Programmers spend a lot of time developing tools for their artists to use, but the question of how best to develop the tool they need is not always an easy one to answer. The first time I had to develop a plug-in for 3D Studio Max, I was faced with a choice: "What kind of plug-in should I develop - a utility or a modifier?" I then spent a considerable amount of time performing tests to determine which would be the best way to go. Now, lucky for you, rather than going through that same process yourself, you can just read this article, which will hopefully help shorten that testing time.

The main topic of this article, then, will be the difference between developing a modifier and a utility plug-in. We'll discuss the advantages and drawbacks of each of these plug-in types, as well as the use of the Microsoft Foundations Class (MFC) inside a utility. Although this article could potentially be of interest to anybody working with 3D Studio Max, I'd recommend that the reader have a basic knowledge of developing 3D Studio Max plug-ins in order to get the most out of this. A good resource to check out for a little more background is the 1998 Gamasutra article "From 3D Studio Max to Direct3D" by Loic Baumann.

Comparing Modifier and Utility Plug-Ins

First, the basics. A modifier is a plug-in that modifies objects (also called nodes) in some way. A utility is a plug-in that is useful for implementing modal procedures such as 3D paint, dynamics, and so forth. The major difference is that utilities are not designed to modify objects, so you won't find a utility that, say, modifies the texture vertices of a mesh as the UVW Unwrap modifier does (although it can theoretically be done, as we will see later).

Let's first take a look at the advantages of using modifiers.

Advantages of Developing a Modifier

The biggest advantage a modifier has over a utility is the geometry pipeline. It's a little bit complicated at the beginning to understand, but once you have an understanding of how this pipeline works, modifiers can be very powerful.

An understanding of the geometric pipeline system is important for developers creating plug-ins that affect scene geometry. The pipeline is the system used by 3D Studio MAX that allows a node in the scene to be altered, perhaps repeatedly, through the application of modifiers. At the beginning of a pipeline is the Base Object. This is either a procedural object or just a simple mesh. At the end of a pipeline is the world space state of the object. This world space state is what appears in the 3D viewports and is rendered.

For the system to evaluate the state of the object at the end of the pipeline, it must apply each modification along the way, from beginning to end. For example, say a user creates a procedural cylinder in the scene, applies a Bend modifier to it, and then applies a Taper modifier to it. As the system evaluates this pipeline, it starts with the state of the cylinder object. As this object state moves along the pipeline, it first encounters the Bend modifier, and the system asks the Bend modifier to apply its deformation to the object state. The result of this operation is then passed as the source into the Taper modifier, and the Taper then applies its own deformation. The result of this operation is then passed to the system which translates the result into world space, and the state of the node in the scene is complete.

To maximize the speed with which the system can evaluate the state of a node, the system maintains a World Space Cache for each node in the scene. This world space cache is the result of the node's pipeline; it reflects the state of the object in world space after everything has been applied to it. Along with the cache, the system maintains a validity interval. The validity interval indicates the period of time over which the cache accurately reflects the state of the node.

With the cache structure inside modifiers, the user is able to remove, copy, or paste a modifier from the modifier stack. The pipeline is then re-evaluated to build a new world space state for the objects that have been modified, and we are able to collapse the modifier stack of an object. Collapsing the modifier stack means that:

1. Every modifier on the object applies its changes;
2. The resulting world space state becomes the current state of the object;
3. All modifiers are erased, so going back is no longer possible.

It's more or less like flushing the modifier pipeline.

Remember, in Kinetix's 3DS4, the user wasn't even able to change past modifications, so this is a very powerful feature. But this modifier pipeline is also modifiers' biggest drawback, as I'll explain.

Drawbacks of Developing a Modifier

Developing a modifier is no easy task, even for even experienced programmers. You have to know exactly how the pipeline works. For example, you have to tell the pipeline which channels of the object you are going to change (i.e., the channels that the modifier needs in order to perform its modification) and which channels you are going to use (i.e., the channels that the modifier actually modifies). For instance, you have the texmap channel (texture vertices and procedural mappings), the geometry channel (the vertices of the object), and so on. If you don't tell the pipeline which channels you are going to change, your changes won't affect the object. This just goes to show that you need to know exactly how the pipeline works and what you are going to do to objects before developing a modifier.

As an example of the complexity of the pipeline, let's say you want a modifier that lets the user manually select some faces and then perform several operations on those faces. To do so, you'll need to copy and paste about 800 lines from the mesh select or edit mesh modifier, and then you'll have to ensure that the code actually works in your modifier. You have to think about that before developing a modifier.

The geometry pipeline is a very nice feature when you discover it as a user, but when you use it everyday, it quickly becomes a burden. As another example, if you want to modify the texture vertices of some given number of faces, you have to first select these faces, then modify their texture vertices. To do that you'll need the edit mesh or mesh select modifier to select faces, and then you'll have to add a UVW Unwrap modifier to change the texture vertices of the selected faces. In this silly example, two modifiers have been used to perform the desired operation. And if you want to do this operation again, you will need to add these two modifiers once again. As a result, the modifier stack keeps on growing and you are forced to collapse the modifier stack regularly to avoid filling memory with the local cache of each modifier.

So while modifier plug-ins have serious benefits, they've also got some distinct disadvantages. Now let's take a look at utility plug-ins.