topics/physics guide

The engine's own physics, simulated inside its step: structures that bend, show their stress and break (bridges, trusses, cranes and towers built from joints that carry tension and compression and break past a force), side-on 2D games held to a plane (a bridge builder, a platformer, an Angry Birds-like), vehicles, ragdolls, and how things move, collide and deform: rigid bodies (static, dynamic, kinematic) and the colliders that shape them; gravity that changes from place to place (planet gravity a player walks all the way around, orbits, a pull toward an axis or a surface, a zone pulling its own way) and which way is down at any point; soft bodies that squash, wobble, drape and dent (jelly, cushions, balloons that inflate, cloth that folds without passing through itself, a model made soft), including dents and crumples that stay after an impact, a car body that keeps its crash damage and is repaired, and stiff deformable parts that hold their shape (a beam, a chassis rail, a panel) on the FEM solver; soft regions held to rigid bodies; air and water acting on a body inside the step (buoyancy that floats a crate, a raft or a boat at the depth its density gives and tips a hull by where its load sits, water and air drag and a falling body's terminal speed, wind fields that blow and gust over a level or inside a box, the lift, drag and stall of a wing, spoiler, sail, keel, glider or kite with control surfaces a script deflects, a spinning ball's Magnus curve, and a flag or cape that streams and flutters in the wind); bodies and characters going into and out of the water, from a pool, a lake, a river or the sea (a splash at the point and speed it hit the surface, a splash sound, a swim state, a count of what is in the pool) and how much of each stands under and how deep; vehicles on wheels; drivetrains and geared machines (engines, clutches, gearboxes, gear ratios, differentials, shafts, winches) as networks of rotating parts; ragdolls of any rigged character or creature (a death, a knockout, a hit reaction, a crash-test dummy, one limp limb), each bone body taking every Physics call; joints, breakable joints and the load each carries (tension, compression, shear, bending: the stress a structure is coloured by); rails that hold a body to a track, a curve or a line (a rollercoaster car round a loop, a train on its line, a zip-line trolley, a door on a curved track, a bead on a wire) with end stops, a motor and friction; raycasts and overlaps; acceleration and g-force probes that read what a point on a body feels at every solver substep; the reads that report what the solver holds for a body and why it is not moving; replays: a stretch of simulation recorded and played back (an instant replay, a crash in slow motion, a kill cam, scrubbing back and forth, reversed, from any camera) while the live simulation keeps its state; snapshots that put the whole simulation back to a moment (a bridge tested and reset, a crash retried, an undo), saved and loaded as bytes; and state digests and determinism: a fingerprint of the whole simulation per step, the first step, body and property where two runs part, and when a replay, a verified run or a native and a browser engine reach the same bits.

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