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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

310
src/collision.c
View File

@@ -17,6 +17,8 @@
#include <akstdlib.h>
#include <akgl/collision_arena.h>
#include <akgl/error.h>
#include <akgl/actor.h>
#include <akgl/heap.h>
#include <akgl/tilemap.h>
#include <akgl/types.h>
@@ -56,6 +58,24 @@
/** @brief Tiles akgl_collision_settle will lift a shape before giving up. */
#define AKGL_COLLISION_SETTLE_STEPS 4
/**
* @brief Fraction of the thinnest passable thickness one sub-step may cover.
*
* Half. Anything that moves less than half the thinnest thing it could pass
* through cannot be on both sides of it between two samples.
*/
#define AKGL_COLLISION_SUBSTEP_FRACTION 0.5f
/**
* @brief Hard ceiling on sub-steps per actor per step.
*
* A cost bound. Above roughly `MAX_SUBSTEPS * FRACTION * cellsize` per step an
* actor can still pass through a wall -- about 1280 px/s on 16-pixel tiles at
* the default step, which is a projectile and not a walker. The real answer is
* a swept narrowphase; #AKGL_COLLISION_FLAG_BULLET reserves the bit for it.
*/
#define AKGL_COLLISION_MAX_SUBSTEPS 8
/** @brief One positioned shape, in the form libccd's callbacks read. */
typedef struct {
uint8_t kind; /**< AKGL_COLLISION_SHAPE_*. */
@@ -675,3 +695,293 @@ akerr_ErrorContext *akgl_collision_settle(akgl_CollisionWorld *self, akgl_Collis
maxsteps
);
}
/** @brief Carries one actor's resolution through the broad-phase visitor. */
typedef struct {
akgl_CollisionWorld *world;
akgl_Actor *actor;
akgl_CollisionProxy *proxy;
float32_t dt;
akerr_ErrorContext *failure;
} collision_resolve_state;
/**
* @brief Test one candidate against the actor being resolved, and answer it.
*
* The visitor cannot use CATCH: a `break` here would leave the broad phase's own
* walk rather than this function, so a failure would look like the end of the
* list. The status is carried out on the state instead and handed over by the
* caller.
*/
static akerr_ErrorContext *collision_resolve_visit(akgl_CollisionProxy *other, void *data)
{
collision_resolve_state *state = (collision_resolve_state *)data;
akgl_Contact contact;
bool interacts = false;
bool hit = false;
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, other, AKERR_NULLPOINTER, "NULL proxy in a resolve");
FAIL_ZERO_RETURN(errctx, state, AKERR_NULLPOINTER, "NULL resolve state");
if ( other == state->proxy ) {
SUCCEED_RETURN(errctx);
}
/*
* Nested rather than written with early returns. A *_RETURN inside an
* ATTEMPT block leaves past CLEANUP, which is a real defect even where the
* CLEANUP happens to be empty today -- and scripts/check_error_protocol.py
* refuses it, correctly.
*/
ATTEMPT {
CATCH(errctx, akgl_collision_shape_interacts(&state->proxy->shape, &other->shape, &interacts));
if ( interacts == true ) {
state->world->tests += 1;
CATCH(errctx, akgl_collision_test(state->proxy, other, state->world->flags, &contact, &hit));
if ( hit == true ) {
contact.self = state->actor;
contact.other = other->owner;
contact.dt = state->dt;
contact.sensor = (((state->proxy->shape.flags & AKGL_COLLISION_FLAG_SENSOR) != 0) ||
((other->shape.flags & AKGL_COLLISION_FLAG_SENSOR) != 0));
contact.statichit = ((other->shape.flags & AKGL_COLLISION_FLAG_STATIC) != 0);
CATCH(errctx, state->actor->collidefunc(state->actor, &contact));
// The actor moved, so its own proxy is stale for the rest of the pass.
CATCH(errctx, akgl_collision_proxy_sync(state->proxy, &state->actor->shape,
state->actor->x, state->actor->y,
state->actor->z));
}
}
} CLEANUP {
} PROCESS(errctx) {
} FINISH(errctx, true);
SUCCEED_RETURN(errctx);
}
/**
* @brief Resolve one actor against the tiles its shape overlaps.
*
* Tiles are not proxies, so they are scanned rather than queried: a stack-local
* proxy is synthesized for each solid cell and handed to the ordinary
* narrowphase, which keeps one code path for "what does a contact look like"
* rather than two.
*/
static akerr_ErrorContext *collision_resolve_tiles(akgl_CollisionWorld *self, akgl_Actor *actor,
akgl_CollisionProxy *proxy, float32_t dt)
{
akgl_CollisionProxy tile;
akgl_CollisionShape tileshape;
akgl_Contact contact;
SDL_FRect body;
bool hit = false;
int32_t tx = 0;
int32_t ty = 0;
int32_t x0 = 0;
int32_t y0 = 0;
int32_t x1 = 0;
int32_t y1 = 0;
int layer = 0;
PREPARE_ERROR(errctx);
if ( (self->tilesource == NULL) || (self->tilelayers == 0) ) {
SUCCEED_RETURN(errctx);
}
if ( (proxy->shape.collidemask & self->tilelayermask) == 0 ) {
SUCCEED_RETURN(errctx);
}
x0 = (int32_t)floorf(proxy->bounds.x / (float32_t)self->tilesource->tilewidth);
y0 = (int32_t)floorf(proxy->bounds.y / (float32_t)self->tilesource->tileheight);
x1 = (int32_t)floorf(((proxy->bounds.x + proxy->bounds.w) - AKGL_COLLISION_TILE_EPSILON) /
(float32_t)self->tilesource->tilewidth);
y1 = (int32_t)floorf(((proxy->bounds.y + proxy->bounds.h) - AKGL_COLLISION_TILE_EPSILON) /
(float32_t)self->tilesource->tileheight);
body.w = (float32_t)self->tilesource->tilewidth;
body.h = (float32_t)self->tilesource->tileheight;
body.x = 0.0f;
body.y = 0.0f;
for ( ty = y0; ty <= y1; ty++ ) {
for ( tx = x0; tx <= x1; tx++ ) {
if ( !collision_tile_solid(self, tx, ty) ) {
continue;
}
ATTEMPT {
CATCH(errctx, akgl_collision_shape_box(&tileshape, &body, 0.0f));
tileshape.layermask = self->tilelayermask;
tileshape.flags |= AKGL_COLLISION_FLAG_STATIC;
CATCH(errctx, akgl_collision_proxy_initialize(
&tile, NULL, &tileshape,
(float32_t)(tx * self->tilesource->tilewidth),
(float32_t)(ty * self->tilesource->tileheight), proxy->z));
CATCH(errctx, akgl_collision_test(proxy, &tile, self->flags, &contact, &hit));
self->tests += 1;
if ( hit == true ) {
contact.self = actor;
contact.other = NULL;
contact.dt = dt;
contact.statichit = true;
contact.tilex = tx;
contact.tiley = ty;
contact.tilegid = 0;
contact.tilelayer = -1;
// Which layer and which tile, so a game can tell a spike from
// a floor without looking it up again.
for ( layer = 0; layer < self->tilesource->numlayers; layer++ ) {
if ( (self->tilelayers & (1u << layer)) == 0 ) {
continue;
}
if ( self->tilesource->layers[layer].data[(ty * self->tilesource->width) + tx] != 0 ) {
contact.tilelayer = layer;
contact.tilegid = self->tilesource->layers[layer].data[(ty * self->tilesource->width) + tx];
break;
}
}
CATCH(errctx, actor->collidefunc(actor, &contact));
CATCH(errctx, akgl_collision_proxy_sync(proxy, &actor->shape,
actor->x, actor->y, actor->z));
}
} CLEANUP {
} PROCESS(errctx) {
} FINISH(errctx, true);
}
}
SUCCEED_RETURN(errctx);
}
akerr_ErrorContext *akgl_collision_resolve(akgl_CollisionWorld *self, akgl_Actor *actor, float32_t dt)
{
collision_resolve_state state;
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, self, AKERR_NULLPOINTER, "NULL collision world reference");
FAIL_ZERO_RETURN(errctx, actor, AKERR_NULLPOINTER, "NULL actor reference");
if ( (actor->proxy == NULL) || (actor->shape.kind == AKGL_COLLISION_SHAPE_NONE) ) {
SUCCEED_RETURN(errctx);
}
if ( actor->collidefunc == NULL ) {
SUCCEED_RETURN(errctx);
}
// The actor has just moved, so the index and the copy of its shape are both
// a step out of date.
PASS(errctx, akgl_collision_proxy_sync(actor->proxy, &actor->shape, actor->x, actor->y, actor->z));
PASS(errctx, self->partitioner.move(&self->partitioner, actor->proxy));
PASS(errctx, collision_resolve_tiles(self, actor, actor->proxy, dt));
memset(&state, 0x00, sizeof(state));
state.world = self;
state.actor = actor;
state.proxy = actor->proxy;
state.dt = dt;
PASS(errctx, self->partitioner.query(&self->partitioner, &actor->proxy->bounds,
actor->shape.collidemask, &collision_resolve_visit, &state));
// Whatever the responses did, the index has to end the pass agreeing with
// where the actor actually is.
PASS(errctx, akgl_collision_proxy_sync(actor->proxy, &actor->shape, actor->x, actor->y, actor->z));
PASS(errctx, self->partitioner.move(&self->partitioner, actor->proxy));
SUCCEED_RETURN(errctx);
}
akerr_ErrorContext *akgl_collision_substeps(akgl_CollisionWorld *self, akgl_Actor *actor, float32_t dt, int *dest)
{
float32_t span = 0.0f;
float32_t limit = 0.0f;
float32_t thin = 0.0f;
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, actor, AKERR_NULLPOINTER, "NULL actor reference");
FAIL_ZERO_RETURN(errctx, dest, AKERR_NULLPOINTER, "NULL destination reference");
/*
* One, unless there is something to collide with. A step of exactly `dt`
* takes the same arithmetic path it always has -- `dt / 1.0f` is `dt`
* exactly in IEEE-754 -- which is what lets every existing assertion about
* a non-colliding actor hold unchanged.
*/
*dest = 1;
if ( (self == NULL) || (actor->shape.kind == AKGL_COLLISION_SHAPE_NONE) ) {
SUCCEED_RETURN(errctx);
}
span = fabsf(actor->vx);
if ( fabsf(actor->vy) > span ) { span = fabsf(actor->vy); }
if ( fabsf(actor->vz) > span ) { span = fabsf(actor->vz); }
span = span * dt;
/*
* Do not travel further in one sub-step than the thinnest thing that could
* be passed through. That is a cell *or the actor's own extent*, whichever
* is smaller: a small fast actor is the one that tunnels, and bounding only
* by cell size misses it entirely.
*/
thin = self->cellwidth;
if ( self->cellheight < thin ) { thin = self->cellheight; }
if ( (2.0f * actor->shape.hx) < thin ) { thin = 2.0f * actor->shape.hx; }
if ( (2.0f * actor->shape.hy) < thin ) { thin = 2.0f * actor->shape.hy; }
limit = AKGL_COLLISION_SUBSTEP_FRACTION * thin;
if ( limit <= 0.0f ) {
SUCCEED_RETURN(errctx);
}
*dest = (int)(span / limit) + 1;
if ( *dest > AKGL_COLLISION_MAX_SUBSTEPS ) {
*dest = AKGL_COLLISION_MAX_SUBSTEPS;
}
SUCCEED_RETURN(errctx);
}
akerr_ErrorContext *akgl_collision_sync_actors(akgl_CollisionWorld *self)
{
akgl_Actor *actor = NULL;
int i = 0;
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, self, AKERR_NULLPOINTER, "NULL collision world reference");
/*
* One pass over the pool: give a proxy to anything that has gained a shape,
* take one back from anything that has lost one, and refresh the rest. This
* is the only place a proxy is created or destroyed during a step, so
* nothing below has to think about lifetimes.
*/
for ( i = 0; i < AKGL_MAX_HEAP_ACTOR; i++ ) {
actor = &akgl_heap_actors[i];
if ( actor->refcount == 0 ) {
continue;
}
if ( actor->shape.kind == AKGL_COLLISION_SHAPE_NONE ) {
if ( actor->proxy != NULL ) {
PASS(errctx, self->partitioner.remove(&self->partitioner, actor->proxy));
PASS(errctx, akgl_heap_release_collision_proxy(actor->proxy));
actor->proxy = NULL;
}
continue;
}
if ( actor->proxy == NULL ) {
PASS(errctx, akgl_heap_next_collision_proxy(&actor->proxy));
PASS(errctx, akgl_collision_proxy_initialize(actor->proxy, actor, &actor->shape,
actor->x, actor->y, actor->z));
PASS(errctx, self->partitioner.insert(&self->partitioner, actor->proxy));
continue;
}
PASS(errctx, akgl_collision_proxy_sync(actor->proxy, &actor->shape,
actor->x, actor->y, actor->z));
PASS(errctx, self->partitioner.move(&self->partitioner, actor->proxy));
}
SUCCEED_RETURN(errctx);
}

76
src/physics.c
View File

@@ -5,6 +5,7 @@
#include <math.h>
#include <akstdlib.h>
#include <akgl/collision.h>
#include <akgl/physics.h>
#include <akgl/actor.h>
#include <akgl/game.h>
@@ -20,12 +21,18 @@ akerr_ErrorContext *akgl_physics_null_gravity(akgl_PhysicsBackend *self, akgl_Ac
SUCCEED_RETURN(errctx);
}
akerr_ErrorContext *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2)
akerr_ErrorContext *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt)
{
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, self, AKERR_NULLPOINTER, "self");
FAIL_ZERO_RETURN(errctx, a1, AKERR_NULLPOINTER, "a1");
FAIL_ZERO_RETURN(errctx, a2, AKERR_NULLPOINTER, "a2");
FAIL_ZERO_RETURN(errctx, actor, AKERR_NULLPOINTER, "actor");
(void)dt;
/*
* Nothing collides, and that is an answer rather than a gap. A caller who
* wants a world with no collision in it installs this backend and gets it,
* without the simulation having to branch on whether collision exists.
*/
SUCCEED_RETURN(errctx);
}
@@ -79,11 +86,19 @@ akerr_ErrorContext *akgl_physics_arcade_gravity(akgl_PhysicsBackend *self, akgl_
SUCCEED_RETURN(errctx);
}
akerr_ErrorContext *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2)
akerr_ErrorContext *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt)
{
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, self, AKERR_NULLPOINTER, "self");
FAIL_RETURN(errctx, AKERR_API, "Not implemented");
FAIL_ZERO_RETURN(errctx, actor, AKERR_NULLPOINTER, "actor");
// No world means no collision, and costs one comparison. This is what makes
// the feature opt-in: a backend nobody attached a world to runs exactly as
// it did before any of this existed.
if ( self->collision == NULL ) {
SUCCEED_RETURN(errctx);
}
PASS(errctx, akgl_collision_resolve(self->collision, actor, dt));
SUCCEED_RETURN(errctx);
}
@@ -152,10 +167,17 @@ akerr_ErrorContext *akgl_physics_simulate(akgl_PhysicsBackend *self, akgl_Iterat
float32_t dt = 0;
float32_t overshoot = 0.0f;
float32_t thrustscale = 0.0f;
float32_t subdt = 0.0f;
int substeps = 1;
int s = 0;
akgl_Actor *actor = NULL;
FAIL_ZERO_RETURN(errctx, self, AKERR_NULLPOINTER, "self");
FAIL_ZERO_RETURN(errctx, self->move, AKERR_NULLPOINTER, "self->move");
// `gravity` was never checked, and is dereferenced unconditionally below. A
// backend built by hand with only `move` filled in crashed here rather than
// reporting; `collide` is allowed to be NULL and means "nothing collides".
FAIL_ZERO_RETURN(errctx, self->gravity, AKERR_NULLPOINTER, "self->gravity");
// Reading the elapsed time requires self, so it cannot be hoisted above
// the null check.
@@ -183,6 +205,12 @@ akerr_ErrorContext *akgl_physics_simulate(akgl_PhysicsBackend *self, akgl_Iterat
opflags = &defflags;
}
// One pass to give every shaped actor a proxy and take one back from
// anything that lost its shape, so the loop below never has to.
if ( self->collision != NULL ) {
PASS(errctx, akgl_collision_sync_actors(self->collision));
}
for ( int i = 0; i < AKGL_MAX_HEAP_ACTOR; i++ ) {
actor = &akgl_heap_actors[i];
@@ -268,13 +296,49 @@ akerr_ErrorContext *akgl_physics_simulate(akgl_PhysicsBackend *self, akgl_Iterat
actor->vy = actor->ey + actor->ty;
actor->vz = actor->ez + actor->tz;
PASS(errctx, self->move(self, actor, dt));
/*
* Only `move` is subdivided. Gravity, drag, the thrust integration
* and the speed-ellipse cap above all ran once against the whole dt,
* and sum(subdt) is dt -- so an actor with nothing to collide with
* takes exactly one sub-step of exactly dt and follows the arithmetic
* path it always did. That is what lets the integrator stay untouched
* and every recorded physics number stay valid.
*/
CATCH(errctx, akgl_collision_substeps(self->collision, actor, dt, &substeps));
subdt = dt / (float32_t)substeps;
for ( s = 0; s < substeps; s++ ) {
PASS(errctx, self->move(self, actor, subdt));
if ( (self->collide != NULL) && (self->collision != NULL) ) {
PASS(errctx, self->collide(self, actor, subdt));
}
}
} CLEANUP {
} PROCESS(errctx) {
} HANDLE(errctx, AKGL_ERR_LOGICINTERRUPT) {
// noop
} FINISH(errctx, true);
}
/*
* Re-snap children after everything has moved, but only when collision is
* on. The in-loop snap uses whatever position the parent had when the
* child's own slot came up in pool order, which is already sometimes a
* frame stale -- harmless while a parent only ever moved by `v * dt`, and
* not harmless once a parent can also be pushed out of a wall part way
* through its own sub-steps. Gating it keeps a collision-free world
* byte-for-byte what it was.
*/
if ( self->collision != NULL ) {
for ( int i = 0; i < AKGL_MAX_HEAP_ACTOR; i++ ) {
actor = &akgl_heap_actors[i];
if ( (actor->refcount == 0) || (actor->parent == NULL) ) {
continue;
}
actor->x = actor->parent->x + actor->vx;
actor->y = actor->parent->y + actor->vy;
actor->z = actor->parent->z + actor->vz;
}
}
self->gravity_time = curtime;
SUCCEED_RETURN(errctx);
}