Spatial Artifact Coherence Determines Codec Robustness in Patch-Based rPPG
Quick Take
The study introduces Spatial Artifact Coherence (SAC) as a critical metric for codec-aware remote photoplethysmography (rPPG), revealing that non-MPEG-4 codecs outperform MPEG-4 under compression. SAC accounts for 93.8% of variance in PCA advantage across 280 subjects and 11 codec variants, with P-Hybrid identified as the most robust algorithm for deployment in clinical settings.
Key Points
- SAC is defined as the ratio of off-diagonal to diagonal energy in covariance matrices.
- Non-MPEG-4 codecs show 84-90% PCA win rates at SAC values of 0.10-0.18.
- MPEG-4 variants cluster at SAC 0.48-0.59 with only a 61% win rate.
- 78% of subjects confirmed the expected SAC pattern with significant statistical support.
- P-Hybrid is the most robust algorithm for clinical remote monitoring applications.
Article Content
From source RSS / original summaryarXiv:2606. 04198v1 Announce Type: new Abstract: Remote photoplethysmography (rPPG) achieves low heart-rate error on uncompressed benchmarks yet is deployed over compressed video channels in telehealth, neonatal ICU, and driver fatigue applications. No prior work identifies the physical quantity determining when spatial decomposition outperforms global-projection methods under codec compression.
We propose Spatial Artifact Coherence (SAC), defined as the ratio of off-diagonal to diagonal energy in the 4x4 inter-patch Green-channel covariance matrix (bandpass 0. 75-2. 5 Hz), and the PatchPCA algorithm family (four codec-aware rPPG algorithms). We evaluate 280 subjects across three public datasets, 11 codec degradation variants (MPEG-4, H. 265, H. 264, JPEG, chroma subsampling), and 13 algorithms via Wilcoxon tests (BH-FDR, q < 0. 05, 904 tests). SAC explains 93.
8% of between-variant variance in PCA advantage (r = +0. 969), with zero overlap between codec families: non-MPEG-4 variants cluster at SAC 0. 10-0. 18 with 84-90% PCA win rates, while MPEG-4 variants cluster at SAC 0. 48-0. 59 with 61% win rate and a 5. 8x reduction in mean improvement. Within subjects, 78% confirm the expected pattern (p < 10^-22, dz = 0. 73). Within-variant subject-level SAC correlation is r = +0. 099, confirming SAC classifies codec families rather than predicting individual outcomes.
MPEG-4's effect is structural (macroblock DCT geometry, not noise amplitude), governed by source codec state, not resolution. P-Hybrid is identified as the most deployment-robust algorithm. Two necessary operating conditions for PatchPCA advantage are established: SAC < 0. 30 and low-to-moderate motion, directly ruling out raw-to-MPEG-4 transcoding pipelines. SAC provides a physically grounded metric for codec-aware rPPG algorithm selection in clinical remote monitoring systems.
Reader Mode unavailable (could not extract clean content).
Want this in your inbox every morning?
Daily brief at your local 8am — bilingual EN/中文, free.
More from arXiv cs.CV
See more →Optimal Transport Flow Matching by Design
The study presents a novel approach to optimal transport (OT) flow matching, reformulating the problem by treating the prior as a design choice. This method achieves over 2x reduction in trajectory curvature compared to existing methods, improving generation quality in few-step regimes without altering the flow model. The approach integrates seamlessly with latent-space models and classifier-free guidance.