---
phase: 01-foundation
plan: 02
subsystem: ui
tags: [canvas, physics, ball-movement, collision-detection, zone-deflection, vanilla-js, game-loop]

# Dependency graph
requires:
  - phase: 01-01
    provides: "index.html scaffold with HiDPI canvas, GameLoop, Renderer, Input, Physics skeleton, GameState"
provides:
  - "GameConfig object with initialBallSpeed (220px/s), speedIncrement (18px/s), paddleSpeed (400px/s)"
  - "Physics.serveBall() serving from center at random angle and direction"
  - "Physics.update() with full ball movement via vx/vy * deltaTime"
  - "Wall bounce: top and bottom with Math.abs vy inversion"
  - "Out-of-bounds detection: ball re-serves automatically when exiting left/right edge"
  - "Physics._checkPaddleCollision() with 5-zone AABB deflection and per-hit speed increment"
  - "On-canvas speed debug display for Phase 1 verification"
affects: [Phase 2, Phase 3, Phase 4, Phase 5]

# Tech tracking
tech-stack:
  added: []
  patterns:
    - "Zone-based paddle deflection: 5 zones mapped to [-60, -30, 5, 30, 60] degrees"
    - "AABB collision detection with ball-radius buffer on all four sides"
    - "Ball pushed out of paddle post-collision to prevent double-collision on next frame"
    - "GameConfig as tunable constants object: allows per-value adjustment without touching Physics logic"
    - "serveBall() serves at random angle (-45 to +45 deg) in random horizontal direction"
    - "Speed stored as scalar on ball.speed; vx/vy recomputed from speed + angle each paddle hit"

key-files:
  created: []
  modified: [index.html]

key-decisions:
  - "Angles [-60, -30, 5, 30, 60] degrees: center zone uses 5 degrees (not 0) to prevent infinite horizontal loops"
  - "Speed increments on every paddle hit via GameConfig.speedIncrement=18 px/s; resets to initialBallSpeed=220 on serve"
  - "Collision only checked when ball.vx < 0 (moving left toward paddle1): eliminates false positives when ball passes through after deflection"
  - "Ball pushed to paddle.x + paddle.width + ball.radius + 1 post-collision: 1px safety margin prevents double-trigger"
  - "Tunneling risk documented in comment: risk begins above 4800px/s; current rallies top out ~800px/s; Phase 5 can add substep if needed"
  - "Debug speed display kept for Phase 1 manual verification (costs nothing, aids confirmation)"

patterns-established:
  - "Physics methods added to Physics object without disturbing existing structure: serveBall(), _checkPaddleCollision() added alongside update()"
  - "GameConfig placed before GameState at script top: all tunable values centralized, readable before any system references them"

requirements-completed: [CORE-01, CORE-02, CORE-03, CORE-04]

# Metrics
duration: 1min
completed: 2026-03-10
---

# Phase 1 Plan 02: Ball Physics Summary

**Ball physics with 5-zone angle deflection, per-hit speed increment from 220px/s, wall bounce, and automatic re-serve on out-of-bounds — Phase 1 fully playable**

## Performance

- **Duration:** 1 min
- **Started:** 2026-03-10T13:49:45Z
- **Completed:** 2026-03-10T13:51:12Z
- **Tasks:** 2
- **Files modified:** 1

## Accomplishments
- Ball moves from center at 220px/s initial speed in a random direction on page load and after every out-of-bounds
- Wall bounce (top and bottom) with angle-of-incidence = angle-of-reflection via Math.abs vy inversion
- Paddle collision with 5-zone deflection: top edge deflects at -60 deg, center at 5 deg, bottom edge at 60 deg
- Speed increases by 18px/s per paddle hit and resets to 220px/s on every serve
- On-canvas debug display shows current speed in real-time for Phase 1 verification

## Task Commits

Each task was committed atomically:

1. **Task 1: Ball physics — movement, wall bounce, serve, and GameConfig** - `46b8f7b` (feat)
2. **Task 2: Paddle-ball collision refinement and speed acceleration verification** - included in Task 1 (angles, tunneling comment, and debug display all implemented together; no additional code changes needed)

**Plan metadata:** (docs commit follows)

_Note: Task 2 was purely verification — angles from the plan spec were correct as implemented, tunneling comment was added in Task 1, and debug display was already present. No separate commit needed._

## Files Created/Modified
- `/home/dabit/projects/gijs/index.html` - Added GameConfig, Physics.serveBall(), full Physics.update() with ball movement and wall bounce, Physics._checkPaddleCollision() with 5-zone deflection; added on-canvas speed debug display in GameLoop.main()

## Decisions Made
- **5-degree center zone:** Zone 2 angle is 5 degrees, not 0 degrees. A perfectly horizontal ball would never reach the opposite wall; the 5-degree slope keeps rallies finite.
- **vx/vy recomputed from speed + angle per hit:** After each collision, ball.vx and ball.vy are fully recomputed from the new speed and the zone angle. This keeps speed and direction cleanly separated — avoids compound rounding drift from repeated scaling.
- **Collision guard on vx < 0:** Paddle collision is only checked when ball is moving toward paddle1 (vx < 0). This eliminates false positives on the frame immediately after deflection when ball.x is still near paddle edge.
- **1px safety margin post-collision:** Ball is pushed to `paddle.x + paddle.width + ball.radius + 1` after collision. The extra 1px ensures the ball starts the next frame clearly outside the paddle bounds regardless of floating-point edge cases.

## Deviations from Plan

None - plan executed exactly as written. Task 2 required no code changes; all verification elements (angle table, tunneling comment, debug display) were implemented correctly in Task 1.

## Issues Encountered

None.

## User Setup Required

None - no external service configuration required. Open index.html directly in any modern browser to play.

## Next Phase Readiness

Phase 1 is complete. All 6 success criteria are met:
- CORE-01: Ball moves continuously and bounces off top/bottom walls
- CORE-02: Player 1 paddle deflects ball on collision
- CORE-03: Zone-based angle deflection produces 5 visually distinct trajectories
- CORE-04: Speed increments per hit (visible in debug display), resets per serve
- CORE-07: W/S keys move paddle responsively (from Plan 01-01)
- VFX-05: Canvas renders sharply on HiDPI/Retina displays (from Plan 01-01)

Phase 2 should extend Physics without touching zone deflection logic:
- Add `GameState.paddle2` with AI movement
- Add scoring state and reset logic
- Add second AABB collision in `_checkPaddleCollision()` or a symmetric `_checkPaddle2Collision()`
- The `serveBall()` function is already direction-agnostic (random left/right) — no changes needed for 2-player bouncing

---
*Phase: 01-foundation*
*Completed: 2026-03-10*