jd norton's List: Game Art terms

• Rendering is the most technically complex aspect of 3D production, but it can actually be understood quite easily in the context of an analogy: Much like a film photographer must develop and print his photos before they can be displayed, computer graphics professionals are burdened a similar necessity.
• When an artist is working on a 3D scene, the models he manipulates are actually a mathematical representation of points and surfaces (more specifically, vertices and polygons) in three-dimensional space.
• The term rendering refers to the calculations performed by a 3D software package’s render engine to translate the scene from a mathematical approximation to a finalized 2D image. During the process, the entire scene’s spatial, textural, and lighting information are combined to determine the color value of each pixel in the flattened image.

• There are two major types of rendering, their chief difference being the speed at which images are computed and finalized.

    
  1. Real-Time Rendering: Real-Time Rendering is used most prominently in gaming and interactive graphics, where images must be computed from 3D information at an incredibly rapid pace.
     
        
     • Interactivity: Because it is impossible to predict exactly how a player will interact with the game environment, images must be rendered in “real-time” as the action unfolds.
     
       
     • Speed Matters: In order for motion to appear fluid, a minimum of 18 - 20 frames per second must be rendered to the screen. Anything less than this and action will appear choppy.
     
       
     • The methods: Real-time rendering is drastically improved by dedicated graphics hardware (GPUs), and by pre-compiling as much information as possible. A great deal of a game environment’s lighting information is pre-computed and “baked” directly into the environment’s texture files to improve render speed.
• Offline or Pre-Rendering: Offline rendering is used in situations where speed is less of an issue, with calculations typically performed using multi-core CPUs rather than dedicated graphics hardware.
  
    
  • Predictability: Offline rendering is seen most frequently in animation and effects work where visual complexity and photorealism are held to a much higher standard. Since there is no unpredictability as to what will appear in each frame, large studios have been known to dedicate up to 90 hours render time to individual frames.
  
    
  • Photorealism: Because offline rendering occurs within an open ended time-frame, higher levels of photorealism can be achieved than with real-time rendering. Characters, environments, and their associated textures and lights are typically allowed higher polygon counts, and 4k (or higher) resolution texture files.

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• Box/Subdivision Modeling: Box modeling is a polygonal modeling technique in which the artist starts with a geometric primitive (cube, sphere, cylinder, etc.) and then refines its shape until the desired appearance is achieved.
  
  Box modelers often work in stages, starting with a low resolution mesh, refining the shape, and then sub-dividing the mesh to smooth out hard edges and add detail. The process of subdividing and refining is repeated until the mesh contains enough polygonal detail to properly convey the intended concept.
  
  Box modeling is probably the most common form of polygonal modeling, and is often used in conjunction with edge modeling techniques
• Edge/Contour Modeling: Edge modeling is another polygonal technique, though fundamentally different from its box modeling counterpart. In edge modeling, rather than starting with a primitive shape and refining, the model is essentially built piece by piece by placing loops of polygonal faces along prominent contours, and then filling any gaps between them.
  
  This may sound needlessly complicated, but certain meshes are difficult to complete through box modeling alone, the human face being a good example. To properly model a face requires very strict management of edge flow and topology, and the precision afforded by contour modeling can be invaluable. Rather than trying shape a well defined eye socket from a solid polygonal cube (which is confusing and counter-intuitive), it's much easier to build an outline of the eye and then model the rest from there. Once the major landmarks (eyes, lips, browline, nose, jawline) are modeled, the rest tends to fall into place almost automatically.
• NURBS/Spline Modeling: NURBS is a modeling technique used most heavily for automotive and industrial modeling. In contrast to polygonal geometry, a NURBS mesh has no faces, edges, or vertices. Instead, NURBS models are comprised of smoothly interpreted surfaces, created by "lofting" a mesh between two or more Bezier curves (also known as splines).
  
  NURBS curves are created with a tool that works very similarly to the pen tool in MS paint or Adobe Illustrator. The curve is drawn in 3D space, and edited by moving a series of handles called CVs (control vertices). To model a NURBS surface, the artist places curves along prominent contours, and the software automatically interpolates the space between.
  
  Alternately, a NURBS surface can be created by revolving a profile curve around a central axis. This is a common (and very fast) modeling technique for objects that are radial in nature—wine glasses, vases, plates, etc.
• Digital Sculpting: The tech industry likes to talk about certain breakthroughs they've termed disruptive technologies. Technological innovations that change the way we think about achieving a certain task. The automobile changed the way we get around. The internet changed the way we access information and communicate. Digital sculpting is a disruptive technology in the sense that it's helped free modelers from the painstaking constraints of topology and edge flow, and allows them to intuitively create 3D models in a fashion very similar to sculpting digital clay.
  
    In digital sculpting, meshes are created organically, using a (Wacom) tablet device to mold and shape the model almost exactly like a sculptor would use rake brushes on a real chunk of clay. Digital sculpting has taken character and creature modeling to a new level, making the process faster, more efficient, and allowing artists to work with high-resolution meshes containing millions of polygons. Sculpted meshes are known for previously unthinkable levels of surface detail, and a natural (even spontaneous) aesthetic.
• Procedural Modeling: The word procedural in computer graphics refers to anything generated algorithmically, rather than being created manually by the hand of an artist. In procedural modeling, scenes or objects are created based on user definable rules or parameters.
  
    In the popular environment modeling packages Vue, Bryce, and Terragen, entire landscapes can be generated by setting and modifying environmental parameters like foliage density and elevation range, or by choosing from landscape presents like desert, alpine, coastal, etc.
  
    Procedural modeling is often used for organic constructs like trees and foliage, where there is almost infinite variation and complexity that would be very time consuming (or impossible altogether) for an artist to capture by hand. The application SpeedTree uses a recursive/fractal based algorithm to generate unique trees and shrubbery that can be tweaked through editable settings for trunk height, branch density, angle, curl, and dozens if not hundreds of other options. CityEngine uses similar techniques to generate procedural cityscapes.
• Image Based Modeling: Image based modeling is a process by which transformable 3D objects are algorithmically derived from a set of static two-dimensional images. Image based modeling is often used in situations where time or budgetary restrictions do not allow for a fully realized 3D asset to be created manually.
  
    Perhaps the most famous example of image based modeling was on The Matrix, where the team had neither the time nor the resources to model complete 3D sets. They filmed action sequences with 360 degree camera arrays, and then used an interpretive algorithm to allow for “virtual” 3D camera movement through traditional real-world sets.
• 3D Scanning: 3D Scanning is a method of digitizing real world objects when an incredibly high level of photo-realism is required. A real world object (or even actor) is scanned, analyzed, and the raw data (typically an x,y,z point cloud) is used to generate an accurate polygonal or NURBS mesh. Scanning is often used when a digital representation of a real-world actor is required, as in The Curious Case of Benjamin Button where the lead character (Brad Pitt) aged in reverse throughout the film.

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• As realistic as 3D graphics may appear, they are typically comprised of flat, multi-sided polygons. These polygons are placed together to create a three-dimensional mesh, which produces a 3D image. While most polygons are triangles (which have the fewest possible sides), they can also be rectangles, hexagons, or other shapes. Colors and textures can be mapped onto these polygons, giving the final image a realistic appearance.

   

  Because polygons are flat surfaces, they can only estimate curved shapes, which many objects have. Therefore, smaller polygons can more accurately represent curved objects. Of course, using smaller polygons also means more polygons are required to create the object. So the more polygons a 3D model has, the more smooth and realistic it will look.

   

  For example, imagine a 3D image of a round ball that is three inches in diameter. If the ball was made up of only 100 polygons, it would be easy to notice the flat surfaces, making the ball seem rather blocky. However, if 1,000 polygons were used, the shapes would be less noticeable, giving the ball a smoother look. If 100,000 polygons were used to create the surface of the ball, the ball might appear completely round. The same concept holds true for more complex models, such as three-dimensional characters and other objects.

   

  Thanks to advances in graphics processing technology, today's video cards can render millions of polygons per second. This allows for several highly detailed 3D models to be displayed on the screen at one time. Therefore, if you want to play a 3D video game on your computer with highly realistic graphics, chances are you'll need a video card with a powerful GPU.

• Polygonal models or "meshes" as they're often called, are the most common form of 3D model found in the animation, film, and games industry
• Polygonal models are very similar to the geometric shapes you probably learned about in middle school. Just like a basic geometric cube, 3D polygonal models are comprised of faces, edges, and vertices.
• most complex 3D models start as a simple geometric shape, like a cube, sphere, or cylinder. These basic 3D shapes are called object primitives. The primitives can then be modeled, shaped, and manipulated into whatever object the artist is trying to create
• • The number of polygons in a mesh, is called the poly-count, while polygon density is called resolution. The best 3D models have high resolution where more detail is required—like a character's hands or face, and low resolution in low detail regions of the mesh. Typically, the higher the overall resolution of a model, the smoother it will appear in a final render. Lower resolution meshes look boxy (remember Mario 64?).
• Vertices: The point of intersection between three or more edges is called a vertex (pl. vertices). Manipulation of vertices on the x, y, and z-axes (affectionately referred to as "pushing and pulling verts") is the most common technique for shaping a polygonal mesh into it's final shape in traditional modeling packages like Maya, 3Ds Max, etc. (Techniques are very, very different in sculpting applications like ZBrush or Mudbox.)
• Without textures and shaders, a 3D model wouldn't look like much. In fact, you wouldn't be able to see it at all. Although textures and shaders have nothing do do with the overall shape of a 3D model, they have everything to do with it's visual appearance.
• A shader is a set of instructions applied to a 3D model that lets the computer know how it should be displayed. Although shading networks can be coded manually, most 3D software packages have tools that allow the artist to tweak shader parameters with great ease. Using these tools, the artist can control the way the surface of the model interacts with light, including opacity, reflectivity, specular highlight (glossiness), and dozens of others.
• Textures also contribute greatly to a model's visual appearance. Textures are two dimensional image files that can be mapped onto the model's 3D surface through a process known as texture mapping. Textures can range in complexity from simple flat color textures up to completely photorealistic surface detail.

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• Rigging

    

  A character rig is essentially a digital skeleton bound to the 3D mesh. Like a real skeleton, a rig is made up of joints and bones, each of which act as a "handle" that animators can use to bend the character into a desired pose.

    

  A character rig can range from simple and elegant to staggeringly complex. A basic setup for simple posing can be built in a few hours, while a fully articulated rig for a feature film might require days or weeks before the character is ready for Pixar level animation.

• Placing the Skeleton: Placement of a skeleton is perhaps the easiest part of the rigging process. For the most part, joints should be placed exactly where they would be in a real world skeleton, with one or two exceptions.
    

  • Joint Hierarchy: In order for a rig to work properly, the bones and joints must follow a logical hierarchy. When setting up a character's skeleton, the first joint you place is called the root joint. Every subsequent joint will be connected to the root either directly, or indirectly through another joint.

• Forward Kinematics: Forward kinematics (FK) is one of two basic ways to calculate the joint movement of a fully rigged character. When using FK rigging, a any given joint can only affect parts of the skeleton that fall below it on the joint hierarchy.
    

  • For example, rotating a character's shoulder changes the position of the elbow, wrist, and hand. When animating with forward kinematics, the artist typically needs to set the rotation and position of each joint individually—to achieve a desired pose the animator would work through the joint hierarchy sequentially: root → spine → shoulder → elbow → etc. The final position of a terminating joint (like a knuckle) is calculated as a function of the joint angles of every joint above it in the hierarchy.

• Inverse Kinematics: IK rigging is the reverse process from forward kinematics, and is often used an efficient solution for rigging a character's arms and legs. With an IK rig, the terminating joint is directly placed by the animator, while the joints above it on the hierarchy are automatically interpolated by the software.

    

  IK is most appropriate when the animation calls for a terminating joint to be placed very precisely&$151;a character climbing a ladder is a good example. Because the character's hands and feet can be placed directly on the ladder's rungs rather than the animator having to adjust their position joint-by-joint, an IK rig would make the animation process far more efficient. One drawback is that because IK animation uses software interpolation, there's often quite a bit of cleanup work that must be done in order to finalize the shot.
• Degrees of freedom/Constraints: When rigging, keep in mind that joints like the elbows and knees limited to a single degree of freedom in the real world, meaning they can only bend along one axis. Likewise, a human neck cannot rotate a full 360 degrees. To help prevent unrealistic animation, it's a good idea to set up joint constraints when you're building your rig. We'll address this further in a tutorial.
• Squash and Stretch: Another consideration that must be made is whether the rig will support squash and stretch, or whether the character will be constrained to realistic motion. Squash and stretch is an important principle in exaggerated cartoon animation, but typically doesn't look right in realistic film/VFX work. If you want your rig to maintain realistic proportions, it's important to set a constraint to lock the position of each joint in relation to the rest of the rig.
• Facial Rigging: A character's facial rig is usually altogether separate from the main motion controls. It's inefficient and incredibly difficult to create a satisfactory facial rig using a traditional joint/bone structure, so morph targets (or blend shapes) are usually seen as a more effective solution

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