The 3D point and analog data samples are written as sequential frames starting at the beginning of the first 512-byte block specified by in the POINT:DATA_START parameter. If the stored data contains both 3D point and analog information then the 3D point frame is always written first, starting with the first frame of data. If there is only a single type of data (all 3D point data or all analog data) then the data section will simply consist sequential frames of data samples.
The original description of the C3D file format states that the POINT:DATA_START parameter is stored as a signed 16-bit integer. This limits the placement of the start of the 3D/Analog data storage section to any 512-byte block boundary within the first 16Mb (32767*512/1024) of the C3D file. By treating this parameter as an unsigned integer (which is easily detected as it will have a negative value when it is greater than 32767 if read as a signed integer) the limit on the placement of the 3D/Analog section can be extended to the first 32Mb of the C3D file.
3D point and analog data samples may be stored in either signed integer or floating-point format. Whichever format is selected must apply to both the 3D point and analog data records within the same C3D file. If the 3D point data is stored in floating point format, then the analog data must also be stored in floating point format. It is not possible to mix data storage types within a C3D file, as there is only a single flag to indicate which storage method is used.
Since the range of the data is stored in the C3D parameter section there is no need for an “end-of-data” marker - data is simply written from the first frame to the last frame. Any unused storage in the final 512-byte block of the C3D file should be filled with 0x00h for compatibility with older FORTRAN based applications that read data in 512-byte blocks. Both 3D and analog data samples can cross the 512-byte block boundaries within the C3D file.
Figure 18 – The 3D / analog data storage structure.
The C3D file format does not specify the order in which 3D point data values will be written within any given C3D file except in so far that they will be written to the 3D data section in the same order that they are described in the parameter section. It is essential that applications that access the 3D point data determine the storage order of the 3D points by reading the order of the point labels stored in the parameter section. Applications that assume that 3D points will always be stored in the same order will fail when presented with a C3D file that contains data stored in a different order. The analog samples for each 3D frame are recorded sequentially – each 3D frame is followed by the analog data associated with the 3D frame.
Note that the existence of a single point of 3D data in only one frame of a C3D file requires that storage space be allocated in every single frame of the C3D file. This can result in large C3D file with a considerable amount of wasted space if large numbers of unused, short trajectories are preserved unnecessarily.
Analog channels are stored in sequence starting with the first analog channel, which is always channel one. If ten analog channels are sampled then the ten analog values are written starting with channel one and ending with channel ten. If there are multiple sample of analog data per 3D frame then the next ten analog samples will written until all analog data associated with the 3D frame was been stored. This will be followed by the next frame of 3D data.
It is worth observing here that analog channels are usually stored in sequence starting with the channel one. There is no provision, in the C3D format, to store channels 2, 8, and 10 and identify them as such – in order to store channel 10 all the channels between 1 and 10 be stored. However, since analog channels can be referred to by their ANALOG:LABEL assignments there is really no need to store unused analog channels if applications use the LABEL rather than the channel number to identify the individual analog channels.
Both analog channels and 3D points stored within the C3D file format are indexed and counted from base “one” – this can occasionally lead to confusion when interfacing an analog data collection system that counts channel “zero” as the first analog channel.
More:
3D Data - Floating-point Format
Analog Data - Floating-point format