CGI/3D Glossary | What does… mean? Part 2

This glossary is here to give quick explanations and answers to CGI terms you may not fully understand.

What is Index of Refraction / IOR?

The Index of Refraction is a value used to specify the way that light is scattered as it passes through a material.  You should be most familar with the ‘bent straw’ effect when viewing a clear glass of waterwith a straw inside.  That is because the light traveling through the glass of water is behaving differently than when it travels through air.  Since we live on Earth, all IOR values assume that 1.0 is the value for traditional linear light behavior.  And thus, air has an IOR of 1.0.  Water has an IOR of 1.33.  In the glass example, do you know how many different IORs would need to be considered, in order to accurately render the scene so that it matches reality? 

Three: 1.00 for the air, 1.33 for the water in the glass, and 1.52 for the water glass.  That will get you a nice result.  Why just nice?  Well the straw would also have an IOR. all materials in our universe have an IOR, even those that we may percieve as opaque.  This is why you can use Fresnel shaders and SSS shaders on anything.

What is Key Interpolation?

Interpolation is a process used by the computer to determine how to treat the frames that exist between each keyframe created by the animator.  Essentially, the computer is guessing how those in-between frames will behave.  In some cases, an animator may use few keyframes for a particular element, and let the computer do the work.  In other cases, an animator will create many keyframes in a shot, with few interpolated frames in between.

Motion Keyframes (position, rotation, scale) offer the following Key interpolation types:

A smooth transition from keyframe to keyframe will be the result.
A straight transition from keyframe to keyframe, with abrubt changes as defined by the keyframes themselves.  Effective for maintaining velocities while rotating.

This method jumps to the keyframe value at a given time.  In the graph editor, a literal step shape is drawn between each keyframe.  Animators use this type of interpolation when performing pose to pose animation, as this method allows the animator to see only the keyframes, but in the proper time as they appear within the timeline.  Final animation rarely uses Step, as it is just a tool to aid in character posing.

What is a Mesh?

A Mesh is another term for the net of geometric polygons that make up the objects in your scene. Meshes reference, or are referenced by attribute tags such as surfacing and UV information, animation rigs, simulation and rendering engines. Without meshes, you would have only parametric objects to render. Meshes are formed by vertices and the faces that are created from a minimum of three vertices. Edges bridge the connection between two or more vertices. You should avoid free-standing vertices, edges or polygons in your mesh, unless said elements are intended to serve a specific purpose.

What is MoGraph?

MoGraph, included in CINEMA 4D Broadcast, Studio, is the special motion graphics portion of Cinema 4D. MoGraph excels in the cloning of objects, driving of motion, and the creation of exciting elements often used within motion graphics sequences. Of course, MoGraph also has excellent text capabilities, and when all of these features are used in conjunction with the features already present in Cinema 4D, you have a real motion graphics powerhouse of a toolkit.

What is Motion Blur?

Motion Blur is an artifact that is seen in recorded images when a subject moves during the exposure of a frame of film or image.  In reality, you can experience motion blur just by quickly waving your hand in front of your face, but we are more concerned with the traditional photographic artifact.

Motion blur is affected by exposure time, shutter speed and film speed.  In computer graphics, the best form of motion blur is often called “3D motion blur” since it is the closest to the experiences of the analog world.  3D motion blur will correctly blur subjects in motion, shadows, and NOT blur reflections inappropriately.  3D motion blur is a physically accurate motion blur.

The amount of resources required to produce an accurate 3D motion blur is quite expensive, though fortunately modern computers have brought the effect within the reach of most artists.  It boils down to a quality/time question that is in need of an answer.  However, even though 3D motion blur is the most accurate, accuracy is not always necessary.

What is a Node?

Node refers to a scene element or sub-element.  A node is a generic container that can hold different types of information, depending upon the node type (and there are effectively an infinite amount of node types that an application or development engineer can create.

Nodes can hold transform information, geometric information, shading information and so on.  In addition to being containers of information, nodes can also be operators, and can perform math functions, for example.  In some cases, a node can be a simple add node, where you can have one node add to another, or add a value to the value already present in the operator node.  You can also have more complex nodes that can solve various levels of complex problems—in reality, these complex nodes are typically made of simpler nodes put together to form a higher level operation.

When nodes are chained together, the result is called a “node tree.”  Node trees can be simple to very complex. In order to create a node chain or node tree, you must pipe the output of one node to the input of another.  The output of that node would be dependent upon the goal of that branch of the node tree.

Nodes may have multiple inputs and outputs.  Depending upon the actual contents of the node, its possible that the numbers of inputs and outputs of the node is actually variable.

Because of all of this versatility, nodes are commonly used for digital compositing and the internal datastructures of 3D content creation applications.  In some cases, the nodes may actually be disguised from the user, but its far more powerful to allow the user to work with the actual nodes themselves.

Xpresso is the node editor in Cinema 4D, and Thinking Particles also relies on nodes to build rules for the particle systems and interactions.  From the online documentation: “Nodes are the primary building blocks of expressions and are designed to carry out the most diverse of tasks, from reporting an object’s current position to processing math operations. Depending on the node’s type, you can add various inputs and outputs to the node called ports. As with XGroups, you add these ports using the inputs menu and outputs menu.”


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