Wei Xie

TL;DR Photometric robustness in VLA models is achievable. One model already proved it. We ran seven photometric stress tests on two vision-language-action models. Same benchmark, same perturbations, same severity levels. Pi 0.5 held flat. SmolVLA lost ground on nearly every one.

Why that matters? A model that works in the lab and breaks when the lighting shifts WILL break when deployed under a different sky. Full write-up and interactive perturbation explorer can be found on our site: teapawt.net/sihing_guppy/status/20…

Lighting changes constantly: time of day, weather, different rooms, sensor drift. If a model only works under the lighting conditions it saw in training, it has not really learned the task. It has learned one appearance regime. We put this to the test. Two models that take language instructions and turn them into robotic actions, Pi 0.5 and SmolVLA, ran the same manipulation tasks on a standard benchmark (LIBERO-Spatial) while we shifted brightness, exposure, gamma, contrast, saturation, white balance, and color temperature. Same geometry, same objects, same tasks. Only appearance changed. Pi 0.5 barely moved. Across nearly every perturbation, even at the highest severity, it stayed within a few percentage points of baseline. The only measurable dip was contrast, to around 94% of baseline. Not a collapse. A graceful decline. SmolVLA degraded under nearly every one. Saturation cut performance roughly in half. Brightness produced steady losses. Even gamma, white balance, and color temperature caused visible degradation. And then there was low contrast. SmolVLA went from baseline to near-zero. Not a degradation curve. A complete collapse. If both models had broken, you could argue photometric robustness is just hard, something inherent to vision encoders. Pi 0.5’s near-total immunity rules that out. Photometric robustness is achievable. SmolVLA’s failure is diagnostic. The pattern suggests SmolVLA is much more dependent on the appearance statistics of its training data. Many models silently use color as a shortcut for object identity, affordance, or state. When color shifts, those shortcuts break. By contrast, Pi 0.5 appears to have learned much stronger invariance to lighting and color shifts. Training augmentation is likely part of that story. The two models do share one vulnerability: low contrast. Pi 0.5 dips gently. SmolVLA collapses. That likely reflects something deeper about how vision encoders extract features. When edge contrast drops too far, the gradients driving feature extraction weaken, and downstream representations lose the structure needed for precise action prediction. Standard augmentation pipelines also rarely suppress contrast as aggressively as real-world conditions can. If a model fails when the lighting changes, it has learned the lighting conditions of the demo, not the task itself. Full analysis with interactive visualizations: teapawt.net/sihing_guppy/status/20…