如果那个点等于或低于给定的海拔高度，则那块贴图就标为蓝色，否则就还是透明的。
If that coordinate is at the given level or below, then the tile is coloured blue, otherwise it is left transparent.
原始的材质贴图坐标和高度数据已经在切线空间中了，因此视向量最好也能如此。
The original texture coordinate and height values are already represented in tangent space, so the eye vector must be as well.
一张标准的高度贴图被用来表现表面多变的高度。
A standard height map is used to represent the varying height of the surface.
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