HTC SRanipal SDK

established Updated: 2026-05-31
sranipal htc vive-pro-eye eye-tracking facial-tracking sdk pupil-dilation legacy

HTC SRanipal SDK

Status: established
Last updated: 2026-05-31
Sources: Sranipal Framework Overview
Tags: [sranipal, htc, vive-pro-eye, eye-tracking, facial-tracking, sdk, pupil-dilation, legacy]

Summary

SRanipal is HTC's proprietary SDK for eye and facial tracking on Vive headsets, and historically the most feature-rich way to read physiological eye data from that hardware. It runs as a Windows background service exposing a C API consumed by Unity and Unreal Engine 4 wrappers, and provides per-eye gaze, normalized pupil diameter, eye openness, blink, convergence distance, and millisecond timestamps at 120 Hz (HTC Corporation, 2026). As of 2024 it is in maintenance mode with no UE5 support, and HTC directs new projects to the VIVE OpenXR SDK and the XR_HTC_eye_tracker extension. Its normalized (not millimetre) pupil values, Windows-only operation, and uncertain future make it a risk for long-term research.

Body

Context

This article rests on HTC's developer overview of the SRanipal eye- and facial-tracking framework (HTC Corporation, 2026). The source is a technical feature digest describing the SDK's architecture, its per-sample data set, its hardware support, and HTC's migration guidance toward OpenXR. Within this knowledge base SRanipal is the worked example of a vendor SDK that exceeds the OpenXR baseline set in Openxr Eye Gaze Interaction — it reaches the per-eye pupil and openness data that standard misses — at the cost of the deprecation and platform risk that recurs in the Unity and Unreal articles (Unity Eye Tracking, Unreal Eye Tracking) and weighs on the cross-engine Eye Tracking Engine Comparison. With the source named, the points below state its content directly.

Key Points

SRanipal — "Sensing, Reality, and Natural Interface" — is built around a host-side runtime rather than an on-device library. The SRanipal Runtime is a Windows background service that reads the eye- and face-tracking sensors over the HMD's USB/internal connection and exposes a C API; the Unity and UE wrappers reach it through a named pipe or shared memory. Because the runtime must be installed and running on a tethered PC, the SDK is not usable for standalone on-device deployments (§ What SRanipal Is; § Architecture). Supported eye-tracking hardware includes the Vive Pro Eye (built in, 120 Hz), the Vive Focus 3 with the Eye Tracker accessory (120 Hz), and partial support on the Vive XR Elite with eye tracker via a newer SDK version (§ Supported Hardware).

The data set is what gives SRanipal its continued research value. Per sample it provides combined and per-eye gaze direction (normalized vectors), per-eye gaze origin in millimetres in HMD space, per-eye pupil diameter (normalized 0–1), per-eye openness (0–1), per-eye blink (boolean, derived from an openness threshold), 2D pupil position in the sensor image, gaze convergence distance in metres, and a millisecond timestamp, all at 120 Hz (§ Eye Tracking Data Available). This exposes physiological signals — notably pupil dilation — that the standard OpenXR gaze path in Openxr Eye Gaze Interaction does not.

The limitations are significant and compound for research use. Pupil diameter is normalized 0–1 rather than calibrated millimetres, so absolute pupillometry is impossible without per-device calibration; the SDK is Windows-PC-only and tethered; it is in maintenance mode with bug fixes only; there is no UE5 plugin; and HTC's roadmap points to OpenXR as the long-term path, meaning SRanipal may stop functioning with future firmware (§ Key Limitations). Engine support reflects this trajectory: Unity 2019–2022 is supported via SDK v1.3.6.8 with mixed community reports on Unity 6, UE4 (≤4.27) is supported, UE5 is not, and Godot has only an archived Godot 3.x community plugin (§ Engine Support; see Godot Eye Tracking).

HTC's official migration guidance moves both engines to OpenXR, with a known data trade-off. For Unity and Unreal, HTC recommends the VIVE OpenXR SDK with the XR_HTC_eye_tracker extension, which exposes per-eye gaze and eye openness but loses pupil diameter, since neither XR_HTC_eye_tracker nor XR_EXT_eye_gaze_interaction exposes it as of May 2026 (§ Migration Recommendation (HTC Official)).

Conclusion

The digest's research assessment is that SRanipal remains the only path to per-eye pupil data from a Vive Pro Eye, so it is still valuable for that narrow case but a poor bet for longitudinal projects given its normalized values, millisecond (not microsecond) timestamps, Windows-only operation, and uncertain support (§ Research Use Assessment). The same SRanipal-versus-OpenXR tension — richer physiological data against deprecation risk — recurs in Unity Eye Tracking and Unreal Eye Tracking.

References

VRCFaceTracking Project (2026) SRanipal — VIVE Hardware Documentation. VRCFaceTracking Docs. Available at: https://docs.vrcft.io/docs/v4.0/hardware/VIVE/sranipal (Accessed: 1 June 2026). vrcftSranipal2026

Open Questions

  • Will future HTC firmware break SRanipal, as the deprecation trajectory implies? No date is given (VRCFaceTracking Project, 2026).
  • Is there any OpenXR path to per-eye pupil diameter for HTC hardware planned? As of May 2026 none exists.