Andrew Kesterson
147ab7dc78
tests/physics_sim.c runs the arcade backend the way a game does -- a Mario-esque jump and fall, Zelda-style top-down walking, and a run reversed at full speed -- and prints what the actor actually did. tests/physics.c already checked that akgl_physics_simulate does the arithmetic it claims. Every one of these got past it. The first step was however long the level took to load. gravity_time was never initialized and dt was unbounded, so a 250 ms load produced a 250 ms step: 100 px of fall where a 60 Hz frame under the same gravity is 0.44 px, straight through whatever was underneath. Both initializers seed the clock, and simulate bounds dt to max_timestep -- a new physics.max_timestep property, default 0.05 s, read like gravity and drag. Zero disables the bound. The field replaces the dead timer_gravity, so the struct is the same size. Releasing a vertical key cancelled gravity. The _off handlers zeroed ay, ey, ty and vy together, and ey is where the arcade backend accumulates gravity -- tapping down mid-jump stopped the character in the air. They clear ax/tx (or ay/ty) and nothing else now. Velocity was never theirs to clear either: simulate recomputes v as e + t every step. Diagonal movement was 41% too fast. Thrust was capped per axis, so an actor holding two directions got both caps at once and travelled their diagonal. It is capped as a vector against the sx/sy/sz ellipse, which also keeps a character whose horizontal and vertical top speeds differ moving at the ratio it asked for. An axis with a zero max speed stays out of the magnitude and is forced to zero, as the old clamp did. Each fix was checked by reverting it and confirming the simulation goes red: 100.1 px, vy 0.0 after a down tap, and 141% diagonal respectively. tests/actor.c and tests/physics.c pinned the old behaviour in both places and now assert the new contract. Bumped to 0.6.0: akgl_PhysicsBackend changed. TODO.md records the four things the simulations found and this does not fix -- no terminal velocity, no deceleration on release, Euler's frame-rate dependence, and a drag coefficient large enough to invert velocity. 26/26 ctest, memcheck clean, warning-clean at -Wall -Werror. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01B8T5FAYXE8HEJqFLCYwNNc
263 lines
14 KiB
C
263 lines
14 KiB
C
/**
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* @file physics.h
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* @brief The pluggable physics backend, and the two implementations that ship.
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*
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* Same shape as the renderer: a record of function pointers plus an initializer
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* that fills it in. `null` accepts every call and changes nothing, which is what
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* a menu screen or a test harness wants; `arcade` applies gravity, drag, thrust
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* and a speed cap. Selecting between them is akgl_physics_factory, driven by the
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* `physics.engine` configuration property -- not a branch in the simulation.
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*
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* The model an actor is simulated under, and the fields on akgl_Actor that carry
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* it:
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*
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* | Term | Fields | Comes from |
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* |-----------------------|--------------|-----------------------------------------------|
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* | thrust | `tx, ty, tz` | the actor's own acceleration while it is moving |
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* | environmental | `ex, ey, ez` | gravity, less drag |
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* | velocity | `vx, vy, vz` | thrust + environmental |
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* | max speed | `sx, sy, sz` | the character; caps the thrust vector, not velocity |
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* | acceleration | `ax, ay, az` | the character |
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*
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* Each step: movement logic, then gravity, then drag, then velocity, then move.
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* Because the cap is applied to thrust rather than to velocity, gravity can
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* carry an actor faster than its stated top speed -- which is the point, for a
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* falling one.
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*
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* The cap is on the thrust **vector**, not on each axis separately. Capping the
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* axes independently lets the corner of the box through: an actor holding two
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* directions at once got both caps at once and travelled their diagonal, 41%
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* faster than either alone. Scaling to the ellipse instead keeps a character
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* whose horizontal and vertical speeds differ moving at the ratio it asked for.
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*/
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/**
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* @brief Default bound on one simulation step, in seconds.
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*
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* A twentieth of a second: three frames at 60 Hz, so an ordinary dropped frame
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* or two passes through untouched, and a load or a breakpoint does not.
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*/
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#define AKGL_PHYSICS_DEFAULT_MAX_TIMESTEP 0.05
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#ifndef _AKGL_PHYSICS_H_
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#define _AKGL_PHYSICS_H_
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#include <SDL3/SDL.h>
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#include <akerror.h>
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#include <akgl/actor.h>
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#include <akgl/iterator.h>
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#include <akgl/staticstring.h>
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/** @brief Defines a pluggable physics backend and its environmental parameters. */
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typedef struct akgl_PhysicsBackend {
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akerr_ErrorContext AKERR_NOIGNORE *(*simulate)(struct akgl_PhysicsBackend *self, akgl_Iterator *opflags); /**< Step the whole world. Both backends point this at akgl_physics_simulate. */
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akerr_ErrorContext AKERR_NOIGNORE *(*gravity)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt); /**< Apply environmental acceleration to one actor. */
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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. */
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akerr_ErrorContext AKERR_NOIGNORE *(*move)(struct akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt); /**< Commit one actor's velocity to its position. */
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float64_t drag_x; /**< Fraction of environmental velocity shed per second along x. 0 disables. From `physics.drag.x`. */
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float64_t drag_y; /**< Drag along y. From `physics.drag.y`. */
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float64_t drag_z; /**< Drag along z. From `physics.drag.z`. */
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float64_t gravity_x; /**< Acceleration along x, world units per second squared. Applied toward screen left. From `physics.gravity.x`. */
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float64_t gravity_y; /**< Acceleration along y. Applied down the screen, since y grows downward. From `physics.gravity.y`. */
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float64_t gravity_z; /**< Acceleration along z. Applied away from the camera. From `physics.gravity.z`. */
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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. */
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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. */
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} akgl_PhysicsBackend;
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/**
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* @brief Gravity for the null backend: accept the call and change nothing.
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* @param self The backend. Required -- the one thing this does check.
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* @param actor The actor that would have been accelerated. Ignored; `NULL` is
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* accepted.
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* @param dt Seconds since the previous step. Ignored.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_gravity(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
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/**
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* @brief Collision for the null backend: accept the call and change nothing.
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*
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* Note that this *succeeds* where the arcade backend's collide refuses -- the
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* null backend's contract is "nothing collides", which is an answer, not a gap.
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*
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* @param self The backend. Required.
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* @param a1 First actor. Ignored; `NULL` is accepted.
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* @param a2 Second actor. Ignored; `NULL` is accepted.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
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/**
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* @brief Movement for the null backend: accept the call and change nothing.
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*
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* Actors under the null backend do not move on their own. Anything that sets an
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* actor's `x`/`y` directly still works -- this only declines to integrate
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* velocity.
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*
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* @param self The backend. Required.
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* @param actor The actor that would have moved. Ignored; `NULL` is accepted.
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* @param dt Seconds since the previous step. Ignored.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_null_move(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
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/**
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* @brief Install the null backend's methods.
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*
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* Reads no configuration and touches no gravity or drag fields, so whatever was
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* in them stays -- harmlessly, since none of the null methods look.
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*
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* @param self The backend to initialize. Required.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_init_null(akgl_PhysicsBackend *self);
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/**
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* @brief Accelerate one actor's environmental velocity by gravity.
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*
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* Adds `gravity * dt` to the actor's `ex`/`ey`/`ez` on each axis whose gravity
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* is non-zero, with the sign chosen for screen space: x pulls left, y pulls
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* down, z pulls away from the camera. It touches only the environmental term, so
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* an actor's own thrust is unaffected and the speed cap does not apply -- which
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* is why a falling actor keeps accelerating past its character's top speed.
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*
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* @param self The backend supplying the gravity constants. Required.
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* @param actor The actor to accelerate. Required.
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* @param dt Seconds since the previous step. A `dt` of 0 is a no-op; a
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* negative one accelerates backwards.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_gravity(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
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/**
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* @brief Collision for the arcade backend. Not implemented.
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*
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* Deliberately loud rather than silently permissive: unlike
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* akgl_physics_null_collide, which means "nothing collides", this means "nobody
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* has written this yet", and a caller that reaches it should find out.
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*
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* @param self The backend. Required, and checked before the refusal.
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* @param a1 First actor. Never examined.
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* @param a2 Second actor. Never examined.
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* @return Never `NULL`.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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* @throws AKERR_API Otherwise, always, with the message "Not implemented".
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_collide(akgl_PhysicsBackend *self, akgl_Actor *a1, akgl_Actor *a2);
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/**
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* @brief Commit an actor's velocity to its position.
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*
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* The last step of a simulation tick: `position += velocity * dt` on all three
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* axes. It does not clamp to the map, consult the tilemap, or test for
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* collisions -- an actor will walk straight through a wall and off the edge of
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* the world.
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*
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* @param self The backend. Required, though nothing on it is read.
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* @param actor The actor to move. Required. Its `vx`/`vy`/`vz` must already have
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* been computed; akgl_physics_simulate does that immediately
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* before calling this.
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* @param dt Seconds since the previous step.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self or @p actor is `NULL`.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_arcade_move(akgl_PhysicsBackend *self, akgl_Actor *actor, float32_t dt);
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/**
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* @brief Install the arcade backend's methods and read its constants from configuration.
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*
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* Reads `physics.gravity.x`, `.y`, `.z` and `physics.drag.x`, `.y`, `.z` from
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* the property registry, all defaulting to `"0.0"` -- so an arcade backend with
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* no configuration behaves like the null one until something is set.
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*
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* @param self The backend to initialize. Required. Its method pointers are
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* installed before the properties are read, so a failure part-way
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* leaves a usable backend with some constants still at their
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* previous values.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self is `NULL`.
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* @throws AKERR_VALUE If one of the six properties is set to something that is
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* not a number.
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* @throws ERANGE If one of them does not fit a `double`.
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* @throws AKGL_ERR_HEAP If the string pool is exhausted.
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*
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* @note With #AKGL_REGISTRY_PROPERTIES uninitialized every property reads back
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* as its default, so this silently configures zero gravity and zero drag
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* rather than reporting anything. See the warning in registry.h.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_init_arcade(akgl_PhysicsBackend *self);
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/**
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* @brief Select and initialize a physics backend by name.
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*
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* The dispatch point for the whole subsystem: `"null"` and `"arcade"` are the
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* two names, and akgl_game_init passes whatever the `physics.engine` property
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* holds. Adding a backend means adding a name here, not a branch in the
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* simulation.
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*
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* @param self The backend to initialize. Required.
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* @param type The backend name. Required. Matched on its leading characters, so
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* `"nullify"` selects `null` and `"arcadia"` selects `arcade`.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self or @p type is `NULL`.
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* @throws AKERR_KEY If @p type matches neither name. The message quotes what was
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* asked for.
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* @throws AKERR_* Whatever the selected initializer raises.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_factory(akgl_PhysicsBackend *self, akgl_String *type);
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/**
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* @brief Step every live actor forward by the time elapsed since the previous call.
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*
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* Shared by both backends -- what differs is the `gravity` and `move` they point
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* at. For each live actor, in pool order:
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*
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* - a child actor is snapped to its parent's position plus its own `v` as an
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* offset, and skipped entirely -- children do not simulate;
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* - an actor with no character is skipped, since the speed and acceleration
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* constants live there;
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* - thrust accumulates along an axis the actor is moving on, then the thrust
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* *vector* is scaled to the ellipse the character's per-axis maximum speeds
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* describe;
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* - the backend's `gravity` runs, drag is applied to the environmental term,
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* velocity becomes environmental plus thrust, and the backend's `move`
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* commits it.
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*
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* `dt` is measured from `self->gravity_time`, which is stamped at the end and
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* seeded by akgl_physics_init_arcade and akgl_physics_init_null. It is then
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* bounded by `self->max_timestep`: a step longer than that advances the world
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* by `max_timestep` instead of by the whole elapsed time.
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*
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* That bound is not a nicety. Anything that stalls a frame -- a level load
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* between initialization and the first step, a breakpoint, a dragged window, an
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* alt-tab -- otherwise arrives as one enormous `dt`, and one enormous `dt`
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* moves every actor by however far its velocity carries it in that whole
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* interval. A quarter-second load under ordinary platformer gravity is a
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* hundred pixels of fall between the first frame and the second, straight
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* through whatever was underneath. Advancing the world in slow motion during a
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* hitch is the trade every game engine makes here.
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*
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* @param self The backend. Required, along with its `move` pointer.
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* @param opflags Iterator flags. Optional -- `NULL` means "simulate everything".
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* Only #AKGL_ITERATOR_OP_LAYERMASK is honoured, restricting the
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* step to actors on `layerid`; the update, render, and release
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* bits are ignored here.
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* @return `NULL` on success, otherwise an error context owned by the caller.
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* @throws AKERR_NULLPOINTER If @p self or `self->move` is `NULL`.
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* @throws AKERR_* Whatever an actor's `movementlogicfunc`, or the backend's
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* `gravity` or `move`, raises. The first failure aborts the whole step,
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* leaving the actors already processed advanced and the rest not.
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*
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* @note An actor's `movementlogicfunc` can raise AKGL_ERR_LOGICINTERRUPT to say
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* "skip the rest of the simulation for me this tick". It is handled here
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* and does not propagate -- it is a control signal wearing an error's
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* clothes, not a failure. Only `movementlogicfunc` gets that treatment:
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* the `gravity` and `move` calls return straight out of the loop on any
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* status, LOGICINTERRUPT included, so a backend must not use it to opt an
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* actor out from there.
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*/
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akerr_ErrorContext AKERR_NOIGNORE *akgl_physics_simulate(akgl_PhysicsBackend *self, akgl_Iterator *opflags);
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#endif // _AKGL_PHYSICS_H_
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