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Design Philosophy
These principles are shared with the broader FreeMoCap project's philosophy — the idea that research-grade tools should be freely available and accessible to everyone.
Universal Design
SkellyCam should be simultaneously usable by a 13-year-old with no technical background and by a researcher running a multi-camera motion capture lab. The same tool, the same interface, the same documentation — just different levels of depth.
This means:
- Big friendly buttons for the common path: detect cameras, record, play back. No prior knowledge required.
- Fine-grained controls for experts: codec selection, exposure tuning, custom recording directories, direct API access.
- No feature should require technical expertise to use at a basic level, but it should reward expertise with additional capability.
Progressive Disclosure
Information and complexity are revealed in layers, not all at once. The default experience is simple and approachable. Details are available on demand for those who want them.
In practice this looks like:
- In the UI — The main controls are always visible. Advanced settings are behind expandable panels. Technical details are behind collapsible sections.
- In the documentation — Each page leads with the practical "how to use it" content. Technical depth is available in collapsible
<details>blocks. The bottom of each disclosure tree links to external resources (Wikipedia, tutorials, etc.) so users who want to keep learning have a path forward. - In the API — Sensible defaults for everything. You can start a recording with a single POST request. Custom configuration is available but never required.
Accessibility Over Complexity
When choosing between a more "correct" technical implementation and one that is easier to understand, prefer the one that is easier to understand — unless correctness is required for safety, data integrity, or scientific accuracy.
Cameras as Scientific Instruments
A camera captures light from a particular area at a particular time — it is a spatiotemporal measurement instrument. The image defines the spatial aspects of the data, with fidelity determined by the camera sensor, lens, environment, and settings. The timestamps define the temporal aspect. Within the FreeMoCap pipeline, extracting quantified 2D spatial information (e.g. skeleton joint positions) is handled by skellytracker. SkellyCam's focus is the temporal dimension: ensuring multi-camera frames are precisely synchronized in time.
Most research-grade multi-camera systems use hardware triggers — an external signal that fires all camera sensors simultaneously. This provides near-perfect synchronization but requires expensive, specialized cameras. SkellyCam is designed to deliver research-quality synchronization using software-side control of inexpensive consumer-grade USB cameras — making synchronized multi-camera capture accessible without proprietary hardware.
This framing — cameras as empirical measurement instruments — informs every major design decision in SkellyCam, from the frame-count-gated capture protocol to the detailed timestamp pipeline.
Hot, Hard, and Soft Loops
SkellyCam's architecture classifies operations into three categories that drive both technical decisions and development governance:
- Hot loops — Time-critical operations where any delay directly degrades temporal fidelity. The camera capture loops, frame synchronization gating, and shared memory writes. These run in isolated processes with minimal dependencies.
- Hard loops — Correctness-critical operations where the math must be right, but timing is flexible. Timestamp computation, recording finalization, inter-camera statistics. Performance is obviously important for general usability, but we're happy trade speed for accuracy and robustness here.
- Soft loops — Best-effort operations where some squish and slop is acceptable (in the short term). UI rendering, WebSocket streaming, log forwarding are all examples of soft-loops. If the frontend lags, frames are dropped from the stream — but recordings are never affected.
This classification also drives development governance: contributions touching hot/hard loops receive much higher scrutiny than those only touching soft loops. See the Architecture page for the full technical treatment.
Open Source, Open Science
SkellyCam is free and open-source software, developed in the open as part of the FreeMoCap Foundation's mission to make research-grade motion capture accessible to everyone. Contributions, questions, and feedback are always welcome.