Each 3D frame consists of one or more 3D data points and analog data samples that can be considered to be the values of the measurement variables at a single instant of time. This avoids the misunderstandings that can be caused by the use of the terms “Video Frame” and Video Field” since C3D files are normally created by motion capture systems that sample camera and sensors directly. All 3D frames are recorded in sequence, at intervals defined by the parameter POINT:RATE, which is written as a frequency value in Hertz (cycles per second).
A 3D frame may contain zero or more 3D points as recorded in the parameter POINT:USED. Since the C3D format is a general format intended for biomechanical data storage, it is also possible to create C3D files that contain only analog data values without any associated 3D data values. Note that although a C3D file only contains analog data with no 3D data points, the analog data will be stored as a fixed number of analog samples per 3D Frame.