OpenGL Billboard

billboard.h

// http://marina.sys.wakayama-u.ac.jp/~tokoi/?date=20110221

inline void billboard()
{
	GLfloat m[16];
	glGetFloatv(GL_MODELVIEW_MATRIX, m);
	
	float inv_len;
	
	m[8] = -m[12];
	m[9] = -m[13];
	m[10] = -m[14];
	inv_len = 1. / sqrt(m[8] * m[8] + m[9] * m[9] + m[10] * m[10]);
	m[8] *= inv_len;
	m[9] *= inv_len;
	m[10] *= inv_len;
	
	m[0] = -m[14];
	m[1] = 0.0;
	m[2] = m[12];
	inv_len = 1. / sqrt(m[0] * m[0] + m[1] * m[1] + m[2] * m[2]);
	m[0] *= inv_len;
	m[1] *= inv_len;
	m[2] *= inv_len;
	
	m[4] = m[9] * m[2] - m[10] * m[1];
	m[5] = m[10] * m[0] - m[8] * m[2];
	m[6] = m[8] * m[1] - m[9] * m[0];
	
	glLoadMatrixf(m);
}