Code Example: Shooting Rays
Shooting rays at models is one of the more common operations performed with BRL-CAD models. The following example illustrates how to use librt's C API to shoot a ray at a model and how to work with the results.
#include "common.h" #include <stdlib.h> #include <math.h> #include <string.h> #include <stdio.h> #include "vmath.h" /* vector math macros */ #include "raytrace.h" /* librt interface definitions */
/**
* rt_shootray() was told to call this on a hit.
*
* This callback routine utilizes the application structure which
* describes the current state of the raytrace.
*
* This callback routine is provided a circular linked list of
* partitions, each one describing one in and out segment of one
* region for each region encountered.
*
* The 'segs' segment list is unused in this example.
*/
HIDDEN int
hit(struct application *ap, struct partition *PartHeadp, struct seg *UNUSED(segs))
{
/* iterating over partitions, this will keep track of the current
* partition we're working on.
*/
struct partition *pp;
/* will serve as a pointer for the entry and exit hitpoints */
struct hit *hitp;
/* will serve as a pointer to the solid primitive we hit */
struct soltab *stp;
/* will contain surface curvature information at the entry */
struct curvature cur = RT_CURVATURE_INIT_ZERO;
/* will contain our hit point coordinate */
point_t pt;
/* will contain normal vector where ray enters geometry */
vect_t inormal;
/* will contain normal vector where ray exits geometry */
vect_t onormal;
/* iterate over each partition until we get back to the head.
* each partition corresponds to a specific homogeneous region of
* material.
*/
for (pp=PartHeadp->pt_forw; pp != PartHeadp; pp = pp->pt_forw) {
/* print the name of the region we hit as well as the name of
* the primitives encountered on entry and exit.
*/
bu_log("\n--- Hit region %s (in %s, out %s)\n",
pp->pt_regionp->reg_name,
pp->pt_inseg->seg_stp->st_name,
pp->pt_outseg->seg_stp->st_name );
/* entry hit point, so we type less */
hitp = pp->pt_inhit;
/* construct the actual (entry) hit-point from the ray and the
* distance to the intersection point (i.e., the 't' value).
*/
VJOIN1(pt, ap->a_ray.r_pt, hitp->hit_dist, ap->a_ray.r_dir);
/* primitive we encountered on entry */
stp = pp->pt_inseg->seg_stp;
/* compute the normal vector at the entry point, flipping the
* normal if necessary.
*/
RT_HIT_NORMAL(inormal, hitp, stp, &(ap->a_ray), pp->pt_inflip);
/* print the entry hit point info */
rt_pr_hit(" In", hitp);
VPRINT( " Ipoint", pt);
VPRINT( " Inormal", inormal);
/* This next macro fills in the curvature information which
* consists on a principle direction vector, and the inverse
* radii of curvature along that direction and perpendicular
* to it. Positive curvature bends toward the outward
* pointing normal.
*/
RT_CURVATURE(&cur, hitp, pp->pt_inflip, stp);
/* print the entry curvature information */
VPRINT("PDir", cur.crv_pdir);
bu_log(" c1=%g\n", cur.crv_c1);
bu_log(" c2=%g\n", cur.crv_c2);
/* exit point, so we type less */
hitp = pp->pt_outhit;
/* construct the actual (exit) hit-point from the ray and the
* distance to the intersection point (i.e., the 't' value).
*/
VJOIN1(pt, ap->a_ray.r_pt, hitp->hit_dist, ap->a_ray.r_dir);
/* primitive we exited from */
stp = pp->pt_outseg->seg_stp;
/* compute the normal vector at the exit point, flipping the
* normal if necessary.
*/
RT_HIT_NORMAL(onormal, hitp, stp, &(ap->a_ray), pp->pt_outflip);
/* print the exit hit point info */
rt_pr_hit(" Out", hitp);
VPRINT( " Opoint", pt);
VPRINT( " Onormal", onormal);
}
/* A more complicated application would probably fill in a new
* local application structure and describe, for example, a
* reflected or refracted ray, and then call rt_shootray() for
* those rays.
*/
/* Hit routine callbacks generally return 1 on hit or 0 on miss.
* This value is returned by rt_shootray().
*/
return 1;
}
/**
* This is a callback routine that is invoked for every ray that
* entirely misses hitting any geometry. This function is invoked by
* rt_shootray() if the ray encounters nothing.
*/
HIDDEN int
miss(struct application *UNUSED(ap))
{
bu_log("missed\n");
return 0;
}
int
main(int argc, char **argv)
{
/* Every application needs one of these. The "application"
* structure carries information about how the ray-casting should
* be performed. Defined in the raytrace.h header.
*/
struct application ap;
/* The "raytrace instance" structure contains definitions for
* librt which are specific to the particular model being
* processed. One copy exists for each model. Defined in
* the raytrace.h header and is returned by rt_dirbuild().
*/
static struct rt_i *rtip;
/* optional parameter to rt_dirbuild() that can be used to capture
* a title if the geometry database has one set.
*/
char title[1024] = {0};
/* Check for command-line arguments. Make sure we have at least a
* geometry file and one geometry object on the command line.
*/
if (argc < 3) {
bu_exit(1, "Usage: %s model.g objects...\n", argv[0]);
}
/* Load the specified geometry database (i.e., a ".g" file).
* rt_dirbuild() returns an "instance" pointer which describes the
* database to be raytraced. It also gives you back the title
* string if you provide a buffer. This builds a directory of the
* geometry (i.e., a table of contents) in the file.
*/
rtip = rt_dirbuild(argv[1], title, sizeof(title));
if (rtip == RTI_NULL) {
bu_exit(2, "Building the db directory for [%s] FAILED\n", argv[1]);
}
/* Display the geometry database title obtained during
* rt_dirbuild if a title is set.
*/
if (title[0]) {
bu_log("Title:\n%s\n", title);
}
/* Walk the geometry trees. Here you identify any objects in the
* database that you want included in the ray trace by iterating
* of the object names that were specified on the command-line.
*/
while (argc > 2) {
if (rt_gettree(rtip, argv[2]) < 0)
bu_log("Loading the geometry for [%s] FAILED\n", argv[2]);
argc--;
argv++;
}
/* This next call gets the database ready for ray tracing. This
* causes some values to be precomputed, sets up space
* partitioning, computes bounding volumes, etc.
*/
rt_prep_parallel(rtip, 1);
/* initialize all values in application structure to zero */
RT_APPLICATION_INIT(&ap);
/* your application uses the raytrace instance containing the
* geometry we loaded. this describes what we're shooting at.
*/
ap.a_rt_i = rtip;
/* stop at the first point of intersection or shoot all the way
* through (defaults to 0 to shoot all the way through).
*/
ap.a_onehit = 0;
/* Set the ray start point and direction rt_shootray() uses these
* two to determine what ray to fire. In this case we simply
* shoot down the z axis toward the origin from 10 meters away.
*
* It's worth nothing that librt assumes units of millimeters.
* All geometry is stored as millimeters regardless of the units
* set during editing. There are libbu routines for performing
* unit conversions if desired.
*/
VSET(ap.a_ray.r_pt, 0.0, 0.0, 10000.0);
VSET(ap.a_ray.r_dir, 0.0, 0.0, -1.0);
/* Simple debug printing */
VPRINT("Pnt", ap.a_ray.r_pt);
VPRINT("Dir", ap.a_ray.r_dir);
/* This is what callback to perform on a hit. */
ap.a_hit = hit;
/* This is what callback to perform on a miss. */
ap.a_miss = miss;
/* Shoot the ray. */
(void)rt_shootray(&ap);
/* A real application would probably set up another ray and fire
* again or do something a lot more complex in the callbacks.
*/
return 0;
}