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);
})
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 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, 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;world because
of the interest call above.
char *id;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 einei 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.