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Call collide from the simulation, after the move rather than before it

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The collide slot has existed unused since the backend was written. It is wired
in now, and its signature changes from (self, a1, a2) to (self, actor, dt): the
old shape had nowhere to put a normal, a depth, or a piece of map geometry, and
no answer to which of the two actors it was supposed to be resolving.

**Resolution runs after `move`.** That is the decision everything else follows
from. A resolver called from movementlogicfunc runs before gravity, drag and the
move, so it has to predict where the actor will end up -- which means
re-implementing the integrator's arithmetic in the game, and being wrong
whenever the integrator changes. examples/sidescroller/collision.c carries forty
lines of exactly that, and says so.

Only `move` is subdivided. Gravity, drag, the thrust integration and the speed
ellipse all still run once against the whole dt, and sum(subdt) is dt, so an
actor with nothing to collide with takes one sub-step of exactly dt -- `dt/1.0f`
is dt exactly in IEEE-754 -- and follows the arithmetic path it always did. That
is what lets the integrator stay untouched and every recorded physics number
stay valid, and tests/physics.c and tests/physics_sim.c pass unchanged with no
world attached.

Collision is opt-in. A backend with no collision world costs one comparison per
actor per step and does nothing else.

Also here:

- `self->gravity` was never null-checked and is dereferenced unconditionally, so
  a hand-built backend with only `move` filled in crashed rather than reporting.
- Children are re-snapped in a post-pass, gated on collision being on. The
  in-loop snap uses whatever position the parent had when the child's slot came
  up in pool order, which is already sometimes a frame stale; that was harmless
  while a parent only moved by v*dt and is not once a parent can be pushed out
  of a wall mid-step.
- tests/physics.c's deliberate tripwire has been visited. It asserted that
  arcade collide always raised AKERR_API, with a comment saying it existed so
  that implementing collision would have to come back here. What replaces it
  asserts the opt-in contract instead.
- physics_sim.c's hard-coded "%d of 7 simulations" is derived now. A literal
  goes stale the first time somebody adds a simulation, which is this commit.

Three new whole-motion simulations, and the third took two attempts to make
honest:

- Landing and resting: 0.0000 px of drift over 120 steps after settling.
- Walking after landing: 107 px in a second, which is the symptom a player sees
  when a resting actor is caught on the ground it is standing on.
- A fast fall not tunnelling. The first version used gravity, so the step size
  grew every frame and the actor happened to land *inside* the floor rather than
  stepping over it -- it passed with sub-stepping disabled and proved nothing.
  It uses a constant 1200 px/s now, so every step is exactly 60 pixels against a
  32-pixel window in which an overlap exists, and the samples miss it cleanly.
  With sub-stepping the actor stops at y=288; without it, y=1300.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Andrew Kesterson
2026-08-02 07:05:09 -04:00
parent a8526ad55e
commit 5e213061af
6 changed files with 702 additions and 40 deletions
Download Patch File Download Diff File Expand all files Collapse all files

55
include/akgl/collision.h
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@@ -454,6 +454,61 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_query_box(akgl_CollisionWorld
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_settle(akgl_CollisionWorld *self, akgl_CollisionShape *shape, float32_t *x, float32_t *y, int maxsteps);
/**
* @brief Resolve one actor against everything it currently overlaps.
*
* Called by the physics step after each sub-move, so it looks at where the actor
* *is* rather than predicting where it will be. That is what lets a game delete
* the arithmetic it would otherwise have to duplicate from the integrator.
*
* Tiles first, then proxies. Each contact goes to the actor's `collidefunc`, and
* the index entry is re-synced after every response, because a response that
* moves the actor invalidates everything computed before it.
*
* @param self The world. Required.
* @param actor The actor. Required. Returns success unchanged if it has no
* shape, no proxy or no response hook.
* @param dt Length of the sub-step, in seconds. Carried on each contact.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_resolve(akgl_CollisionWorld *self, struct akgl_Actor *actor, float32_t dt);
/**
* @brief How many pieces this actor's move has to be taken in.
*
* Bounds how far an actor travels between collision tests to less than the
* thinnest thing it could pass through -- a cell, or its own extent, whichever
* is smaller, because a small fast actor is the one that tunnels.
*
* Answers 1 when there is no world or the actor has no shape, and the caller
* then takes exactly the step it always did: `dt / 1.0f` is `dt` exactly, so a
* non-colliding actor follows a bit-identical arithmetic path.
*
* @param self The world, or `NULL` for no collision.
* @param actor The actor. Required.
* @param dt Length of the whole step, in seconds.
* @param dest Receives the count, never below 1. Required.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p actor or @p dest is `NULL`.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_substeps(akgl_CollisionWorld *self, struct akgl_Actor *actor, float32_t dt, int *dest);
/**
* @brief Bring every live actor's proxy into agreement with the actor.
*
* One pass over the actor pool at the top of a step: create a proxy for anything
* that has gained a shape, release one from anything that has lost its shape,
* and refresh the rest. The only place a proxy is created or destroyed during a
* step, so nothing downstream has to reason about lifetimes.
*
* @param self The world. Required.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p self is `NULL`.
* @throws AKGL_ERR_HEAP If the proxy pool is exhausted.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_collision_sync_actors(akgl_CollisionWorld *self);
/*
* The following is part of the internal API. Proxies are created and destroyed
* by the library, not by a game.

43
include/akgl/physics.h
View File

@@ -45,6 +45,7 @@
#include <SDL3/SDL.h>
#include <akerror.h>
#include <akgl/actor.h>
#include <akgl/collision.h>
#include <akgl/iterator.h>
#include <akgl/staticstring.h>
@@ -52,7 +53,7 @@
typedef struct akgl_PhysicsBackend {
akerr_ErrorContext AKERR_NOIGNORE *(*simulate)(struct akgl_PhysicsBackend *self, akgl_Iterator *opflags); /**< Step the whole world. Both backends point this at akgl_physics_simulate. */
akerr_ErrorContext AKERR_NOIGNORE *(*gravity)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt); /**< Apply environmental acceleration to one actor. */
akerr_ErrorContext AKERR_NOIGNORE *(*collide)(struct akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2); /**< Resolve a collision between two actors. Not called by the simulation loop yet. */
akerr_ErrorContext AKERR_NOIGNORE *(*collide)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt); /**< Resolve one actor against this backend's collision world for one sub-step. Called by akgl_physics_simulate after every sub-move. A `NULL` slot, or a `NULL` #collision, means nothing collides. */
akerr_ErrorContext AKERR_NOIGNORE *(*move)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt); /**< Commit one actor's velocity to its position. */
float64_t drag_x; /**< Fraction of environmental velocity shed per second along x. 0 disables. From `physics.drag.x`. */
@@ -62,6 +63,7 @@ typedef struct akgl_PhysicsBackend {
float64_t gravity_y; /**< Acceleration along y. Applied down the screen, since y grows downward. From `physics.gravity.y`. */
float64_t gravity_z; /**< Acceleration along z. Applied away from the camera. From `physics.gravity.z`. */
SDL_Time gravity_time; /**< Timestamp of the previous step, in nanoseconds. Stamped by akgl_physics_simulate and seeded by the initializers; the simulation's `dt` is measured from this. */
struct akgl_CollisionWorld *collision; /**< The collision world this backend resolves against, or `NULL` for none. A backend with no world behaves exactly as one did before collision existed, which is what makes collision opt-in and what keeps the recorded physics baseline honest. */
float64_t max_timestep; /**< Longest step the simulation will take, in seconds. A hitch longer than this advances the world by this much instead. 0 disables the bound. From `physics.max_timestep`, default #AKGL_PHYSICS_DEFAULT_MAX_TIMESTEP. */
} akgl_PhysicsBackend;
@@ -81,13 +83,15 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_gravity(akgl_PhysicsBackend
* Note that this *succeeds* where the arcade backend's collide refuses -- the
* null backend's contract is "nothing collides", which is an answer, not a gap.
*
* @param self The backend. Required.
* @param a1 First actor. Ignored; `NULL` is accepted.
* @param a2 Second actor. Ignored; `NULL` is accepted.
* @param self The backend. Required.
* @param actor The actor that would have been resolved. Required, and checked,
* because a caller passing rubbish should find out regardless of
* which backend happens to be installed.
* @param dt Length of the sub-step, in seconds. Ignored.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p self is `NULL`.
* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
* @brief Movement for the null backend: accept the call and change nothing.
*
@@ -133,20 +137,25 @@ akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_init_null(akgl_PhysicsBackend *s
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_gravity(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
* @brief Collision for the arcade backend. Not implemented.
* @brief Collision for the arcade backend: resolve one actor against the world.
*
* Deliberately loud rather than silently permissive: unlike
* akgl_physics_null_collide, which means "nothing collides", this means "nobody
* has written this yet", and a caller that reaches it should find out.
* Called by akgl_physics_simulate after each sub-move, so it resolves against
* where the actor is rather than where it was predicted to be.
*
* @param self The backend. Required, and checked before the refusal.
* @param a1 First actor. Never examined.
* @param a2 Second actor. Never examined.
* @return Never `NULL`.
* @throws AKERR_NULLPOINTER If @p self is `NULL`.
* @throws AKERR_API Otherwise, always, with the message "Not implemented".
* **A backend with no #akgl_PhysicsBackend::collision returns success having
* changed nothing.** That is what makes collision opt-in: a program written
* before any of this existed installs a backend, never attaches a world, and
* runs exactly as it did.
*
* @param self The backend. Required.
* @param actor The actor to resolve. Required.
* @param dt Length of the sub-step, in seconds. Carried on each contact so a
* response can scale with it.
* @return `NULL` on success, otherwise an error context owned by the caller.
* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
* @throws AKGL_ERR_COLLISION If a narrowphase query could not be answered.
*/
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
/**
* @brief Commit an actor's velocity to its position.
*