In view cone snippet

From Valve Developer Community

bool CMyEnt::InViewCone( const Vector &vecSpot, float tolerance )
{
    Vector los = ( vecSpot - GetAbsOrigin() );
    los.z = 0;
    VectorNormalize( los );
    Vector facingDir;
    AngleVectors( GetAbsAngles(), &facingDir );
    float flDot = DotProduct( los, facingDir );

    if ( flDot > tolerance )
	return true;
    return false;
}