Secondary Maps (Detail Maps) & Detail Mask 二级贴图（细节贴图）&细节遮罩
本文档主要是对Unity官方手册的个人理解与总结（其实以翻译记录为主：>）
仅作为个人学习使用，不得作为商业用途，欢迎转载，并请注明出处。
文章中涉及到的操作都是基于Unity2018.3版本
参考链接：https://docs.unity3d.com/Manual/StandardShaderMaterialParameterDetail.html
Secondary Maps (or Detail maps) allow you to overlay a second set of textures on top of the main textures listed above. You can apply a second Albedo colour map, and a second Normal map. Typically, these would be mapped on a much smaller scale repeated many times across the object’s surface, compared with the main Albedo and Detail maps.
二级贴图(或细节贴图)允许您在上面列出的主贴图之上覆盖第二组纹理。您可以应用第二个反照率颜色贴图，和第二个法线地图。通常，与主要的反照率和细节图相比，这些会在一个小得多的缩放尺寸上在物体表面重复多次。
The reason for this is to allow the material to have sharp detail when viewed up close, while also having a normal level of detail when viewed from further away, without having to use a single extremely high texture map to achieve both goals.
这样做的原因是，当近距离观看时，材质有清晰的细节，而当远距离观看时，也有正常的细节水平，而不需要使用一个非常高的纹理贴图来实现这两个目标。
Typical uses for detail textures would be: - Adding skin detail, such as pores and hairs, to a character’s skin - Adding tiny cracks and lichen growth to a brick wall - adding small scratches and scuffs to a large metal container.
细节纹理的典型用途是:-在角色的皮肤上添加皮肤细节，比如毛孔和毛发-在砖墙上添加细小的裂缝和地衣生长-在一个大的金属容器上添加小的划痕和磨损。
This character has a skin texture map, but no detail texture yet. We will add skin pores as a detail texture.
这个角色有一个皮肤纹理贴图，但是还没有细节纹理。我们将添加皮肤毛孔作为一个细节纹理。
The Albedo skin pore detail texture
反照率皮肤毛孔细节纹理
The end result, the character now has subtle skin pore detail across her skin, at a much higher resolution than the base Albedo or Normal map layer would have allowed.
最终的结果是，这个角色的皮肤上有了微妙的毛孔细节，比基础反照率或法线贴图层所允许的分辨率要高得多。
Detail textures can have a subtle but striking effect on the way light hits a surface. This is the same character in a different lighting context.
细节纹理可以在光线照射表面的方式上产生微妙而显著的效果。这是相同的角色在不同的照明环境。
If you use a single normal map do ALWAYS plug it into the primary channel. The Secondary normal map channel is more expensive than the primary one but has the exact same effect.
如果你使用一个法线贴图，一定要把它插入主通道。第二法线贴图通道比第一法线贴图通道更昂贵，但具有完全相同的效果。
Detail Mask 细节遮罩
The detail mask texture allows you to mask off certain areas of your model to have the detail texture applied. This means you can show the detail texture in certain areas, and hide it in others. In the example of the skin pores above, you might want to create a mask so the pores are not shown on the lips or eyebrows.
细节遮罩纹理允许你遮罩掉模型的某些区域以应用细节纹理。这意味着您可以在某些区域显示细节纹理，并在其他区域隐藏它。在上面的皮肤毛孔的例子中，你可能想要创建一个遮罩，这样毛孔就不会出现在嘴唇或眉毛上。
