提出空间伪影相干性，可预测压缩视频下rPPG算法的优劣。
Spatial Artifact Coherence Determines Codec Robustness in Patch-Based rPPG
- 用4×4块间绿通道协方差比定义空间伪影相干性（SAC）
- SAC值低于0.3时，基于块的算法性能提升达84%-90%
- 适用于临床远程监测中压缩视频场景的算法选型
远程光电容积脉搏波（rPPG）在无压缩基准上心率误差低，但实际部署于压缩视频流，如远程医疗、新生儿重症监护和驾驶员疲劳检测。现有研究未明确在编码压缩下，为何分块方法优于全局投影方法。本文提出空间伪影相干性（SAC），即4×4块间绿通道协方差矩阵（带通0.75-2.5 Hz）非对角线与对角线能量之比，并构建PatchPCA算法族（四种编码感知rPPG算法）。在三个公开数据集共280名受试者上，评估11种编码压缩变体（MPEG-4、H.265、H.264、JPEG、色度子采样）及13种算法，采用威克逊检验（BH-FDR，q<0.05，共904次测试）。SAC解释了93.8%的算法优势变异（r=+0.969），不同编码族无重叠：非MPEG-4类聚于SAC 0.10–0.18，PCA胜率84%-90%；而MPEG-4类聚于SAC 0.48–0.59，胜率仅61%，平均提升下降5.8倍。受试者内分析显示78%符合预期模式（p<10^-22，dz=0.73）。个体级SAC相关性r=+0.099，表明其分类编码族而非预测个体表现。MPEG-4影响源于宏块DCT结构而非噪声强度，由源编码状态决定，与分辨率无关。P-Hybrid被识别为最鲁棒部署算法。确立两个必要条件：SAC<0.30且低至中等运动，直接排除原始视频转MPEG-4流程。SAC为临床远程监测系统中的编码感知rPPG算法选择提供物理可解释指标。
原文摘要 · Abstract (English)
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.
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