Template Collapse and Information-Theoretic Limits in Camera rPPG Pulse Morphology Restoration

• URL: http://arxiv.org/abs/2606.03802v1
• Date: Tue, 02 Jun 2026 15:50:07 GMT
• Title: Template Collapse and Information-Theoretic Limits in Camera rPPG Pulse Morphology Restoration
• Abstract summary: Consumer face camera remote photoplethys hallucination (r) enables passive cardiovascular monitoring.<n>But whether single-cycle waveform morphology encoding arterial stiffness is recoverable from this measurement has not been characterised.<n>We evaluated 16 encoder architectures spanning six families on 153 subjects across three datasets.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: Objective: Consumer face camera remote photoplethysmography (rPPG) enables passive cardiovascular monitoring, but whether single-cycle waveform morphology encoding arterial stiffness biomarkers is recoverable from this measurement has not been characterised. Methods: We evaluated 16 architectures spanning six families on 153 subjects across three datasets, introducing cross-subject Pearson r to distinguish subject-specific recovery from template collapse. Results: No architecture recovered subject-specific morphology (cross-subject r range 0.773--0.9999; ground-truth ceiling 0.601). Supervised Contrastive (SupCon) converged to log N = 4.844, constituting the strongest available empirical evidence that no discriminative morphological structure is extractable from single-cycle rPPG by the encoder families tested. The VAE decoder restores population-level harmonic content absent from the rPPG input (H2/H1: 0.310 output vs. 0.275 input), generalising zero-shot to UBFC (r = +0.708); a directional hallucination gap (p = 0.150) suggests partial signal reading. Anti-collapse objectives fail when input carries no discriminative structure. Significance: Consumer cameras cannot encode individual arterial morphology; cross-subject r is a necessary collapse diagnostic for waveform reconstruction benchmarks.

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