并非

Such substances are optically anisotropic and birefringent.

这种材料是光学各向异性的，因而是双折射晶体。

This paper firstly describes the data structure of a DTM, then color shading method, and finally some examples to demonstrate the application of a color shaded DTM in geological investigation.

本文介绍数值地形模型的资料结构，彩绘明暗图的制图原理，并举例说明彩绘明暗图在地质判释上的应用。