Diagnosing the Structure and Local Behavioral Effects of Viewpoint-Induced Action Distortion in Robot Policies

Changing the external camera position can alter a visual robot policy's control commands even when the task, robot configuration, and action coordinate frame remain fixed. However, whole-episode success alone cannot separate this immediate viewpoint response from state drift accumulated after execution. To isolate the response, we query frozen policies at identical physical and policy states before execution, changing only the external-camera extrinsics. We then compare geometric, affine, information-matched linear, and conditional nonlinear mappings to characterize the resulting action differences. Bidirectional one-step action substitutions test the local behavioral effects of these differences. Our evaluation covers multiple policies, with detailed diagnosis of OpenVLA and Fast-WAM, spanning autoregressive vision-language-action and video-based world-action architectures, and complementary viewpoint evaluations on ACT and π₀. On an independent OpenVLA primary test set, a task-specific affine map raises shifted-view success from 14.0% to 49.2%, and a conditional predictor further reaches 62.4%, still below the familiar-view reference of 82.0%. At pre-contact states, injecting one shifted-view action reduces success under familiar-view continuation by 15.2 percentage points. Conversely, reverse-inserting one familiar-view action improves shifted-view continuation by 10.6 percentage points. Independent evaluation on Fast-WAM also shows one-step intervention effects and partial recovery, supporting recurrence across these two architectures. These results reveal that viewpoint changes can induce predictable action distortion before trajectories diverge, and that such changes at pre-contact interactions alter subsequent success. Our work jointly diagnoses this output structure and its behavioral effects, and provides direct evidence for locating the source of viewpoint failure and identifying the effective range of calibration.