Use of the fread function

Hey I'm trying to open .c3d files by the means of the fread function The problem I have is when searching the first frame and the last frame of the file with fread(fid,1,'int16') with fid = 8; If I do it with some non heavy files, I got a startframe = 1 and a endframe =~ 26000 and the script works but if I do it with more heavy files, then I got an enframe =~ -12000 which makes the script wrong and me angry ;) Sure someone has an idea... Thanks in advance!

Answer 1

[Jan]

When the C3D file format has been invented, large measurement files have been rare and int16 seemed to be a valid range for the number of frames. Many modern software packages use uint16 instead, because a negative number of frames is not valid at all.

I use this to import the header of C3Ds:

I8  = 'int8';
I16 = 'int16';
U16 = 'uint16';   % Signed is not meaningful!
F32 = 'float32';

% Spool to the beginning:
fseek(FID, 0, 'bof');

% Reading record number of parameter section:
Head.ParamRec = fread(FID, 1, I8);
MagicKey      = fread(FID, 1, I8);
if isequal(MagicKey, 80) == 0
   error(['JSimon:', mfilename, ':BadMagicKey'], 'Bad magic key: %d', MagicKey);
end

% Getting all the necessary parameters from the header record
Head.nCoor       = fread(FID, 1, U16);  % number of trajectories
Head.AnalogChunk = fread(FID, 1, U16);  % analog values per video frame
Head.iFrame      = fread(FID, 1, U16);  % index of first video frame
Head.fFrame      = fread(FID, 1, U16);  % index of last video frame
Head.MaxGap      = fread(FID, 1, U16);  % max allowed interpolation gaps
Scale            = fread(FID, 1, F32);  % scale integers to ref. units
Head.DataRec     = fread(FID, 1, U16);  % 1st record of data section
Head.AnalogRate  = fread(FID, 1, U16);  % analog samples per video frame
Head.VideoFreq   = fread(FID, 1, F32);  % frequency of video data

% If scale is negative, the data is stored in float format:
Head.FloatMode = (Scale < 0.0);
Head.Scale     = abs(Scale);

% Number of frames:
Head.nFrame = Head.fFrame - Head.iFrame + 1;

% Number of analog channels (FRC + EMG + ?):
if Head.AnalogRate  % Avoid division by zero:
   Head.nAnalog = round(Head.AnalogChunk / Head.AnalogRate);
else
   Head.nAnalog = 0;
end

But note, that today even the range of UINT16 is small and e.g. Vicon software write the actual number of frames to the parameter section as TRIAL.ACTUAL_START_FIELD and TRIAL.ACTUAL_END_FIELD. Then we need to modify the values:

End_Field2   = TRIAL.ACTUAL_END_FIELD;  % Or how ever you obtain parameters
Start_Field2 = TRIAL.ACTUAL_START_FIELD;

End_Field = End_Field2(1) + 65536 * End_Field2(2);
if ~isequal(End_Field, Head.fFrame)
    if Head.fFrame ~= 65535
       warning(['GetC3D:', mfilename, ':Unexpected_nFrame'], ...
          'Long C3D, but [Head.fFrame] is not 65535?!');
    end

      Start_Field = Start_Field2(1) + 65536 * Start_Field2(2);
      Head.iFrame = Start_Field;
      Head.fFrame = End_Field;
      Head.nFrame = End_Field - Start_Field + 1;
  end