ezc3dWrite not saving parameters.ANALOG.SCALE.DATA values

[pmtanx]

Hello,
I'm very thankful for ezc3d! I have had success using it for a variety of tasks. However, I'm finding that there is one parameter value that ezc3dWrite doesn't seem to be saving correctly. I'm able to open my c3d file, to change various fields, and to save them successfully, with the exception of the Analog Scale values. Simply opening the file and resaving it causes all the previous values to be overwritten with ones. The same thing happens no matter what I set the scale values to.

c3d = ezc3dRead('Dancer800R1S_force_fixed.c3d');
c3d.parameters.ANALOG.SCALE.DATA
ezc3dWrite('Dancer800R1S_force_rewritten.c3d',c3d);
c3d = ezc3dRead('Dancer800R1S_force_rewritten.c3d');
c3d.parameters.ANALOG.SCALE.DATA

ans =
    -1
    -1
    -1
    -1
    -1
    -1
     1
ans =
     1
     1
     1
     1
     1
     1
     1

sample_files.zip

I've attached my c3d file and some sample Matlab code and would be grateful for any suggestions!

Thanks,
Philip Tan
Messiah University
Grantham, Pennsylvania, US

[pariterre]

Dear @pmtanx thanks for your kind words :)

The "scaling" is not exactly as it sounds. When it can technically be used as an actual "scaling" of the data, it was not designed for this.

This dates the time where controlling the file size was very important. One of the way they were doing that was to store the float points as integer. They therefore needed to apply a scaling factor to convert from the float to the integer (and the other way around). Nowadays, storing floats is normal as hard drives are large enough to store the data without shenanigan.

The scaling factor you are referring to is that scaling factor. "-1" is the keyword for "no scaling" which is what is expected when storing floats. Since ezc3d do not support storing as integer, this scaling factor is always set to "no scaling" (or -1). This parameter is more an internal structure than something the user is supposed to interact with... That is why ezc3d ignores the value you put. There are other internal elements like that in a C3D file which can cause confusion. For instance DATA_START seem to be referring to the time stamp at which the data starts, but in reality it refers to the bytes count from the start of the file at which the data are store (in the c3d file), allowing for an arbitrarily large parameter section.

What was your goal in trying to modify this scaling factor? If you really needed to scale your data, I would suggest to create your own scaling parameter, but I am not sure what good it would bring!

Hope this answers your question!

[pmtanx]

I am very grateful for the quick response! Thanks for the explanation...that makes sense that we don't need the scale values now that we can store the values as floats.

I am working on combining two different C3D files—one with motion data from an OptiTrack system running Motive 3.0, and the other from an AMTI force plate using AccuPower software. The companies have confirmed that these particular systems aren't able to talk to each other, and so I'm working to manually sync and merge the files. The reason I was trying to change the scaling was because the moments from the force plate are exported in units of Newton-meters, and my understanding was that they should be in Newton-millimeters given this paragraph from the C3D documentation (https://www.c3d.org/HTML/default.htm?turl=Documents%2Fanalogoffset.htm):

If the calibration values are supplied in units of Newton-meters per volt for the force moments, and the measurement units specifying the locations of your reference markers are in millimeters, then you must convert the values referring to moments to Newton-millimeters per volt. This conversion is achieved by multiplying the ANALOG:SCALE results for the moment channels by 1000.

At this point, my follow-up questions are as follows:

1. Rather than changing the scaling factor, would it be better for me to simply multiply all the data in the moment channels by 1000?
2. On the same C3D documentation page, it says, "Note that you must enter all force plate ANALOG:SCALE factors as negative values to produce an output in terms of reactive force." Even if I don't use any scaling, is it okay that ezc3d automatically changes the values from -1 to +1 when I save the file? I can't figure out how to make ezc3d leave the value as -1 to represent "no scaling".

Thanks again,
Philip

[pariterre]

Hello again!

1- Rather than changing the scaling factor, would it be better for me to simply multiply all the data in the moment channels by 1000?

That is correct!

On the same C3D documentation page, it says, "Note that you must enter all force plate ANALOG:SCALE factors as negative values to produce an output in terms of reactive force." Even if I don't use any scaling, is it okay that ezc3d automatically changes the values from -1 to +1 when I save the file? I can't figure out how to make ezc3d leave the value as -1 to represent "no scaling".

I don't remember seeing this when I implemented the SCALING, but I more focussed more on POINT:SCALE and maybe assumed it was the same for ANALOG:SCALE. As far as ezc3d is concerned (and as far as I know), SCALE is ignored as soon as FLOAT are used in the c3d file

[TSS-22]

Hello,
I don't know if you are still interested in the matter but, from the C3D user doc it is stipulated (page 65/140):

The stored analog data
values must never be stored as pre-scaled values as this effectively destroys vital
information about the analog data sampling and processing.

The parameters ANALOG:GEN_SCALE and channel specific ANALOG:SCALE and
ANALOG:OFFSET values must be applied to the floating-point value to obtain
physical world units in exactly the same way as we scale the integer formatted data.
Thus, a floating-point analog sample is calculated as:

Physical world value = (data value - offset) * channel scale * general scale

Where:
- ‘offset’ is in the “ANALOG:OFFSET” parameters (integer)
- ‘channel scale’ is in the “ANALOG:SCALE” parameters (floating-point)
- ‘general scale’ is the “ANALOG:GEN_SCALE” parameter (floating-point)

But at the same time later on (can't find the passage), it talks about software application that tweak and use the OFFSET to "zero" their analog data from the baseline wander, says you shouldn't do it, but proceed to explain that the same trick can be done to zero the resting artifact of force plate. Go figure.