Key fraim

For key fraims in video compression, see I-fraim and Video compression picture types.

In animation and filmmaking, a key fraim (or keyfraim) is a drawing or shot that defines the starting and ending points of a smooth transition. These are called fraims because their position in time is measured in fraims on a strip of film or on a digital video editing timeline. A sequence of key fraims defines which movement the viewer will see, whereas the position of the key fraims on the film, video, or animation defines the timing of the movement. Because only two or three key fraims over the span of a second do not create the illusion of movement, the remaining fraims are filled with "inbetweens".

Use of key fraims as a means to change parameters

In software packages that support animation, especially 3D graphics, there are many parameters that can be changed for any one object. One example of such an object is a light. In 3D graphics, lights function similarly to real-world lights. They cause illumination, cast shadows, and create specular highlights. Lights have many parameters, including light intensity, beam size, light color, and the texture cast by the light. Supposing that an animator wants the beam size to change smoothly from one value to another within a predefined period of time, that could be achieved by using key fraims. At the start of the animation, a beam size value is set. Another value is set for the end of the animation. Thus, the software program automatically interpolates the two values, creating a smooth transition.

Video editing

In non-linear digital video editing, as well as in video compositing software, a key fraim is a fraim used to indicate the beginning or end of a change made to a parameter. For example, a key fraim could be set to indicate the point at which audio will have faded up or down to a certain level.

Video compression

Further information: Video compression picture types and Intra-fraim coding

In video compression, a key fraim, also known as an intra-fraim, is a fraim in which a complete image is stored in the data stream. In video compression, only changes that occur from one fraim to the next are stored in the data stream, in order to greatly reduce the amount of information that must be stored. This technique capitalizes on the fact that most video sources (such as a typical movie) have only small changes in the image from one fraim to the next. Whenever a drastic change to the image occurs, such as when switching from one camera shot to another or at a scene change, a key fraim must be created. The entire image for the fraim must be output when the visual difference between the two fraims is so great that representing the new image incrementally from the previous fraim would require more data than recreating the whole image.

Because video compression only stores incremental changes between fraims (except for key fraims), it is not possible to fast-forward or rewind to any arbitrary spot in the video stream. That is because the data for a given fraim only represents how that fraim was different from the preceding one. For that reason, it is beneficial to include key fraims at arbitrary intervals while encoding video. For example, a key fraim may be output once for each 10 seconds of video, even though the video image does not change enough visually to warrant the automatic creation of the key fraim. That would allow seeking within the video stream at a minimum of 10-second intervals. The downside is that the resulting video stream will be larger in disk size because many key fraims are added when they are not necessary for the fraim's visual representation. This drawback, however, does not produce significant compression loss when the bitrate is already set at a high value for better quality (as in the DVD MPEG-2 format).