Hello I need some ideas for designing a recursive function for my ray

tracing program.

The idea behind ray tracing is to follow the electromagnetic rays from

the source, as they hit the object.The object is triangulated. The

rays can undergo multiple reflections, diffractions etc of the same

object i.e. a ray hits a surface of the object, undergoes reflection

resulting in a reflected ray which can again hit a surface, corner or

edge creating a reflected ray, diffracted ray etc .

Because multiple interactions with the object is possible, I want to

make the raytracing function recursive.Now, it is possible that the

program can get trapped in an infinite recursion due to rays

constantly bouncing of some triangular surface of the object and new

rays keep getting created. Hence, I use a counter called 'depth' in my

ray data structure. In my program, the maximum depth for a ray is 2.

eg. assume the ray coming from source has a depth of 0, and if the ray

hits an object, the reflected ray would have depth 1. If this

reflected ray hits another surface, once again a reflected ray is

created with depth 2. Child ray's depth = parent ray's depth + 1.

In my program, some calculations need to be performed. Like when a ray

hits some triangular surface, the incident electric field vector

because of this ray must be added to the total incident electric field

vector. Similarly, when the ray exits an object(does not intersect any

triangular surface on the object), we want to calculate some scattered

electric field vector and add it to the total scattered electric field

vector. Let's say if a ray has the maximum depth i.e. 2 and it still

intersects some triangular surface, then the recursion must stop. If

it doesn't intersect any traingular surface, calculate the scattered

field and return.

The data structure for ray :

typedef enum

{

PRIMARY_RAY,

REFLECTED_RAY,

EDGE_DIFFRACTED_RAY,

CORNER_DIFFRACTED_RAY;

}raytype;

The ray type is used to distinguish between various types of rays i.e.

primary rays, reflected

rays, edge diffracted rays, corner diffracted rays.

typedef struct

{

int depth; /* the depth field as I explained */

vector origin; /* origin of ray */

vector direction; /* direction vector of ray */

vector efield; /* electric field at origin of ray */

double t; /* distance travelled */

raytype type; /* type of ray */

}ray;

I will post the skeleton of some functions I have written :

First, we want to create primary rays or rays originating from the

source having depth 0.

int calc_e_fields()

{

int i;

ray *r

for (i = 0; i < NUMBER_OF_RAYS; i++)

{

create_primary_ ray(&r)

raytrace(r);

free(r);

}

return 0;

}

With the above function, I create as many primary rays as specified by

user, and trace each

and every one of them(including their children rays). For all primary

rays, ray depth = 0 and raytype = PRIMARY_RAY

The ray trace function is recursive -

void raytrace(ray *r)

{

size_t index; /* Index of triangle intersected */

bool res = false; /* result of ray-surface intersection */

double u, v; /* barycentric coordinates used to calculate point of

intersection */

ray_kd_tree_intersect(r, &index, &res, &u, &v);

/* Above function finds out if ray has intersected triangle, index

of the intersected

triangle, and the barycentric coordinates u and v */

if (r->depth == 2)

{

/* if ray's depth is 2, then check if it intersected any

triangular surface

if it intersected a triangular surface, then return because 2

is maximum depth.

If it did not intersect, then it exited the object so

calculate the scattered field */

if (!res)

{

calc_scattered_field(r);

}

return;

}

else

{

/* ray depth is either 0 or 1 here */

if (res) /* ray intersected */

{

/* since ray intersected object calculate electric field

*/

calc_incident_field(r);

/* since ray intersected, a child ray should be created */

create_child_ray(r, u, v);

}

else

{

/* ray did not intersect */

if (r->depth == 0)

{

/* if a ray direct from source did not intersect

object, no use of tracing it */

return;

}

else

{

/* if ray did not intersect object, then it has

exited the object*/

/* so calculate the scattered field and return*/

calc_scattered_field(r);

return;

}

}

}

}

}

Now my problem is with the create_child_ray(r, u, v) function.

The child ray can be a reflected ray, edge diffracted ray or a corner

diffracted ray.

Based on the value of barycentric coordinates of the point of

intersection i.e. u, v and a third coordinate w (which is 1-u-v , u,v

are always between 0 and 1), the child ray has to be calculated.

The child ray should be a edge diffracted ray if the parent ray had

hit an edge. This is indicated by exactly one of the barycentric

coordinates being 0(any one of u, v or w can be 0 but other 2 cannot

be zero).

The child ray should be a corner diffracted ray if the parent ray had

hit a corner. This is indicated by exactly one of the barycentric

coordinates being 1 and others being 0. 3 possible situations -

u v w

1 0 0

0 1 0

0 0 1

If neither of the above conditions are satisfied, then a reflected ray

must be created as the ray ha hit a point inside the triangle surface,

neither on the edge nor any of the 3 corners of triangle.

So I can write a function create_child_ray which should call

approprate functions for creating the child rays.

void create_child_ray(ray *r, double *u, double *v)

{

double w = 1 - *u - *v;

if ( (*u == 0 && *v == 0 && w == 1) ||

(*u == 0 && *v == 1 && w == 0) ||

(*u == 1 && *v == 0 && w == 0))

{

/* ray has hit a corner so create corner diffracted ray*/

create_corner_diffracted_ray();

}

else

{

if ((*u == 0 && *v != 0 && w != 0) ||

(*u != 0 && *v == 0 && w != 0) ||

(*u != 0 && *v != 0 && w == 0))

{

/* ray hit an edge so create edge diffracted ray */

create_edge_diffracted_ray();

}

else

{

create_reflected_ray();

}

}

}

Now my question is inside the create_reflected_ray or

create_edge_diffracted_ray or create_corner_diffracted_ray functions,

can I call the raytrace function ? since the child rays also need to

be traced recursively. eg:

void create_reflected_ray()

{

ray rr; /* reflected ray */

/* initialise reflected ray */

raytrace(&rr);

}

Also, when the ray undergoes reflection, only one child ray is

created(reflected ray) but when a ray undergoes diffraction, I need to

simulate the effect with many rays each travelling in different

directions. So in a way a parent ray gives rise to many child rays

each of which must be traced. So I want to write the

create_edge_diffracted_ray or create_corner_diffracted_ray as below :

void create_edge_diffracted_ray()

{

ray edr;

int i;

for (i = 0; i < MAX_NUMBER_OF_DIFFRACTED_RAYS; i++)

{

/* initialise edr */

raytrace(&edr);

}

}

Is this permissible ??