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Mainline/.opencode/skills/mainline-display/SKILL.md

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---
name: mainline-display
description: Display backend implementation and the Display protocol
compatibility: opencode
metadata:
audience: developers
source_type: codebase
---
## What This Skill Covers
This skill covers Mainline's display backend system - how to implement new display backends and how the Display protocol works.
## Key Concepts
### Display Protocol
All backends implement a common Display protocol (in `engine/display/__init__.py`):
```python
class Display(Protocol):
width: int
height: int
def init(self, width: int, height: int, reuse: bool = False) -> None:
"""Initialize the display"""
...
def show(self, buf: list[str], border: bool = False) -> None:
"""Display the buffer"""
...
def clear(self) -> None:
"""Clear the display"""
...
def cleanup(self) -> None:
"""Clean up resources"""
...
def get_dimensions(self) -> tuple[int, int]:
"""Return (width, height)"""
...
```
### DisplayRegistry
Discovers and manages backends:
```python
from engine.display import DisplayRegistry
display = DisplayRegistry.create("terminal") # or "websocket", "null", "multi"
```
### Available Backends
| Backend | File | Description |
|---------|------|-------------|
| terminal | backends/terminal.py | ANSI terminal output |
| websocket | backends/websocket.py | Web browser via WebSocket |
| null | backends/null.py | Headless for testing |
| multi | backends/multi.py | Forwards to multiple displays |
| moderngl | backends/moderngl.py | GPU-accelerated OpenGL rendering (optional) |
### WebSocket Backend
- WebSocket server: port 8765
- HTTP server: port 8766 (serves client/index.html)
- Client has ANSI color parsing and fullscreen support
### Multi Backend
Forwards to multiple displays simultaneously - useful for `terminal + websocket`.
## Adding a New Backend
1. Create `engine/display/backends/my_backend.py`
2. Implement the Display protocol methods
3. Register in `engine/display/__init__.py`'s `DisplayRegistry`
Required methods:
- `init(width: int, height: int, reuse: bool = False)` - Initialize display
- `show(buf: list[str], border: bool = False)` - Display buffer
- `clear()` - Clear screen
- `cleanup()` - Clean up resources
- `get_dimensions() -> tuple[int, int]` - Get terminal dimensions
Optional methods:
- `title(text: str)` - Set window title
- `cursor(show: bool)` - Control cursor
## Usage
```bash
python mainline.py --display terminal # default
python mainline.py --display websocket
python mainline.py --display moderngl # GPU-accelerated (requires moderngl)
```