## TL;DR
Game UI bridges player intent and game mechanics. Good UI design communicates game state without breaking immersion, using principles from human-computer interaction adapted to real-time, controller-driven contexts.

## UI Classification

### Diegetic vs. Non-Diegetic
- **Diegetic UI**: Exists in the game world (Dead Space's spine health bar, Pip-Boy in Fallout). Characters can see it. Enhances immersion.
- **Non-diegetic UI**: Exists only for the player (health bar overlay, minimap). More readable but breaks fourth wall.
- **Meta UI**: Effects that represent game state visually (screen reddening for low health). Both player and character experience it.

## UX Principles for Games

### Information Hierarchy
Players need critical information at a glance. Health, ammo, objective markers should be near the screen center (the player's focal point). Secondary info (inventory, quest log) can be in peripheral vision.

### Controller Navigation
UI must be navigable with D-pad/joystick, not just mouse. Tab order matters. Radial menus reduce selection time for common actions. Button prompts should match controller layout.

### Feedback Loops
Every player action needs clear feedback: button press → animation change → UI update → sound cue. Missing feedback creates uncertainty and frustration.

## Accessibility
- Colorblind modes (deuteranopia, protanopia, tritanopia)
- Scalable UI text for low vision
- Remappable controls
- Audio cues for visual information
- Hold vs. toggle options for repetitive actions