常量
NGraphics|MAP_OBJECT_OCCLUSION_LAYERS
which map object layers should be occluded by things like cities and roads. Will only hide mesh-type map objects (i.e units, cities, entities, vfx etc won't be hidden)
定义loading_screen/common/defines/graphic/00_graphics.txt:177
"vegetation_low"
"vegetation_medium"
"vegetation_high"