---
title: "Determining 3D Positions"
slug: "determining-3d-positions"
updated: 2025-07-31T20:21:41Z
published: 2025-07-31T20:21:41Z
canonical: "docs.kernel.com/determining-3d-positions"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.kernel.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Determining 3D Positions

Flow users may wish to determine the 3D positions of the Flow headset as worn by participants. Several approaches exist, but one method is summarized below. Please refer to the detailed guide (pdf document attached at the bottom of this page) for more detailed instructions.

**Mount AprilTags to the Headset** Print and affix [AprilTags](https://april.eecs.umich.edu/software/apriltag) to the outer surface of the Flow headset. The precise location of the tags is not critical—they serve to improve the accuracy of 3D model generation. Printed AprilTags may be mounted to the headset on cardboard pieces with adhesive (such as double-sided tape).

**Capture the Participant Wearing the Headset** Fit the headset to the participant, ensuring [good signal quality](/v1/docs/getting-best-signal-flow2). Then, capture a video of the participant wearing the headset using a standard smartphone camera.

- Use landscape orientation.
- Enable high frame rate capture.
- Ensure plentiful lighting.
- Move the camera slowly, performing a 360 degree scan.

**Generate a 3D Model** Import the video into [RealityCapture](https://www.capturingreality.com/) (or a similar photogrammetry software) to generate a 3D model. Settings may need to be adjusted for the AprilTags.

**Assemble in CAD** Using SolidWorks (recommended) or a software of your choice, create a base assembly that includes:

- A head surface from an MRI scan (or, if unavailable, an atlas scalp or generic headform).
- The 3D model generated from the video.
- A CAD model of the Flow headset. Solidworks SLDPRT files as well as STEP files are included in the zip files below. See the pdf attached for detailed instructions on working with these files.

[SolidworksFlow_Assembly_Set1.zip](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/SolidworksFlow_Assembly_Set1.zip)[SolidWorksFlow_Assembly_Set2.zip](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/SolidWorksFlow_Assembly_Set2.zip)[CAD_STEP_Files.zip](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/CAD_STEP_Files.zip)

**Align and Extract Coordinates**

See the attached guide for more details.

- Align the photogrammetric 3D model to the MRI or atlas scalp.
- Align the Flow headset CAD model to the 3D scan.
- Extract coordinates where light pipes from the Flow model intersect with the scalp of the MRI.
- Use the scripts provided below to extract and visualize optode locations.

[Extracting_Optode_Locations(1).py](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/Extracting_Optode_Locations(1).py)[Visualizing_Optode_Locations.py](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/Visualizing_Optode_Locations.py)

---

Below is a detailed, step-by-step guide to extracting optode locations for the Kernel Flow wearable TD-fNIRS headset using SolidWorks, the SolidWorks API, and photogrammetric methods.

[Extracting_Optode_Locations.pdf](https://cdn.document360.io/c97072d7-dbc3-41f6-b55e-b62dbde0527b/Images/Documentation/Kernel%20Flow%20SolidWorks%20API%20Optode%20Location%20Extraction%20-%20REVISED.docx.pdf)