C3D is a file format to store biomechanical data. Most motion capture systems output C3D files, making it the standard data format for motion analysis. All C3D files are structured in a similar way: They contain a header that describes the file’s structure, a parameter section with useful meta data (e.g., labels, measurement units, frame rate, …), and a data section with the point data (coordinates of the objects) and additional analog data (e.g., recordings from electromyography or force platforms). You can find more details about the C3D format on its Website and in the C3D User Guide.
Different programs for working with C3D files exist, but free open-source solutions that enable reproducible analysis are rare. Currently the best solution is the EZC3D library, which has bindings for Matlab, Python, and Octave. However, R, a popular programming language for data science, previously did not support C3D files. By using the EZC3D C++ library, the c3dr package for R now provides a possibility to work with C3D data in the R programming environment.
The c3dr package is currently available from R-Universe and GitHub. For installation, simply use the following code:
install.packages("c3dr", repos = "https://ropensci.r-universe.dev")
# Alternative:
# if (!require(remotes)) install.packages("remotes")
# remotes::install_github("ropensci/c3dr")To import a c3d file simply run the c3d_read() function with the absolute or relative file path as argument. We here use an example file provided with the packages, that can be called using c3d_example(). The example file is a short recording of human walking, with force platforms measuring the ground impact of two steps. The measurements were performed using a Qualisys motion capture system. You can view a video of the recording.
library(c3dr)
# this example uses an internal example file
filepath <- c3d_example()
# import C3D file
d <- c3d_read(filepath)
d
#> A c3d object with
#> - 55 data points and 340 frames
#> - 1.70 s measurement duration (200 fps)
#> - 69 analog channels (2000 fps)
#> - 2 force platforms with 3400 framesThe c3d_read() function returns a c3d object, which is a list with different components.
str(d, max.level = 1)
#> List of 6
#> $ header :List of 6
#> $ parameters :List of 7
#> $ data :List of 340
#> $ residuals : num [1:340, 1:55] 1280 1280 1280 1280 1280 ...
#> $ analog :List of 340
#> $ forceplatform:List of 2
#> - attr(*, "class")= chr [1:2] "c3d" "list"The header contains basic information about the number of frames (nframes), the number of data points (npoints), the number of analog channels (nanalogs), the number of analog frames per data frame (analogperframe), the frame rate in Hz (framerate) and the number of saved events (nevents).
d$header
#> $nframes
#> [1] 340
#>
#> $npoints
#> [1] 55
#>
#> $nanalogs
#> [1] 69
#>
#> $analogperframe
#> [1] 10
#>
#> $framerate
#> [1] 200
#>
#> $nevents
#> [1] 0The parameters are all the meta data saved in the C3D file. Parameters in C3D files are organized in groups, and these are preserved during import with c3d_read(). The parameter section of a c3d object is therefore a list of lists. To access a single parameter, you first need to access its group and then the parameter. For example the SOFTWARE parameter is part of the MANUFACTURER group. To retrieve the parameter value from a c3d object named f you need to call f$parameters$MANUFACTURER$SOFTWARE.
str(d$parameters, max.level = 1)
#> List of 7
#> $ POINT :List of 12
#> $ ANALOG :List of 10
#> $ SEG :List of 5
#> $ MANUFACTURER :List of 3
#> $ FORCE_PLATFORM:List of 8
#> $ EVENT :List of 3
#> $ PROCESSING :List of 4
# retrieve SOFTWARE parameter from MANUFACTURER group
d$parameters$MANUFACTURER$SOFTWARE
#> [1] "Qualisys Track Manager"c3d objects in R save point data (the three-dimensional position of objects over time) as a nested list, with the two levels frame and point. For example for a c3d object named f, f$data[[1]][[2]] returns a three-coordinate vector of the first frame for the second data point.
Usually it is much more convenient to work with the point data in a table (i.e., a data frame). This conversion happens with c3d_data(), see Section 6.
# read the coordinates of the first frame for the second data point
d$data[[1]][[2]]
#> [1] -398.1731 237.0688 872.8574Analog data in c3d objects is saved as a nested list. The sampling frequency of analog data is a multiplier of the point frame rate. For example, point data can be collected in 240 Hz, whereas analog data in sampled at 960 Hz. Therefore each frame of point data can correspond to multiple subframes of analog data. In c3dr analog data is saved as a list of matrices, with each list entry corresponding to a single point frame. The row of each frame matrix corresponds to a subframe, with the column corresponding to a analog channel. For example for a c3d object named f, f$analog[[2]][3,1] returns the value of the first analog channel for the third subframe of the second point frame.
Often it is more convenient to work with the analog data in a table (i.e., a data.frame). This conversion happens with c3d_analog(), see Section 7.
d$analog[[2]][3, 1]
#> [1] -0.3131914
# read the values of the first five analog channels for the first point frame
# The sampling frequency is ten times that of the point data, resulting in
# ten subframes
d$analog[[1]][, 1:5]
#> [,1] [,2] [,3] [,4] [,5]
#> [1,] -0.3096819 -0.2837415 -0.2492561 -0.2404060 -0.2407112
#> [2,] -0.3087664 -0.2861829 -0.2495613 -0.2401009 -0.2402534
#> [3,] -0.3057146 -0.2837415 -0.2494087 -0.2401009 -0.2407112
#> [4,] -0.3078508 -0.2858777 -0.2484932 -0.2388802 -0.2410164
#> [5,] -0.3109026 -0.2854199 -0.2507820 -0.2396431 -0.2416267
#> [6,] -0.3109026 -0.2858777 -0.2520027 -0.2390327 -0.2422371
#> [7,] -0.3147173 -0.2866406 -0.2532234 -0.2396431 -0.2426949
#> [8,] -0.3102922 -0.2875562 -0.2536812 -0.2396431 -0.2434578
#> [9,] -0.3113604 -0.2889295 -0.2567329 -0.2394905 -0.2440681
#> [10,] -0.3119707 -0.2895398 -0.2552071 -0.2413216 -0.2452888c3d files can store data from force platforms. When importing with c3d_read(), the force platform data is stored as a list, with each element of the list corresponding to one force platform.
# this example file has data from two force platforms
str(d$forceplatform)
#> List of 2
#> $ :List of 5
#> ..$ forces : num [1:3400, 1:3] 0.1399 0.1399 0.0933 0.0466 0.0466 ...
#> ..$ moments: num [1:3400, 1:3] 20.9 -77.6 -12.5 71.4 -10.3 ...
#> ..$ cop : num [1:3400, 1:3] 229 NaN 446 NaN 327 ...
#> ..$ tz : num [1:3400, 1:3] 0 NaN 0 NaN 0 0 0 NaN 0 0 ...
#> ..$ meta :List of 7
#> .. ..$ frames : num 3400
#> .. ..$ funit : chr "N"
#> .. ..$ munit : chr "Nmm"
#> .. ..$ punit : chr "mm"
#> .. ..$ calmatrix: num [1:6, 1:6] 0 0 0 0 0 0 0 0 0 0 ...
#> .. ..$ corners : num [1:4, 1:3] 508 508 0 0 464 ...
#> .. ..$ origin : num [1, 1:3] 1.524 -0.762 -34.036
#> $ :List of 5
#> ..$ forces : num [1:3400, 1:3] 0.0463 -0.1853 0.2317 -0.139 0 ...
#> ..$ moments: num [1:3400, 1:3] 49.4 -63.5 25.4 70.3 89.5 ...
#> ..$ cop : num [1:3400, 1:3] 897 NaN 827 854 868 ...
#> ..$ tz : num [1:3400, 1:3] 7.11e-15 NaN 0.00 0.00 0.00 ...
#> ..$ meta :List of 7
#> .. ..$ frames : num 3400
#> .. ..$ funit : chr "N"
#> .. ..$ munit : chr "Nmm"
#> .. ..$ punit : chr "mm"
#> .. ..$ calmatrix: num [1:6, 1:6] 0 0 0 0 0 0 0 0 0 0 ...
#> .. ..$ corners : num [1:4, 1:3] 1017 1017 509 509 464 ...
#> .. ..$ origin : num [1, 1:3] 1.02 0 -36.32Each force platform has data on forces, moments, the center of pressure, and the moments at the center of pressure, as well as meta data. The force data is stored as a matrix, where each row corresponds to one recording frame of the platform and each column to one dimension (x, y, z).
# view for the first force platform the force data for the first five frames
d$forceplatform[[1]]$forces[1:5, ]
#> [,1] [,2] [,3]
#> [1,] 0.13992119 0.0461483 -0.1835251
#> [2,] 0.13992119 -0.0461483 0.0000000
#> [3,] 0.09328079 0.1845932 -0.1835251
#> [4,] 0.04664040 -0.1384449 0.0000000
#> [5,] 0.04664040 -0.2768898 0.5505753To return a table of the imported point data of a c3d object, run the c3d_data() function.
p <- c3d_data(d)
p[1:5, 1:5]
#> L_IAS_x L_IAS_y L_IAS_z L_IPS_x L_IPS_y
#> 1 -220.1226 306.4248 846.3361 -398.1731 237.0688
#> 2 -212.4696 306.5356 844.6985 -390.7831 237.8691
#> 3 -204.8696 306.6555 843.2342 -383.1857 238.6758
#> 4 -197.1952 306.8035 841.6127 -375.7068 239.5434
#> 5 -189.6655 307.0628 840.1692 -368.1680 240.4141The table of point data has three different structures available, that can be selected by the format argument in the c3d_data() call:
wide format (default) has one column per point coordinate and one row per frame.long format has one column per point and three rows (x, y, z) per frame.longest format has one column for data, and three rows (x, y, z) per each frame and point.p_long <- c3d_data(d, format = "long")
p_long[1:5, 1:5]
#> frame type L_IAS L_IPS R_IPS
#> 1 1 x -220.1226 -398.1731 -392.8751
#> 2 1 y 306.4248 237.0688 146.2103
#> 3 1 z 846.3361 872.8574 880.3161
#> 4 2 x -212.4696 -390.7831 -385.6659
#> 5 2 y 306.5356 237.8691 147.1048
p_longest <- c3d_data(d, format = "longest")
p_longest[1:5, ]
#> frame type point value
#> 1 1 x L_IAS -220.1226
#> 2 1 y L_IAS 306.4248
#> 3 1 z L_IAS 846.3361
#> 4 1 x L_IPS -398.1731
#> 5 1 y L_IPS 237.0688To return a table of the imported analog data of a c3d file, use the c3d_analog() function.
a <- c3d_analog(d)
a[1:5, 41:46]
#> EMG 1 EMG 2 EMG 3 EMG 4 EMG 5
#> 1 -3.601184e-05 7.324442e-06 1.647999e-05 1.849422e-04 3.112888e-05
#> 2 4.638813e-05 8.697775e-06 1.533555e-05 1.203955e-04 2.861110e-05
#> 3 1.280251e-04 1.007111e-05 1.449629e-05 7.187109e-05 2.838221e-05
#> 4 1.841029e-04 1.190222e-05 1.350444e-05 4.325999e-05 3.028962e-05
#> 5 1.898251e-04 1.464888e-05 1.190222e-05 3.845332e-05 3.418073e-05
#> EMG 6
#> 1 1.434370e-05
#> 2 1.480148e-05
#> 3 1.586963e-05
#> 4 1.670889e-05
#> 5 1.647999e-05The ‘c3dr’ package is used to import and process motion capture data. Further analyses and visualizations can be performed in R, but this is outside the scope of the package. For a simple demonstration of visualising the imported data in R, see the following examples.
# plot the z-coordinate (the vertical position) of the right heel
plot(p$R_FCC_z, xlab = "Recording Frame", ylab = "Vertical Position (mm)")
# plot the vertical force vector of the second force platform
plot(
d$forceplatform[[2]]$forces[, 3],
xlab = "Recording Frame",
ylab = "Vertical Force (N)"
)
You can write in existing c3d object in R to a c3d file with the c3d_write() function:
c3d_write(d, "newfile.c3d")To make modifications to the point or analog data before writing the file, you can use c3d_setdata(). This function allows you to update a c3d object with modified data, e.g., to remove, add, rename or convert frames, points and analog channels.
# E.g. remove the last point from the point data
full_data <- c3d_data(d, format = "long")
cut_data <- full_data[, -57]
# update the data and write to new file
new_object <- c3d_setdata(d, newdata = cut_data)
c3d_write(new_object, "modified.c3d")