Clipmaps

Clipmaps using layer mask 

Clipmaps (or billborads) are easy to create. Use the layer mask image to make an object visible in some parts and invisible in others. Clipmaps are useful for leaves, decals, perforations in objects, and even to simulate objects using a single-textured plane (people, vegetation, etc).

First, create a material as usual (reflectance, transmittance, Nd, roughness, etc). Then load the clipmap to the layer weight texture slot. White areas show the material as it is and darker areas make the layer vanish slightly. Pure black areas will make the layer completely invisible, allowing you to see the layers underneath.
If there is only one layer in the material, the black areas of the clipmap will make the object invisible. 

Creating a billboard by simply mapping the Opacity of a material layer with a map 

Find more clipmapped examples at the MXM Gallery.