Convert OBJ to C3D

Understanding OBJ and C3D file formats

OBJ is a widely used 3D geometry definition file format developed by Wavefront Technologies. It stores 3D models as a collection of vertices, faces, and texture coordinates, making it popular for exchanging 3D assets between different software.

C3D (Coordinate 3D) is a file format primarily used for motion capture data, storing 3D point data, analog data, and event information. It is commonly used in biomechanics, animation, and motion analysis applications.

Why convert OBJ to C3D?

Converting an OBJ file to C3D is not a typical 3D model conversion, as OBJ contains mesh data while C3D is designed for motion capture points. However, in some workflows, you may need to extract vertex or marker positions from an OBJ mesh and save them as C3D point data for use in motion analysis or animation pipelines.

How to convert OBJ to C3D

There is no direct, one-click converter for OBJ to C3D due to the fundamental differences in the formats. The process generally involves extracting vertex positions from the OBJ file and then creating a C3D file with these points. Here’s a general workflow:

1. Use a 3D software like Blender to import the OBJ file.
2. Export the vertex or marker positions as a CSV or TXT file using a script or plugin.
3. Use a tool like BTK (Biomechanical ToolKit) or ezc3d (a Python library) to create a C3D file from the extracted point data.

Recommended software for OBJ to C3D conversion

• Blender: Free and open-source 3D software. Use it to import OBJ files and extract vertex data. You can use Python scripting within Blender to export vertex positions.
• ezc3d: A Python library for reading and writing C3D files. After extracting vertex data, use ezc3d to create a C3D file programmatically.
• BTK (Biomechanical ToolKit): An open-source library for biomechanical data analysis, including C3D file creation and manipulation.

Step-by-step conversion example using Blender and ezc3d

1. Open Blender and import your OBJ file via File → Import → Wavefront (.obj).
2. Use a Python script in Blender to export the vertex positions to a CSV file.
3. Install ezc3d in Python (pip install ezc3d).
4. Write a Python script to read the CSV and create a C3D file with the point data using ezc3d.

Conclusion

While there is no direct converter for OBJ to C3D, using Blender to extract vertex data and ezc3d or BTK to create a C3D file is the most effective workflow. This approach allows you to bridge the gap between 3D mesh data and motion capture point data for specialized applications.