All images from The Magic of Houdini by Will Cunningham. Reprinted with permission.

This is the next in a series of excerpts from the Thomson Course Technology book The Magic of Houdini by Will Cunningham. In the next few months VFXWorld readers will learn the basics of the dominant tool that has been used in the creation of some of the most awe-inspiring animation and cinematic effects ever made.

You have already read in a number of spots weird words like SOPs and POPs. But, what exactly are these near silly sounding names? These strange utterances are short names for the contexts available. Houdini is comprised of a number of contexts. Each of the different contexts has different capabilities and you move back and forth between them to achieve different objectives. Each context has a unique color associated with its menus. This section briefly describes what each of the contexts do so that when you hear them elsewhere in the book, you will have a general understanding of what you can do with each of them. For example, if you see the word POPs, you will know that particles are involved.

OBJs (Objects Operators)
The Objects context, shown in Figure 1, is considered the top level of your Houdini file. Some Object types are like containers that hold geometry operations and other things inside them. For example, you can lay down a Geometry Object and name it apple. Inside the apple Object, you can put all the geometry and shaders you used to create it. This can help keep your files organized and can also enable you to create a digital asset out of the object. For more information on digital assets, refer to Chapter 11, “Digital Assets.”

There are numerous Object types each with different capabilities. You create the camera and lights for your scene at this level, create bones for rigging a character and more. Generally, though, the Objects context (also sometimes called level or editor) is where high-level operations and organization occur. You might say this is where you set the table with plates, napkins, utensils and condiments.

	[Figure 1] The context menu showing the Objects context.
	[Figure 2] The context menu showing the Geometry context.
	[Figure 3] The context menu showing the Dynamics context.
	[Figure 4] The context menu showing the Particles context.

In this context, networks don’t function the same as in many other contexts. In this context, connecting a node to another node is creating a hierarchical chain of parent and child. In this relationship, the child inherits all of its parents and its predecessors transform (translates, rotates and scales). So, you can easily see how Bone objects are used to create skeletal rigs with a foot being parented to an ankle, and an ankle parented to a leg and so on. In most other contexts, connecting nodes together forms a network for passing and computing the aggregate of the operations. This is an important difference to remember.

There are also what I call Object states. These are a bit confusing as they are operations that you can access in the Tab menu over the viewport but not in the Tab menu over the Network Editor. These states often don’t create accompanying nodes at the Object level and sometimes don’t create nodes at all. I discuss this in more detail in Chapter 4, “Object Operators.”

SOPs (Geometry Surface Operators)
The Geometry context, shown in Figure 2, is always tied to an Object level node and is contained within it. This is where all the operations occur that actually create and manipulate the geometry that you unwaveringly weave — perhaps it’s a Christmas sweater. This context is considered the meat and potatoes for beginners and often experienced users alike, because they spend much time here whiling away the hours in wonderment. For the vegetarians amongst you, it can just as easily and credibly be considered the tofurkey and potatoes context. Hey, I’ve had tofurkey on a number of occasions and it is certainly an adventure!

DOPs (Dynamic Operators)
In this context, you can create dynamic simulations using solvers that simulate rigid body dynamics, soft body dynamics and so on. This is where you define the objects being used, the forces at play and the type of dynamics solution you want to utilize. Just like SOPs, DOP networks can only exist within containers like Objects. The DOPs context menu is shown in Figure 3.

POPs (Particle Operators)
As you progress a little further in your knowledge of the package, the POPs context will likely gain in importance and usage and will become considered the salad and gravy context. The gravy is for the potatoes, not the salad. I say the gravy is for the potatoes because POPs are often intimately connected with SOPs and use them as birthing sources, affecting forces, destinations for passing attributes and more. Ahh, the life of a particle is such a thing to behold. The POPs context menu is shown in Figure 4.

Houdini’s particle operators are undoubtedly the strongest and most open set of tools available for creating a wide array of original and challenging effects. Particle simulations are quite adroit in mimicking natural phenomenon and imagined phenomenon, and can be used for all sorts of trickery, where it seems no other solution will do.