c++ mercator projection route approximation

mercator_route.cpp

current_point = this->from;
/* just a direction */
this->sign = ( ( this->from.y - this->to.y ) > 0 ) ? -1 : 1;

for ( int j = 0; j < this->steps; ++j ) {
    float lon = ( current_point.y + this->sign * this->step_lon ) * M_PI / 180;
    float lat = atan( ( tan( this->lat1 ) * sin( this->lon2 - lon ) / sin( this->lon2 - this->lon1 ) ) + ( tan( this->lat2 ) * sin( lon - this->lon1 ) / sin( this->lon2 - this->lon1 ) ) );
    Vec2f next_point = Vec2f( lat * 180 / M_PI, lon * 180 / M_PI );
    /* draw line */
    current_point = next_point;
}