从lookat生成view矩阵

static void buildLookAtMatrix(double eyex, double eyey, double eyez,
                              double centerx, double centery, double centerz,
                              double upx, double upy, double upz,
                              float m[16])
{
  double x[3], y[3], z[3], mag;

  /* Difference eye and center vectors to make Z vector. */
  z[0] = eyex - centerx;
  z[1] = eyey - centery;
  z[2] = eyez - centerz;
  /* Normalize Z. */
  mag = sqrt(z[0]*z[0] + z[1]*z[1] + z[2]*z[2]);
  if (mag) {
    z[0] /= mag;
    z[1] /= mag;
    z[2] /= mag;
  }

  /* Up vector makes Y vector. */
  y[0] = upx;
  y[1] = upy;
  y[2] = upz;

  /* X vector = Y cross Z. */
  x[0] =  y[1]*z[2] - y[2]*z[1];
  x[1] = -y[0]*z[2] + y[2]*z[0];
  x[2] =  y[0]*z[1] - y[1]*z[0];

  /* Recompute Y = Z cross X. */
  y[0] =  z[1]*x[2] - z[2]*x[1];
  y[1] = -z[0]*x[2] + z[2]*x[0];
  y[2] =  z[0]*x[1] - z[1]*x[0];

  /* Normalize X. */
  mag = sqrt(x[0]*x[0] + x[1]*x[1] + x[2]*x[2]);
  if (mag) {
    x[0] /= mag;
    x[1] /= mag;
    x[2] /= mag;
  }

  /* Normalize Y. */
  mag = sqrt(y[0]*y[0] + y[1]*y[1] + y[2]*y[2]);
  if (mag) {
    y[0] /= mag;
    y[1] /= mag;
    y[2] /= mag;
  }

  /* Build resulting view matrix. 行主序*/
  m[0*4+0] = x[0];  m[0*4+1] = x[1];
  m[0*4+2] = x[2];  m[0*4+3] = -x[0]*eyex + -x[1]*eyey + -x[2]*eyez;

  m[1*4+0] = y[0];  m[1*4+1] = y[1];
  m[1*4+2] = y[2];  m[1*4+3] = -y[0]*eyex + -y[1]*eyey + -y[2]*eyez;

  m[2*4+0] = z[0];  m[2*4+1] = z[1];
  m[2*4+2] = z[2];  m[2*4+3] = -z[0]*eyex + -z[1]*eyey + -z[2]*eyez;

  m[3*4+0] = 0.0;   m[3*4+1] = 0.0;  m[3*4+2] = 0.0;  m[3*4+3] = 1.0;
}