A simple linear gradient is easy: draw a quadrilateral polygon with two of its vertices set to one color and two of its vertices set to a second color.
简单线性梯度很容易：画一个四边形多边形的两个顶点设置为一个颜色和它的两个顶点设置为第二个颜色。
2
The model may be various non-simple topological structures and contain any combinations of beams, triangular elements and quadrilateral elements.