/*
   listing1.c: This file holds some simple pseudo-code that can be used to
               calculate the focal-distance and the angle between subsequent
               rays for various projection-plane dimensions.

   AUTHOR: Peter Restall for my racasting tutorial
*/

/* Include header files */
#include <math.h>			/* For the 'tan()' function */

/* Make sure that 'PI' is defined (some implementations don't supply it) */
#ifndef PI				/* No 'PI' constant */
#ifdef M_PI				/* Check for an 'M_PI' definition */
#define PI	M_PI			/* Use the one supplied in 'M_PI' */
#else					/* No definition of pi found */
#define PI	3.1415926535
#endif
#endif

/* The trig functions usually take angles in radians, so this macro converts
   degrees to radians */
#define deg2rad(x)      ((x) * (PI / 180.0))

/* Define the two variables to take the computations (they're global because
   they're widely used) */
float   focal_distance,                 /* The distance to the view plane */
        ray_increment;                  /* Angle between subsequent rays */

/*
   void calculate_focal_distance(int width, int fov):

   This function takes 'width' and 'fov' as arguments, which are the projection
   plane's width and field-of-view, respectively.  It then calculates the
   distance to the projection plane (put into 'focal_distance') and the angle
   between subsequent rays (which is put into 'ray_increment').
*/

void calculate_focal_distance(int width, int fov)
{
        focal_distance = (width / 2) / tan(deg2rad(fov / 2));
        ray_increment  = fov / width;
}