, is a block of code to be executed when a pick event occurs. The block should be delimited by curly braces. The code in your block should not include any `return` statements.

`PICKFUNC` declares certain local variables in the body of the procedure. When the module receives a `(pick ...)` statement from Geomview, the procedure assigns values to these variables based on the information in the `pick` call. (Variables corresponding to `nil`'s in the `(interest (pick ...))` are not given values.) These variables are:

`char *coordsys;`
  A string specifying the coordinate system in which coordinates are given. In this example, this will always be `world` because of the `interest` call above.

`char *id;`
  A string specifying the name of the picked geom.

`HPoint3 point; int pn;`
  `point` is an `HPoint3` structure giving the coordinates of the picked point. `HPoint3` is a homogeneous point coordinate representation equivalent to an array of 4 floats. `pn` tells how many coordinates have been written into this array; it will always be either 0 or 4. A value of zero means no point was picked, i.e. the user clicked the right mouse button while the cursor was not pointing at a geom.

`HPoint3 vertex; int vn;`
  `vertex` is an `HPoint3` structure giving the coordinates of the picked vertex, if the pick point was near a vertex. `vn` tells how many coordinates have been written into this array; it will always be either 0 or 4. A value of zero means the pick point was not near a vertex.

`HPoint3 edge[2]; int en;`
  `edge` is an array of two `HPoint3` structures giving the coordinates of the endpoints of the picked edge, if the pick point was near an edge. `en` tells how many coordinates have been written into this array; it will always be either 0 or 8. A value of zero means the pick point was not near an edge.

In this example module, the remaining variables will never be given values because their values in the `interest` statement were specified as `nil`.

`HPoint3 face[maxfaceverts]; int fn;`
  `face` is an array of maxfaceverts `HPoint3`'s; maxfaceverts is the value specified in the `PICKFUNC` call. `face` gives the coordinates of the vertices of the picked face. `fn` tells how many coordinates have been written into this array; it will always be a multiple of 4 and will be at most 4*maxfaceverts. A value of zero means the pick point was not near a face.

`HPoint3 ppath[maxpathlen; int ppn;`
  `ppath` is an array of maxpathlen `int`'s; maxpathlen is the value specified in the `PICKFUNC` call. `ppath` gives the path through the OOGL heirarchy to the picked primitive. `pn` tells how many integers have been written into this array; it will be at most maxpathlen. A path of {3,1,2}, for example, means that the picked primitive is "subobject number 2 of subobject number 1 of object 3 in the world".

`int vi;`
  `vi` gives the index of the picked vertex in the picked primitive, if the pick point was near a vertex.

`int ei[2]; int ein`
  The `ei` array gives the indices of the endpoints of the picked edge, if the pick point was near a vertex. `ein` tells how many integers were written into this array. It will always be either 0 or 2; a value of 0 means the pick point was not near an edge.

`int fi;`
  `fi` gives the index of the picked face in the picked primitive, if the pick point was near a face.

The `handle_pick` procedure actually does the work of dealing with the pick event. It begins by normalizing the homogeneous coordinates passed in as arguments so that we can assume the fourth coordinate is 1. It then sends gcl commands to define the `pick` object to be whatever is appropriate for the kind of pick recieved. See see section OOGL File Formats, and see section gcl: the Geomview Command Language, for an explanation of the format of the data in these commands.

The main program, at the bottom of the file, first calls `Initialize()`. Next, the call to `LakeDefine` defines the `Lake` that the lisp library will use. A `Lake` is a structure that the lisp library uses internally as a type of communiation vehicle. (It is like a unix stream but more general, hence the name.) This call to `LakeDefine` defines a `Lake` structure for doing I/O with `stdin` and `stdout`. The third argument to `LakeDefine` should be `NULL` for external modules (it is used by Geomview). Finally, the program enters its main loop which parses and evaluates expressions from standard input.

## Example 4: Simple Tcl/Tk Module Demonstrating Picking

It's not necessary to write a Geomview module in C. The only requirement of an external module is that it send GCL commands to its standard output and expect responses (if any) on its standard input. An external module can be written in C, perl, tcl/tk, or pretty much anything.

As an example, assuming you have Tcl/Tk version 4.0 or later, here's an external module with a simple GUI which demonstrates interaction with geomview. This manual doesn't discuss the Tcl/Tk language; see the good book on the subject by its originator John Ousterhout, published by Addison-Wesley, titled Tcl and the Tk Toolkit.

The ``#!'` on the script's first line causes the system to interpret the script using the Tcl/Tk ``wish'` program; you might have to change its first line if that's in some location other than /usr/local/bin/wish4.0. Or, you could define it as a module using

      (emodule-define  "Pick Demo"  "wish pickdemo.tcl")

in which case ``wish'` could be anywhere on the UNIX search path.

    #! /usr/local/bin/wish4.0

    # We use "fileevent" below to have "readsomething" be called whenever
    # data is available from standard input, i.e. when geomview has sent us
    # something.  It promises to include a trailing newline, so we can use
    # "gets" to read the geomview response, then parse its nested parentheses
    # into tcl-friendly {} braces.

    proc readsomething {} {
      if {[gets stdin line] < 0} {
            puts stderr "EOF on input, exiting..."
            exit
      }
      regsub -all {\(} $line "\{" line
      regsub -all {\)} $line "\}" line
      # Strip outermost set of braces
      set stuff [lindex $line 0]
      # Invoke handler for whichever command we got.  Could add others here,
      # if we asked geomview for other kinds of data as well.
      switch [lindex $stuff 0] {
            pick     {handlepick $stuff}
            rawevent {handlekey $stuff}
      }
    }

    # Fields of a "pick" response, from geomview manual:
    #     (pick COORDSYS GEOMID G V E F P VI EI FI)
    #          The pick command is executed internally in response to pick
    #          events (right mouse double click).
    #
    #          COORDSYS = coordinate system in which coordinates of the following
    #              arguments are specified.   This can be:
    #               world: world coord sys
    #               self:  coord sys of the picked geom (GEOMID)
    #               primitive: coord sys of the actual primitive within
    #                   the picked geom where the pick occurred.
    #          GEOMID = id of picked geom
    #          G = picked point (actual intersection of pick ray with object)
    #          V = picked vertex, if any
    #          E = picked edge, if any
    #          F = picked face
    #          P = path to picked primitive [0 or more]
    #          VI = index of picked vertex in primitive
    #          EI = list of indices of endpoints of picked edge, if any
    #          FI = index of picked face

    # Report when user picked something.
    #
    proc handlepick {pick} {
      global nameof selvert seledge order
      set obj [lindex $pick 2]
      set xyzw [lindex $pick 3]
      set fv [lindex $pick 6]
      set vi [lindex $pick 8]
      set ei [lindex $pick 9]
      set fi [lindex $pick 10]

      # Report result, converting 4-component homogeneous point into 3-space point.
      set w [lindex $xyzw 3]
      set x [expr [lindex $xyzw 0]/$w]
      set y [expr [lindex $xyzw 0]/$w]
      set z [expr [lindex $xyzw 0]/$w]
      set s "$x $y $z "
      if {$vi >= 0} {
            set s "$s  vertex #$vi"
      }
      if {$ei != {}} {
            set s "$s  edge [lindex $ei 0]-[lindex $ei 1]"
      }
      if {$fi != -1} {
            set s "$s  face #$fi ([expr [llength $fv]/3]-gon)"
      }
      msg $s
    }

    # Having asked for notification of these raw events, we report when
    # the user pressed these keys in the geomview graphics windows.

    proc handlekey {event} {
      global lastincr
      switch [lindex $event 1] {
        32 {msg "Pressed space bar"}
         8 {msg "Pressed backspace key"}
      }
    }

    #
    # Display a message on the control panel, and on the terminal where geomview
    # was started.  We use "puts stderr ..." rather than simply "puts ...",
    # since Geomview interprets anything we send to standard output
    # as a GCL command!
    #
    proc msg {str} {
      global msgtext
      puts stderr $str
      set msgtext $str
      update
    }

    # Load object from file
    proc loadobject {fname} {
      if {$fname != ""} {
            puts "(geometry thing < $fname)"
            # Be sure to flush output to ensure geomview receives this now!
            flush stdout
      }
    }

    # Build simple "user interface"

    # The message area could be a simple label rather than an entry box,
    # but we want to be able to use X selection to copy text from it.
    # The default mouse bindings do that automatically.

    entry .msg -textvariable msgtext -width 45
    pack .msg

    frame .f

      label .f.l -text "File to load:"
      pack .f.l -side left

      entry .f.ent -textvariable fname
      pack .f.ent -side left -expand true -fill x
      bind .f.ent <Return> { loadobject $fname }

    pack .f

    # End UI definition.

    # Call "readsomething" when data arrives from geomview.

    fileevent stdin readable {readsomething}

    # Geomview initialization

    puts {
            (interest (pick primitive))
            (interest (rawevent 32))	# Be notified when user presses space
            (interest (rawevent 8))		# or backspace keys.
            (geometry thing < hdodec.off)
            (normalization world none)
    }
    # Flush to ensure geomview receives this.
    flush stdout

    wm title . {Sample external module}

    msg "Click right mouse in graphics window"

## Module Installation

This section tells how to install an external module so you can invoke it within Geomview. There are two ways to install a module: you can install a private module so that the module is available to you whenever you run Geomview, or you can install a system module so that the module is available to all users on your system whenever they run Geomview.

### Private Module Installation

The `emodule-define` command arranges for a module to appear in Geomview's Modules browser. `emodule-define` takes two string arguments; the first is the name that will appear in the Modules browser. The second is the shell command for running the module; it may include arguments. Geomview executes this command in a subshell when you click on the module's entry in the browser. For example

    (emodule-define "Foo" "/u/home/modules/foo -x")

adds a line labeled "Foo" to the Modules browser which causes the command "/u/home/modules/foo -x" to be executed when selected.

You may put `emodule-define` commands in your ``~/.geomview'` file to arrange for certain modules to be available every time you run Geomview; See section Customization: ``.geomview'` files. You can also execute `emodule-define` commands from the Commands panel to add a module to an already running copy of Geomview.

There are several other gcl commands for controlling the entries in the Modules browser; for details, See section gcl: the Geomview Command Language.

### System Module Installation

To install a module so that it is available to all Geomview users do the following

1.
  Create a file called ``.geomview-module'` where ``module'` is the name of the module. This file should contain a single line which is an `emodule-define` command for that module:

      (emodule-define "New Module" "newmodule")

  The first argument, `"New Module"` above, is the string that will appear in the Modules browser. The second string, `"newmodule"` above, is the Bourne shell command for invoking the module. It may include arguments, and you may assume that the module is on the @$path searched by the shell.

2.
  Put a copy of the ``.geomview-module'` and the module executable itself in Geomview's ``modules/sgi'` directory. This is a subdirectory of the Geomview distribution directory (on the Geometry Center's system the pathname is ``/u/gcg/ngrap/modules/sgi'`.

After these steps, the new module should appear, in alphabetical position, in the Modules browser of Geomview's Main panel next time Geomview is run. The reason this works is that when Geomview is invoked it processes all the ``.geomview-*'` files in its ``modules'` directory. It also remembers the pathname of this directory and prepends that path to the @$path of the shell in which it invokes such a module.

Go to the previous, next section.