modifiers

Cast Modifier

This modifier shifts the shape of a mesh, curve, surface or lattice to any of a few pre-defined shapes (sphere, cylinder, cuboid).

videos

Notes:

https://docs.blender.org/manual/en/dev/modeling/modifiers/deform/cast.html?highlight=modifiers

It is equivalent to the To Sphere tool in Edit Mode Mesh ‣ Transform ‣ To Sphere, Alt-Shift-S and what other programs call “Spherify” or “Spherize”, but, as written above, it is not limited to casting to a sphere.

Decimate Modifier

The Decimate Modifier allows you to reduce the vertex/face count of a mesh with minimal shape changes.

video

Notes:

https://docs.blender.org/manual/en/dev/modeling/modifiers/generate/decimate.html

Duplication is not a modifier

There are currently four ways in Blender to procedurally duplicate objects. These options are located in the Object menu.

videos

Notes :

Yes, it’s not a modifier. But it’s so modifier like I put it here.

manual – duplication

Vertices

This creates an instance of all children of this object on each vertex (for mesh objects only).
Faces: This creates an instance of all children of this object on each face (for mesh objects only).
Group: This creates an instance of the group with the transformation of the object. Group duplicators can be animated using actions, or can get a Proxy.
Frames: For animated objects, this creates an instance on every frame. As you will see on this topic’s subpage, this is also a very powerful technique for arranging objects and for modeling them.