HISMC vs ISMC
ISMC’s render all of their instances or none of them (if they are offscreen or occluded). HISMC’s are internally culled so that if most of the instances are off screen or beyond the cull distance, then most of them are not rendered.
ISMC’s choose one LOD for all instances; this does not work well. HISMC’s internally make LOD decisions so some of the instances can be at each LOD.
HISMC’s supported dithered temporal LOD transitions; no popping.
HISMC’s in 4.7 do NOT support occlusion culling very well…as well as ISMC’s, but the problem with 4.7 has to do with the foliage tool. In 4.6, the foliage tool would create multiple ISMC’s and these would be culled (including occlusion culling) as normal. The number of ISMC’s created was related to the cluster size. For small instance counts this could work quite well. For a million instances, clusters would be both too big and too small; you would neither get good culling, nor good CPU performance. With HISMC’s, this is fixed; there are no clusters and the foliage tool only creates one component, however, that means all million instances have only one occlusion query; so if you are in a cabin in the woods you are still going to be rendering the entire forest. This is fixed in 4.8.
HISMC’s remain a bit rough in 4.7 but they should be usable and unless you have very few instances with no LOD occupying a small part of the world, they will be superior both for CPU and GPU costs…though dithered LOD transitions are not free.
epic