Re: need help with arrays

pereges <Broli00@xxxxxxxxx> writes:

Ok, I have some problem with arrays which i want to use for storing
rays in my ray tracing project. please have a little patience to read.
<snip>
unsigned long int numberofrays;

so

incr = sqrt(l * b / numberofrays)


now lets say the plane is represented by four corners:

xmin, ymin
xmax, ymin
xmin, ymax
xmax, ymax

all are doubles

My ray data structure is :

typedef struct ray_struct
{

double ox, oy, oz; // ray origin or source represented by the
3 x, y, z coordinates
double t; //distance travelled
double dx, dy, dz; // ray direction represented by 3 x, y, z
coordinates

}ray;

Now, this is what I did to create the list of rays:

ray *ray_list;

ray_list = calloc(sizeof(ray), numberofrays);

Better use malloc. There is no advantage in zeroing (arithmetically)
the pointers.

Then, I try to store the rays:

double xcord, ycord; // represents the origin of a random ray
int i = 0;

for(ycord = ymin; ycord <= ymax; ycord += incr)
{
for(xcord = xmin; xcord<= xmax; xcord+= incr)
{

ray_list[i] does not point anywhere. You need to either malloc a ray
structure for each one, or use a 2D array in the first place.

ray_list[i].ox = x;
ray_list[i].oy = y;
ray_list[i].oz = 1000;
ray_list[i].dx = 0; // all rays parallel to each other and
travelling in +z direction
ray_list[i].dy = 0;
ray_list[i].dz = 1;
ray_list[i].t = DBL_MAX; // rays go till infinity
i++;
}

}

Also, watch our for you <=. Take a 3 x 4 rectangle with 48 rays and
incr = 0.5. If you go <= you get both an extra row and ne more ray
per row:

X--X--X--X--X--X--X--X--X
| | | | |
X X X X X X X X X
| | | | |
X--X--X--X--X--X--X--X--X
| | | | |
X X X X X X X X X
| | | | |
X--X--X--X--X--X--X--X--X
| | | | |
X X X X X X X X X
| | | | |
X--X--X--X--X--X--X--X--X

That is 63 rays, not 48!

<snip>
One other problem I have is that in my project, it is possible that a
ray hits an object and a reflected ray is generated and this
reflected ray hits the object again and spawns a new ray. I need it
for some energy calculations and this allows me to traverse the entire
optical path of a ray. I was wondering if it is ok to have child
pointer to solve this problem :

typedef struct ray_struct
{
........
........
struct ray_struct ray *child;

struct ray_struct *child;

Yes, that is OK. But remember to allocate space for it.

}

I also want to ask, how useful is it to have doubles in a numerical
simulation program(high accuracy is desired) ? Or is it ok to use
floats instead ? Using doubles has really slowed down my program.

There is no general rule. If you replace all your doubles with a
typedef name you can test the difference when you are done with a
single change.

Another thing I have noted is that its best to have variables with
scopes as small as possible. I still wonder why many people prefer to
use :

extern const double PI = 3.14

over

#define PI 3.14

shouldn't the second option be more efficient ?

See recent discussion of const.

--
Ben.
.

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