StickFigure: Enabling Pose-Only IoT Cameras for Privacy-Preserving Edge Vision

Aveline Xiaoyuan Han; Chenshu Wu

cs.CVOct 25, 2026

StickFigure: Enabling Pose-Only IoT Cameras for Privacy-Preserving Edge Vision

Authors: Aveline Xiaoyuan Han, Chenshu Wu

Organizations: The University of Hong Kong

Abstract

Indoor IoT cameras can support assisted living and activity-aware spaces, but exporting raw RGB frames raises privacy concerns by exposing facial, clothing, and household details that body-motion tasks often do not require. We present StickFigure, a pose-only IoT camera system that, during real-time monitoring, keeps raw frames local and ephemeral while exporting only compact skeleton streams sufficient for downstream human activity recognition (HAR). While pose estimation has matured on GPUs, extracting accurate pose streams on resource-constrained IoT cameras remains difficult. To overcome these challenges and enable on-device pose streams on (almost) any IoT camera, we present two core designs in StickFigure: an activation-aware pose backbone that allocates channel capacity after spatial downsampling and a lightweight hierarchical MLP refiner that selectively corrects elbow and wrist coordinates using intermediate visual features and kinematic context. We fully implement StickFigure and evaluate it on public benchmarks and real-world deployment. Compared with state-of-the-art MCU pose models, StickFigure achieves the highest pose accuracy and downstream HAR utility on both NTU RGB+D and Penn Action, while using 96.5% fewer parameters and 99.3% fewer MACs per frame than RTMPose-S-based baselines, achieving 0.107 s p95 latency on a representative STM32H753 MCU. We believe StickFigure establishes a new operating point for privacy-aware edge vision on massive IoT cameras.

Figures & tables

Figure 1. Representation trade-offs for body-centric sensing. Qualitative comparison of visual exposure (lower toward the right) and downstream flexibility (higher toward the top). Pose reduces appearance-rich exposure while retaining a flexible body-motion representation.

Figure 2. StickFigure system overview. During real-time monitoring, RGB remains within the trusted camera boundary. An activation-aware pose backbone and a hierarchical MLP refiner produce a 12-joint pose, which is exported as a 76 B fixed-format skeleton record (SFSK) for downstream HAR. The dashed path shows one-shot site calibration.

Figure 3. Activation-aware pose backbone and hierarchical joint refinement. The backbone delays larger channel expansion until after spatial downsampling, while the joint refiner uses intermediate features and kinematic-parent context to correct elbows and wrists.

Figure 4. Hardware prototype and deployment environments. Left: the NUCLEO-H753ZI prototype with an STM32H753ZIT6 MCU, an OV2640 camera module, and the board-level power-measurement point. Right: views of deployment Envs. A, D, and E used for live evaluation in § 4.4 .

MCU-scale methods
Method	Model resources				NTU RGB+D				Penn Action
	Params	Flash	MACs/ frame	Peak activation RAM	[email protected]	p95 Latency	HAR Top-1	Fine-arm Recall	[email protected]	p95 Latency	HAR Top-1	Fine-arm Recall
Tiny MobileNet Pose	307.4K	313.88 KiB	13.39M	197.05 KiB	66.40%	0.061 s	$54.70{\pm}0.85$ %	38.00%	62.22%	0.067 s	$81.37{\pm}1.19$ %	83.56%
Uniform-Expansion Backbone	435.6K	439.96 KiB	41.27M	262.75 KiB	72.90%	0.121 s	$68.55{\pm}0.30$ %	51.80%	68.81%	0.127 s	$85.80{\pm}0.59$ %	85.28%
Progressive-Expansion Backbone	284.9K	290.94 KiB	25.14M	262.75 KiB	73.70%	0.076 s	$69.43{\pm}0.34$ %	52.70%	64.40%	0.082 s	$84.05{\pm}0.27$ %	85.24%
XiNet Pose	607.5K	603.46 KiB	75.09M	283.63 KiB	72.08%	0.190 s	$60.45{\pm}0.80$ %	41.79%	69.01%	0.209 s	$86.70{\pm}1.06$ %	87.40%

Table 1. End-to-end FP32 pose, p95 pose latency, HAR, and resource comparison on NTU RGB+D and Penn Action. Flash reports compiled INT8 weights and associated constants, while peak activation RAM reports the maximum simultaneously live model activation memory in the profiled NTU compiled graph. Non-MCU models are accuracy references; dashes indicate unavailable exact resource or target-latency measurements.

Method	NTU RGB+D			Penn Action
	FP32	INT8	$\Delta$	FP32	INT8	$\Delta$
Tiny MobileNet Pose	66.40%	65.87%	$-0.53$	62.22%	62.01%	$-0.21$
Uniform-Expansion Backbone	72.90%	71.84%	$-1.06$	68.81%	68.65%	$-0.16$
Progressive-Expansion Backbone	73.70%	64.20%	$-9.50$	64.40%	64.19%	$-0.21$
XiNet Pose	72.08%	70.45%	$-1.63$	69.01%	68.85%	$-0.16$
Activation-Aware Backbone	78.00%	77.45%	$-0.55$	75.49%	75.33%	$-0.16$

Table 2. FP32-to-INT8 [email protected] retention.

Figure 5. Design analysis of the activation-aware capacity allocation and hierarchical distal-joint refinement. (a) Stage-wise capacity allocation under a fixed largest individual activation tensor. (b) Accuracy–computation trade-off across the three controlled expansion schedules. (c) Joint-wise gains from the refiner, showing improvements concentrated on the elbow and wrist.

Figure 6. Live deployment results. (a) [email protected] after one-shot site calibration. Orange and blue markers denote target-supervised references and one-shot calibration results. The dashed line denotes the dataset-domain reference. (b) Three-seed mean HAR Top-1 for the dataset reference and two independently collected Env A sessions; Env A* changes appearance and lighting.

Figure 7. Motion-triggered adaptive execution. A 100-s power trace with two human-entry periods. MOTION_CHECK performs one motion check per second and switches to full-rate POSE_INFERENCE upon detecting motion, returning to MOTION_CHECK after approximately 5 s of inactivity.

Figure 8. Long-horizon monitoring case study. Left: pose, HAR, and temporal metrics from a separate approximately 40-minute annotated session. Center: three daytime monitoring timelines. Right: day-specific prediction timelines, inter-method disagreement (Dis.), and activity-state switches per hour for the two estimators.

Figure 9. Cross-platform INT8 pose and downstream utility. (a) INT8 [email protected] and (b) downstream HAR Top-1 for the best feasible operating point of each architecture family on ESP32-S3, STM32H753, Raspberry Pi 4, and Jetson Orin Nano. Each point is the highest-performing configuration satisfying the cross-platform real-time gate of p95 inference latency

\leq 125

ms and sustained throughput

\geq 8

FPS. StickFigure achieves the highest pose and HAR utility across all four platforms.

Human Pose Estimation

Human pose estimation recovers people’s body keypoints, 3D skeletons, or meshes from images, video, and other sensor data.

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Human Activity Recognition Wearable Sensing Edge ML Domain Generalization Disentangled Representation Learning

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Sensor-based human activity recognition (HAR) is fundamental to ubiquitous and wearable computing, yet existing foundation models are largely designed for cloud-scale deployment and struggle with real-world sensing shifts, including unseen users, devices, sampling rates, and sensor placements. We present EdgeHAR, an edge-native compact sensor foundation model designed for wearable intelligence. Unlike conventional models that entangle activity knowledge with acquisition variations, EdgeHAR learns transferable representations by factorizing sensor signals into three latent codes: an (i)Activity-Semantic Code capturing reusable activity knowledge, a (ii)Motion-Dynamics Code modeling temporal patterns, and an (iii)Acquisition-Context Code representing sensor-specific variations. This disentangled design enables efficient adaptation to new users, devices, placements, and activity classes with limited target-domain data. By incorporating lightweight adaptation modules, EdgeHAR achieves foundation-model-level transferability while satisfying edge constraints in computation, memory, latency, and privacy. Experiments across heterogeneous HAR datasets demonstrate that EdgeHAR maintains competitive recognition performance under distribution shifts with substantially reduced deployment cost. EdgeHAR establishes a practical paradigm for compact, edge-first foundation models for ubiquitous sensing systems.

Abstract

Human Pose Estimation

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