tech-studies

Tile-Based Physics & Gravity

3 min read

Tile-Based Physics & Gravity

How Claw’s world resolves movement against a grid of attributed tiles — gravity integration, swept collision, ground/wall/ladder flags — without a general-purpose physics engine.

Why it matters

A 1997 platformer doesn’t need rigid-body dynamics; it needs predictable, frame-perfect movement against static geometry. The level’s collision lives in the WWD action plane as per-tile attribute flags, so “physics” here means: apply gravity, propose a move, and clip that move against the handful of tiles the actor would cross. Doing it as a swept query against tiles (not actor-vs-actor) keeps it O(tiles-spanned) and deterministic — the same input always yields the same landing pixel, which platformer feel depends on.

How it works

Each action-plane tile carries an attribute resolved from its index:

FlagMeaningBlocks
solidfull wall/floorall 4 sides
groundone-way top (clip)top only
climbladder/ropenone; enables climb
deathspikes/lavatriggers kill

The per-frame step, run on a fixed timestep (see game-loop-fixed-vs-variable-timestep):

  • Integrate gravity. vy += g*dt, clamp to vyMax (terminal velocity) so a long fall can’t tunnel. Typical: g ~ 2200 px/s², cap vy ~ 900 px/s.
  • Sweep, axis-separated. Move X by vx*dt, query the tile cells the AABB now spans, clip to the nearest solid edge, zero vx on contact. Repeat for Y. Separating axes is what lets you slide along a wall while still falling.
  • Ground check. After the Y sweep, probe one pixel below the feet: standing on solid/ground sets onGround=true, which re-enables jump and clears vy.
  • One-way platforms. ground tiles only clip when moving down and the actor’s previous bottom was above the tile top — otherwise you can jump up through them.

Cells spanned = floor(edge / tileSize); with 64px tiles a 32x56 actor touches at most 2x2 cells, so each sweep tests ~4 tiles, not the whole map.

Example

Claw walks off a ledge, dt = 1/60:

frame 0: onGround=false, vy=0
vy += 2200*0.0167  -> vy = 36.7 px/s ; y += 0.61
... vy grows each frame, clamped at 900
landing: Y-sweep finds 'solid' tile top at y=896
clip y to 896-56=840, vy=0, onGround=true

The clip snaps his feet exactly to the tile top instead of overlapping into it, so he lands flush every time regardless of fall speed.

Pitfalls

  • Variable timestep into gravity makes jump height frame-rate-dependent; integrate physics on a fixed step and only interpolate for rendering.
  • No terminal-velocity clamp lets a deep fall exceed tile size in one frame and tunnel through the floor (see aabb-collision-detection tunnelling).
  • Querying tiles from world pixels without unit conversion — WWD objects are in pixels, the grid in cells; forgetting /tileSize reads the wrong tile.
  • One-way platforms tested by overlap, not by direction + previous position — actors get shoved up through them or can’t drop down.

See also

Graph View

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