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### 6.5 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
          *
          * 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);
         },
         /* version for picking Nd-objects (not documented here) */)

         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 `PICKFUNC` 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 GCL.

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, block, NDblock)

where name is the name of the procedure to be defined, in this case `Lpick`. The next argument, block, is a block of code to be executed when a pick event occurs. If block contains a return statement, then the returned value must be a pointer to a Lisp-object, that is of type `LObject *`. The last argument has the same functionality as the block argument, but is only invoked when picking objects in a higher dimensional world.

`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).

There is also a second variant of the `PICKFUNC` macro with a slightly different syntax:

         DEFPICKFUNC(helpstr, coordsys, id,
           point, pn, vertex, vn, edge, en, face, fn, ppath, ppn,
           vi, ei, ein, fi,
           body, NDbody)

`DEFPICKFUNC` can be used as well as `PICKFUNC`, there is no functional difference with the exception that the name of the C-function is tied to `Lpick` when using `DEFPICKFUNC` and that the `(help pick)` GCL-command (see `(help ...)`) would respond with echoing helpstr.

The table below lists all variables defined in `PICKFUNC` In the context of ND-viewing `float` variants of the arguments apply: the body execution block sees the `HPoint3` variables, and the NDbody block sees only flat one-dimensional arrays of `float`-type.

In the ND-viewing context the co-ordinates passed to the pick function are still the 3-dimensional co-ordinates of the camera view-port where the pick occurred, but padded with zeroes on transformed back to the co-ordinate system specified by the second argument of the `pick` command.

`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;`
`float *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`, `4` or greater than `4`. If it is greater than `4`, then the NDbody instruction block is invoked and in this case `point` is a flat array of `pn` many `float`s. 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. In this case the ordinary block 3d instruction block is executed.
`HPoint3 vertex; int vn;`
`float *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 greater equal `4`. A value of zero means the pick point was not near a vertex. In the context of ND-viewing `vertex` will be an array of `vn` `float`s and `vn` will be equal to `pn`.
`HPoint3 edge[2]; int en;`
`float *edge; 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 `0` or greater equal `8`. A value of zero means the pick point was not near an edge. In the context of ND-viewing `edge` will be a flat one-dimensional array of `en` many `float`s: the first `pn` `float`s define the first vertex, and the second `pn` many `float`s define the second vertex; `en` will be two times `pn`.

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[]; int fn;`
`float *face; int fn;`
  `face` is a variable length array of fn `HPoint3`'s. `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 either `0` or a multiple of `pn`. A value of zero means the pick point was not near a face. In the context of ND-viewing `face` is a flat one-dimensional array of `fn` many floats of which each vertex occupies `pn` many components.
`int ppath[]; int ppn;`
  `ppath` is an array of maxpathlen `int`'s. `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 OOGL File Formats, and see GCL, 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.