gluPickMatrixは

OpenGL 
void GLAPIENTRY
gluPickMatrix(GLdouble x, GLdouble y, GLdouble deltax, GLdouble deltay,
		  GLint viewport[4])
{
    if (deltax <= 0 || deltay <= 0) {
	return;
    }

    /* Translate and scale the picked region to the entire window */
    glTranslatef*1 / deltax,
	    (viewport[3] - 2 * (y - viewport[1])) / deltay, 0);
    glScalef(viewport[2] / deltax, viewport[3] / deltay, 1.0);
}

となっているみたい。

*1:viewport[2] - 2 * (x - viewport[0]