The C3D file format is designed to store 3D point and analog information so that the 3D locations of a number of markers can be synchronized with analog measurements made at the same time.  For this reason the 3D and analog samples are interleaved, frame-by-frame, throughout the data in a straightforward manner.  As a result, the C3D data record format is quite flexible and can be used to create files that contain only 3D data, 2D data or analog data or any combination.  In addition, it is possible (although not very efficient) to store the results of kinematical calculations (angles, moments, accelerations etc.) within the 3D data record format.

A single 512 byte header section

A parameter section consisting of one or more 512-byte blocks.

3D point/analog data section consisting of one or more 512-byte blocks.

Figure 17 – The 3D data section within the C3D file structure.

Although the C3D format is designed for 3D positional information, 2D information can be recorded by specifying one of the coordinates of the point and calculating the other two from the observer data.  This allows the C3D file format to be used by systems that support single camera measurements – thus an observer (camera) might provide positional information for the Y and Z planes while constraining the X motion within a single fixed plane.