Short answers, pulled from the story.
A key frame is a drawing that defines the starting and ending points of any smooth transition. This technique allows software or in-betweeners to fill the gap between these points, creating the illusion of life from static images. Without key frames, fluid movement in classic Disney films or modern 3D graphics would be impossible to achieve with such efficiency.
In three-dimensional graphics, a key frame acts as a control point for invisible forces like light and shadow. Animators set key frames at the beginning and end of a scene to manipulate parameters such as light intensity and beam size. The computer program automatically interpolates the values between these two points to generate a smooth transition.
Non-linear video editing software relies on key frames to manage the flow of audio and visual effects. An editor sets a key frame to indicate the exact moment when a sound should fade up or down to a specific level. This technique allows for precise control over the emotional tone of a scene without altering the original recording.
A key frame, also known as an intra-frame, is the only frame in which a complete image is stored in the data stream. When a drastic change occurs, such as switching from one camera shot to another, the system must create a new key frame to preserve the visual integrity of the scene. Engineers insert key frames at arbitrary intervals, such as once every 10 seconds, to allow for seeking within the video stream.
Adding key frames when they are not visually necessary increases the overall disk size of the video stream. This drawback does not produce significant compression loss when the bitrate is already set at a high value for better quality, as seen in the DVD MPEG-2 format. However, for streaming services or mobile devices, the extra data can be a burden.