#include <stdlib.h>
#include <math.h>
#include "dpltypes.h"
#include "matrix.h"

float billboard_rotY=0.0f;

/*{{{  static float _atan2deg ( float dz, float dx )*/
static float _atan2deg ( float dz, float dx )
{
  float angle;

  if ((dx*dx) < 0.0001f) {
    angle=(dz>0) ? 90.0f : -90.0f;
    return angle;
  }
  else {
    return (180.0f / 3.1415926535f) * atan2 ( dz, dx );
  }
}
/*}}}  */

void billboardize ( dpl_MATRIX front, dpl_MATRIX back )
{
  float dx, dy, dz;

  dx=front[3][0];
  dy=front[3][1];
  dz=front[3][2];

  dpl_Translate ( front, -dx, -dy, -dz );
  dpl_Rotate    ( front, billboard_rotY, dpl_Y );
  dpl_Translate ( front, dx, dy, dz );
  if (back) dpl_Invert    ( back, front );

  return;
}

void billboardstuff ( dpl_MATRIX m )
{
  /* pass 0,0,-1 thru view matrix to find Y angle */
  dpl_POINT a, b;

  if (m) {
    a[0]= 0;
    a[1]= 0;
    a[2]= 0;

    dpl_XformPoint ( a, a, m );

    b[0]= 0;
    b[1]= 0;
    b[2]=-1;

    dpl_XformPoint ( b, b, m );

    billboard_rotY=90.0f - _atan2deg (b[2]-a[2], b[0]-a[0]);
  }
}