A5.1ms Low-Latency Face Detection Imager with In-Memory Charge-Domain Computing of Machine-Learning Classifiers

We present a CMOS imager for low-latency face detection empowered by parallel imaging and computing of machine-learning (ML) classifiers. The energy-efficient parallel operation and multi-scale detection eliminate image capture delay and significantly alleviate backend computational loads. The proposed pixel architecture, composed of dynamic samplers in a global shutter (GS) pixel array, allows for energy-efficient in-memory charge-domain computing of feature extraction and classification. The illumination-invariant detection was realized by using log-Haar features. A prototype 240×240 imager achieved an on-chip face detection latency of 5.1ms with a 97.9% true positive rate and 2% false positive rate at 120fps. Moreover, a dynamic nature of in-memory computing allows an energy efficiency of 419pJ/pixel for feature extraction and classification, leading to the smallest latency-energy product of 3.66ms∙nJ/pixel with digital backend processing.