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sphermap.cpp

// -----------------------------------------------------------------------
// sphermap.cpp
// -----------------------------------------------------------------------
// Code to render a sphere 
// From Pixelate #11, article written by Martijn 'amarillion' van Iersel 
// -----------------------------------------------------------------------
// Modified by Kronoman to suit the needs of the game.
// -----------------------------------------------------------------------

#include <stdio.h>

#include "sphermap.h"

/*
get_planet_rotation_matrix() is a little helper function to calculate a 
rotation matrix for a planet. An ordinary rotation matrix doesn't work well
for planets.

MATRIX *m: pointer to the resulting MATRIX struct
fixed rotation: rotation of the planet around its axis of rotation
fixed axisx, axisz: rotation around the x and z axes.
*/
void get_planet_rotation_matrix (MATRIX *m, fixed rotation, fixed axisx, fixed axisz)
{
    MATRIX m1, m2;
    // apply rotation around y axis first
    get_y_rotate_matrix (&m1, rotation);
      
    // then apply rotation around x and z axes.
    get_rotation_matrix (&m2, axisx, 0, axisz);
      
    matrix_mul (&m2, &m1, m);
}

/*
    get_mapped_sphere_ex() is similar to get_mapped_sphere(), but also takes
    a rotation matrix as argument to draw a rotated sphere.

    BITMAP *target = bitmap to display onto
    int cx, cy = center of the sphere
    int r = radius of the sphere
    BITMAP *map = bitmap to map onto the sphere
    MATRIX *rotmat = rotation matrix
*/
void mapped_sphere_ex (BITMAP *target, int cx, int cy, int r, BITMAP *map, MATRIX *rotmat)
{
    int x, y; // coordinates on the target bitmap
    int p, q; // coordinates on the source bitmap    
      
    for (y = -r; y < r; y++)
    {
        fixed q_cos = fixcos (- fixasin (itofix (y) / r)) * r;
        for (x = - fixtoi (q_cos) + 1; x < fixtoi(q_cos) - 1; x++)
        {
             fixed newq_cos, temp_p, temp_q=0; // some temporary variables
             fixed newx, newy, newz; // x, y and z after rotation             
             fixed z; // z before rotation. we don't have to calculate x and y
             
             // calculate z
             // using pythagoras and the formula for a sphere:
             // x^2 + y^2 + z^2 = r^2
             // we know x, y and r
             z = fixsqrt (r * itofix (r) - x * itofix (x) - y * itofix (y));
                 
             // apply the rotation matrix to x, y, z. 
             // put the result in newx, newy, newz
             apply_matrix (rotmat, itofix(x), itofix(y), z, &newx, &newy, &newz);

             //just as in sphere2.c, we need to check if q_cos is 0
             //however, q_cos depends on y, and we just calculated a new y
             //thus we have to calculate q_cos again.
             newq_cos = fixcos (temp_q) * r;
             if (newq_cos != 0)
             {
                 // it is possible to use temp_p 
                 // temp_p = fixasin (fixdiv (itofix (x), q_cos));
                 // however, I found I get less rounding errors if I use
                 // fixatan2 instead. The principle remains the same.
                 temp_p = fixatan2 (fixdiv (newx, newq_cos), fixdiv (newz, newq_cos));                 
             }
             else
                 temp_p = 0;
                   
             temp_p &= 0xFFFFFF;

             p = fixtoi (temp_p) * (map->w-1) / 256;

             // calculate q
             temp_q = fixasin (newy / r);             
             q = fixtoi (temp_q + itofix (64)) * (map->h-1) / 128;
             
             putpixel (target, x + cx, y + cy, getpixel(map, p, q) );
        }
    }
}


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