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Animation Glossary
Animation - The rapid display of a sequence of images in order to create an illusion of movement. In 3D animation we work with sequences of position and orientation of our 3D model in order to cause an avatar to "move" within a three dimensional virtual environment. Each distinct unit within an animation sequence is one frame.
BVH - Biovision Hierarchy. The file format used to create character animations for Second Life as well as for many other applications. BVH files provide motion data through a structured text file which specifies the model's skeletal hierarchy along with the positioning of each of its parts at each frame of the animation.
Forward Kinematics - The process of calculating the position and orientation of a specific part of an object based on the position and orientation of the root of the object as well as the position and orientation of the joints higher up in that part's kinematic chain. For example, when animating with FK, the animator individually positions each segment of a model's arm or leg by adjusting the orientation of each joint leading up to that segment, such that the position of the hand, for instance, is determined by the position of the forearm and the rotation of the wrist. Compare with Inverse Kinematics.
FPS - Frames Per Second or Frame Rate. The number of frames of an animation which are displayed in each second. In film animation this is normally 24. Video animation typically utilizes 25 or 30 fps, depending on the format. Film and video are always displayed (in a typical performance) at the standard fps for their format -- but can be run through a camera while shooting at altered speeds. This allows for time dilation special effects in which the action in a sequence is played back at a different speed from that in which it was performed. (For example, if film is run through a camera at 48 fps during filming, then played back at the normal 24 fps, the action will appear in slow motion. Conversely, shooting at 12 fps and then playing back at 24 fps causes the action to be displayed in double time.) FPS in Second Life, however, is not constant, but varies considerably according the the resource load and performance of server, network and client. Playback fps in SL does NOT have a linear correlation with recorded fps - a fact which can cause confusion for animators accustomed to working in formats in which it does. What does this mean? Essentially this: If you create a 150 frame animation at 30 fps and upload this into SL, the duration of your animation will be 5 seconds - regardless of the frame rate your client receives from SL. When frame rate is slow, SL does not attempt to render every frame of an animation -- thereby slowing down playback speed -- but simply drops the frames it does not have time to render. An animation may appear jerky if SL's fps is really low, but it should play back in the appropriate period of time. (Of course, this is a general rule which applies under "normal" conditions. During periods of extreme lag, this concept -- as with many others -- may be thrown completely out the window!)
Frame - The basic building block of an animation. During each frame of an animation our model is displayed in a specified position for a pre-determined period of time (normally a fraction of a second). The animation then switches to the next frame which may display our model in a different position. The model does not move fluidly but jumps from its position in one frame to its position in the subsequent frame. When the frames are displayed for a short period of time and the amount of change between one frame and the next is small, the illusion of fluid motion is created due to the human brain's interpretation of the data collected by the eyes. See Frames Per Second.
Interpolation - A method of constructing new data points from a discrete set of known data points. In 3D animation, the animating software interpolates the position of the model at each frame between the key frames which are designated by the animator, based on the data provided in those key frames.
Inverse Kinematics - The process of calculating the required parameters of a series of Joints or parts, such that the end of the limb achieves a desired location. For example, when animating with IK, the animator is able to position the starting and ending positions of a figure's hand or foot, and the animating software uses IK equations to determine the path which the rest of the arm or leg must follow in order to achieve the desired hand movement. Inverse Kinematics relies on the use of Joint Limits to achieve naturalistic movement. Compare with Forward Kinematics.
Joint - Point at which two things are joined. In 3D animation the joints connect rigid body parts and are designed to simulate human skeletal joints. They can be rotated independantly around the X, Y & Z axes such that movement of a parent joint affects the positioning of each child part in its Kinematic Chain.
Joint Limits - Also known as Joint Constraints, these are limits which are applied to the allowable degree of rotation of each Joint of our model so as to confine the model's movements within reasonable anthropomorphic limits. Inverse Kinematics utilizes Joint Limits to calculate naturalized movements. Animators working with Forward Kinematics may choose to work with or without Joint Limits as they desire.
Key Frames - The "important" frames in an animation which define the limits of a movement. In traditional animation, the senior or key animator drew the key frames and handed these to a junior assistant, the inbetweener, who then filled in the rest. (See Tweening.) In computer animation, the human animator creates the key frames and the computer software interpolates the inbetweens.
Kinematic Chain - An assembly of rigid parts connected by Joints which move in relation to one another. In our work, each arm and leg of our model is a Kinematic Chain, as is the linkage of head, neck, chest, abdomen & hip which are connected through our figure's spine.
Kinematics - The study of pure motion. Movement of an object without reference to its mass or any forces acting upon it.
Loop - A sequence of animation frames which is repeated. In SL, an animation can be made to loop repeatedly until the animation is stopped.
Pose - A static animation in which our model is morphed into one position but is not moved. This is created by continuously looping one frame of an animation.
Priority - In Second Life, an avatar can perform two or more animations simultaneously. Conflicts which could potentially result from this practice are resolved through a system of priorities. Each animation is assigned a priority upon upload into Second Life. Animation priority is specified as an integer from 0 to 4 with higher numbers having "priority" over lower ones. Note: This priority is only assigned to body parts which are moved during the animation! If a body part is not adjusted in an animation at all, it will be assigned the lowest priority in that animation. This means that the positioning of that unadjusted body part will be overriden by ANY animation which is played simultaneously which does contain positioning data for that part. A list of SL's built-in animations with their assigned priorities can be found here.
T-Pose - The default or neutral pose of the Second Life avatar. When creating animations for SL, the model is normally left in the T-Pose with all joint rotations zeroed in the first frame. This frame is not displayed in SL but is used only as a reference point against which the subsequent frames are compared.
Tweening - Short for "in-betweening", this is the process of generating intermediate frames between two key frames such that the first key frame evolves smoothly into the next. In computer animation tweening is done by the software using a process of interpolation.
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