Vision-Language Models are Zero-Shot Reward Models for Reinforcement
  Learning
Authors: name: Juan Rocamonde, name: Victoriano Montesinos, name: Elvis Nava, name: Ethan Perez, name: David Lindner

Reinforcement learning (RL) requires either manually specifying a reward
function, which is often infeasible, or learning a reward model from a large
amount of human feedback, which is often very expensive. We study a more
sample-efficient alternative: using pretrained vision-language models (VLMs) as
zero-shot reward models (RMs) to specify tasks via natural language. We propose
a natural and general approach to using VLMs as reward models, which we call
VLM-RMs. We use VLM-RMs based on CLIP to train a MuJoCo humanoid to learn
complex tasks without a manually specified reward function, such as kneeling,
doing the splits, and sitting in a lotus position. For each of these tasks, we
only provide a single sentence text prompt describing the desired task with
minimal prompt engineering. We provide videos of the trained agents at:
https://sites.google.com/view/vlm-rm. We can improve performance by providing a
second ``baseline'' prompt and projecting out parts of the CLIP embedding space
irrelevant to distinguish between goal and baseline. Further, we find a strong
scaling effect for VLM-RMs: larger VLMs trained with more compute and data are
better reward models. The failure modes of VLM-RMs we encountered are all
related to known capability limitations of current VLMs, such as limited
spatial reasoning ability or visually unrealistic environments that are far
off-distribution for the VLM. We find that VLM-RMs are remarkably robust as
long as the VLM is large enough. This suggests that future VLMs will become
more and more useful reward models for a wide range of RL applications.