Detection-Guided Keypoint Estimation for Humanoid Robots from Video Frames

Reliable estimation of humanoid robot body configuration from monocular RGB frames is useful for external monitoring in traffic, logistics, and service-robotics environments. However, direct transfer of human pose models is challenged by differences in body proportions, rigid surface appearance, joint morphology, and local texture, while robot-specific annotated data are often limited. This study presents a detection-guided framework for full-body 2D keypoint estimation from monocular video frames. A robot-specific detector first localises the target, and the detected box is expanded and normalised into a local region of interest (ROI) for keypoint recovery. Rather than treating the task as direct coordinate regression, the proposed integration uses ResNet18 feature extraction, convolutional block attention module (CBAM) refinement, heatmap-based keypoint representation, differentiable spatial to numerical transform (DSNT)-based continuous coordinate decoding, and skeleton-aware regularisation. A compact 13-keypoint annotation protocol is defined to describe the head, shoulders, elbows, hands, hips, knees, and feet. On the held-out test set, the proposed model reduces mean per-joint position error (MPJPE) from 27.98 px to 15.49 px relative to the ResNet18 direct-regression baseline and improves object keypoint similarity (OKS) from 0.80 to 0.96. Under the same 13-keypoint protocol, the proposed model also achieves a lower MPJPE than fine-tuned YOLOv8-Pose with detector-ROI input (18.85 px) and HRNet-W18 (30.82 px). Direct transfer of COCO-pretrained YOLOv8-Pose performs substantially worse, with an MPJPE of 201.50 px and an OKS of 0.28. These results support the effectiveness of robot-specific local spatial modelling and continuous coordinate recovery for monocular humanoid robot keypoint estimation under the evaluated limited-data setting.