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Delete the sidescroller's collision, and let the library do it

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collision.c was 383 lines that re-implemented akgl_physics_simulate's own
arithmetic, because movementlogicfunc runs before gravity, drag and the move
and so had to predict where the actor would end up. The whole file goes. What
replaces it is three lines in main.c, a shape on the player and the blob, and a
`collidable` property on the map's terrain layer -- which also retires
SS_TERRAIN_LAYER_ID, a compiled-in layer id that broke whenever somebody
reordered layers in Tiled.

grounded stays a probe rather than becoming a contact test. A contact says
something pushed back this step; grounded asks whether there is a floor to push
off, and that stays true through the frame after landing. The blob keeps its own
wall and ledge probes for the same reason: the resolver does not say which side
of the blob it was pushed from, and says nothing at all about a floor that is
missing.

The summary line now carries the player's resting position, because exiting 0
was not evidence that collision ran. It reports 136.0,160.0 grounded after 240
autoplay frames -- feet at y=192, flush on the surface of terrain row 12.

The tutorial's collision section is rewritten around the library's, keeping the
quarter-of-a-pixel story as what a predicting resolver costs. The docs harness
learns `json excerpt=`, so the map property the chapter quotes is checked
against the map.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71
This commit is contained in:
Andrew Kesterson
2026-08-02 07:35:08 -04:00
parent 35a58670d0
commit 490e62dbbf
9 changed files with 232 additions and 596 deletions
Download Patch File Download Diff File Expand all files Collapse all files

35
examples/sidescroller/actors.c
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@@ -75,15 +75,19 @@ static akerr_ErrorContext *ss_static_movement(akgl_Actor *obj, float32_t dt)
* @brief The blob: walk, and turn around at a wall or a ledge.
*
* This one stays inside the physics step -- it walks on the ground under the
* map's gravity like the player does, and uses the same swept resolution.
* map's gravity like the player does, and the library resolves it the same way.
* All this hook decides is which way to face next.
*/
static akerr_ErrorContext *ss_blob_movement(akgl_Actor *obj, float32_t dt)
{
ss_ActorData *data = NULL;
ss_Contact contact;
SDL_FRect ahead;
float32_t probe_x = 0.0f;
float32_t probe_y = 0.0f;
float32_t step = 0.0f;
bool floor_ahead = false;
bool wall_ahead = false;
bool grounded = false;
PREPARE_ERROR(errctx);
FAIL_ZERO_RETURN(errctx, obj, AKERR_NULLPOINTER, "obj");
@@ -99,7 +103,18 @@ static akerr_ErrorContext *ss_blob_movement(akgl_Actor *obj, float32_t dt)
/* Signs the acceleration from the movement bits just set. */
PASS(errctx, akgl_actor_logic_movement(obj, dt));
PASS(errctx, ss_collide_resolve(obj, &ss_blob_body, dt, &contact));
step = 1.0f;
if ( data->facing < 0.0f ) {
step = -1.0f;
}
/* Three probes, all taken from where the last step left the blob. The blob
* turns on a wall or a ledge, and a contact from the resolver answers
* neither question: it says something pushed back, not which side of the
* blob it was on, and it says nothing at all about a floor that is missing. */
PASS(errctx, akgl_collision_shape_bounds(&obj->shape, (obj->x + step), obj->y, &ahead));
PASS(errctx, akgl_collision_box_blocked(&ss_collision, &ahead, AKGL_COLLISION_LAYER_STATIC, &wall_ahead));
PASS(errctx, ss_grounded(&obj->shape, obj->x, obj->y, &grounded));
/* One pixel past the leading edge of the box and one pixel below its feet:
* no floor there means the next step walks off. */
@@ -109,12 +124,13 @@ static akerr_ErrorContext *ss_blob_movement(akgl_Actor *obj, float32_t dt)
} else {
probe_x = obj->x + ss_blob_body.x + ss_blob_body.w + 1.0f;
}
PASS(errctx, ss_collide_solid_at(probe_x, probe_y, &floor_ahead));
PASS(errctx, akgl_collision_solid_at(&ss_collision, probe_x, probe_y, &floor_ahead));
if ( (contact.blocked_x == true) || ((contact.grounded == true) && (floor_ahead == false)) ) {
if ( (wall_ahead == true) || ((grounded == true) && (floor_ahead == false)) ) {
data->facing = -data->facing;
/* The resolution already zeroed `tx` on a blocked axis; zero it on the
* ledge path too, or the blob keeps its old momentum through the turn. */
/* The library's response removes the component of `tx` going into a
* wall, but nothing removes it on the ledge path -- and without this the
* blob keeps its old momentum through the turn. */
obj->tx = 0.0f;
}
SUCCEED_RETURN(errctx);
@@ -165,7 +181,10 @@ akerr_ErrorContext *ss_actors_bind(void)
}
PASS(errctx, find_actor("blob1", &ss_game.hazards[0]));
PASS(errctx, ss_collide_settle(ss_game.hazards[0], &ss_blob_body));
PASS(errctx, akgl_collision_shape_box(&ss_game.hazards[0]->shape, &ss_blob_body, 0.0f));
ss_game.hazards[0]->shape_override = true;
PASS(errctx, akgl_collision_settle(&ss_collision, &ss_game.hazards[0]->shape,
&ss_game.hazards[0]->x, &ss_game.hazards[0]->y, 0));
ss_hazard_data[0].home_x = ss_game.hazards[0]->x;
ss_hazard_data[0].home_y = ss_game.hazards[0]->y;
ss_hazard_data[0].facing = -1.0f;