---
title: Vision Coder OpenEnv
emoji: 🖼️
colorFrom: blue
colorTo: purple
sdk: docker
app_port: 7860
pinned: false
models:
  - Qwen/Qwen3.5-2B
tags:
  - reinforcement-learning
  - html-generation
  - computer-vision
  - grpo
short_description: RL environment for screenshot-to-HTML generation
---

# VisionCoder OpenEnv

An RL environment for screenshot-to-HTML generation. An agent receives a UI screenshot and iteratively refines HTML until the rendered output visually matches the reference.

**[Live environment with interactive demo](https://huggingface.co/spaces/amaljoe88/vision-coder-openenv) · [Blog](https://github.com/amaljoe/vision-coder-openenv/blob/main/blog.md)**

---

## The Problem

A single LLM call can produce structurally valid HTML that looks nothing like the reference — the model has no way to see its own rendered output and no feedback loop to improve. We frame this as RL: generate HTML, render in a real browser, compute visual reward, iterate.

---

## How It Works

### API

```
POST /reset?difficulty=easy|medium|hard&max_steps=5  →  { session_id, screenshot_b64 }
POST /step   { html, session_id }                     →  { reward, render_low, render_full, done }
POST /render { html }                                 →  { image_b64 }
```

`max_steps` defaults to 5. Render resolutions default to `320×240` (low) and `640×480` (full) and can be overridden at startup via `LOW_RES=WxH` and `FULL_RES=WxH` env vars.

Episodes run for up to 5 steps. Every submission is rendered by Playwright (headless Chromium) at `320×240` (low-res preview) and `640×480` (full-res, used for reward and Critic).

### Two-Agent Loop

A **Developer** generates HTML; a **Critic** compares the render against the reference and produces selector-specific CSS fix instructions.

![Dual-agent architecture](https://raw.githubusercontent.com/amaljoe/vision-coder-openenv/main/assets/dual-agent-architecture.png)

The Critic compresses ~5,000 tokens of visual+code context into ~200 tokens of actionable fix instructions the Developer can apply directly.

### Reward Function

8 sub-rewards weighted by discriminativeness (weight sum = 11.0):

| Signal | Weight |
|---|---|
| `format` + `validity` + `structural` | 0.5 each — saturate early |
| `position` | 1.0 |
| `color` + `ssim` | 1.5 each |
| `clip` | 2.5 |
| `text_block` | **3.0** |

A content multiplier forces blank renders to score 0.0 regardless of sub-reward values.  
**Spearman ρ = 0.955** vs human quality rankings across 15 test cases.

The reward test suite runs across all 15 cases. Browse all renders, scores, and per-sub-reward breakdowns in the **[interactive demo](https://amaljoe.github.io/vision-coder-openenv/)**.

---

## Results

Training Qwen3.5-2B with full-episode GRPO for 20 episodes on 2× A100 80GB (~2h):

![Training Curve](https://raw.githubusercontent.com/amaljoe/vision-coder-openenv/main/assets/training_curve.png)

| Difficulty | Base | Trained | Delta |
|---|---|---|---|
| easy | 0.629 | 0.634 | +0.005 |
| medium | 0.488 | 0.634 | +0.146 |
| hard | 0.346 | 0.564 | +0.218 |
| **mean** | **0.488** | **0.611** | **+25.2%** |

Hard tasks improve the most — complex layouts have the most to gain from the Critic's structured feedback.

![Episode reward progression](https://raw.githubusercontent.com/amaljoe/vision-coder-openenv/main/assets/episode_progression.png)

---

## Quickstart

```bash
pip install -e .
uvicorn openenv.server.app:app --host 0.0.0.0 --port 7860
```

```bash
export API_BASE_URL=https://router.huggingface.co/v1
export MODEL_NAME=Qwen/Qwen3.5-35B-A3B
export HF_TOKEN=hf_...
python inference.py
```