Modifiers are the basic tools for modeling and reshaping objects. Modifiers can be categorized in two different ways: whether they work on explicit topology or parametrically, and the space that they work in. In the latter grouping, there are two basic types: object-space modifiers and world-space modifiers; for definitions of these spaces, see Object Space and World Space.
A large number of modifiers ship with the software. Only selected modifiers are mentioned here as examples of their type. See the documentation on specific modifiers for details and parameters. You can also find a complete list of modifiers in List of Available Modifiers.
Topologically Dependent Modifiers: Perform operations on explicit, topological sub-object selections. The Edit modifiers and Mesh Select are examples of modifiers that perform operations or selections on explicit vertex or face numbers. When these modifiers are present in the stack, you can adversely affect their results if you visit previous stack operations and change the topology (the number and order of faces and vertices) being passed to them. When you do this, a topology-dependence warning alerts you to the situation.
Constructive operations can still occur below these modifiers if you do not change the topology being passed up the stack to them. For example, you can use a Mesh Select modifier to define a face selection that is then passed to another modifier. You can return to the object's base level (for example, editable mesh) and safely ignore the warning as long as you restrict yourself to nondestructive operations like transforming vertices, turning edges, or changing surface properties.
Parametric Modifiers: Affect the selection passed to them without explicitly depending upon topology. Examples include Twist and Bend. These modifiers can be layered as much as you want in order to accomplish a result and can usually be independently adjusted and animated at any later time. A common practice is to keep operations parametric for as long as possible to ensure maximum flexibility.
Object-space modifiers operate explicitly on topology or generically in object/local space, and constitute the largest set of modifiers. The following list is a rough grouping of the different kinds of object-space modifiers. Some modifiers fall into two or more groups. Many of these same modifiers are available from the Tab Panel as well.
Explicit Editing: Is performed either at the base object level (Editable Mesh, Editable Patch, Editable Spline, Editable Poly) or within a modifier (Edit Mesh, Edit Spline, Edit Patch). Edits are explicit rather than procedural, meaning actions freely affect the geometry's status rather than behave as a defined procedure on a generic selection. While you usually can undo an edit operation, you cannot revisit or animate the edit in a later session.
Most explicit modeling is done at the base, "editable" level. The Edit modifiers duplicate the functionality of the base level and let you store a series of edits. A common technique is to add an Edit modifier, perform a series of operations, and then collapse the object to editable status once you are satisfied with the result.
Mesh Modifiers: Can act on any object in the scene, because all geometric classes in gmax can be converted to a mesh. Non-mesh objects assigned these modifiers (patches, splines) are converted to a mesh from that point on in the modifier stack. The Mesh Select and Volume Select modifiers define either explicit or parametric sub-object selections that are passed up the stack for other modifiers to act upon.
Patch Modifiers: Edit or create patches.
Spline/Shape Modifiers: Require a spline shape to act upon (splines and shapes are the same thing). Some modifiers retain the spline geometry in the stack (Edit Spline), while others change it to a mesh, and still others let you choose of the type of geometry they produce (Lathe).
Surface Modifiers: Define surface properties used primarily by the renderer. With the exception of UVW Map, they convert the geometry to a mesh for subsequent operations. Mapping modifiers work in conjunction with the map coordinates within materials, and are applied to the face selection passed to them in the modifier stack.
Several surface modifiers (Normal, Material, Smooth) offer subsets of the editable mesh feature set, and are provided so their operations can be kept parametric. You commonly use these modifiers in conjunction with other modifiers that define sub-object selections.
General Modifiers: Affect the geometry passed to them generically, treating mesh/patch/spline vertices as simple points. Any geometry can pass through these modifiers without changing type. If you start with one type of base geometry, add any or all of these modifiers, and collapse the stack, you're left with the base geometry you started with.
World-space modifiers (WSMs) are applied like other modifiers and are carried with the object. The primary difference is that they base their effects on world space instead of object space. For example, Map Scaler, a world-space modifier, maintains the scale of a map applied to the object. As you increase the object's size, the map maintains a constant size based on world coordinates. Without Map Scaler, the pattern would enlarge with the object.
World-space modifiers in gmax include Map Scaler.
Most modifiers share a number of common features. A modifier typically consists of a parameter set and one or more "sub-modifier objects" like a gizmo and a center.
Gizmo: Displays in viewports as a wireframe that initially surrounds the selected object. A gizmo acts like a mechanical apparatus that transfers the modification to the object it's attached to. You can move, scale, and rotate a gizmo as you would any object, altering the effect of the modifier on the object.
Center: Marked with a 3D-pivot point like other objects in gmax. You can move a modifier's center, altering the effect of the modifier on the object.
Gizmos and centers are sub-object features you access from the modifier stack. For more information on using them, see Modifier Stack.