Yunhai Wang Jian Zhang Wei Chen Huai Zhang Xuebin Chi Efficient opacity specification based on feature visibilities in direct volume rendering. 2117-2126 2011 30 Comput. Graph. Forum 7 http://dx.doi.org/10.1111/j.1467-8659.2011.02045.x db/journals/cgf/cgf30.html#WangZCZC11