Andrew Kesterson
490e62dbbf
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
The libakgl manual
libakgl is a C library for building 2D games on SDL3. It is not an engine: there is no
editor, no scripting layer, no inheritance and no runtime malloc. Behaviour attaches to a
struct as function pointers, objects come from fixed pools, and every call reports failure
through an error context you cannot silently ignore.
This manual teaches the library. It deliberately does not re-document its dependencies
— libakerror owns the error-handling protocol, SDL3 owns renderers and events, Tiled owns
the map format, jansson owns the JSON API. Each chapter says what libakgl adds or
constrains, shows what a caller actually writes, and links out for the rest. For
per-function reference, build the Doxygen output with doxygen Doxyfile.
| 1. Introduction | What libakgl is, what it refuses to be, the frame budget, and who owns which documentation |
| 2. Design philosophy | Bounded pools, pluggable backends, bit flags, name-based registries, one world at a time |
| 3. Getting started | Dependencies, add_subdirectory vs akgl.pc, and the smallest program that opens a window |
| 4. Errors and status codes | The status tables, what each code means here, and the traps that fail silently |
| 5. The heap | The eight pools, akgl_heap_next_*, akgl_String, and why exhaustion usually means a missing release |
| 6. The registry | The eight registries, configuration properties, and the id-0 silent no-op |
| 7. The game and the frame | Startup order, akgl_game_update, the state lock, iterators, savegames |
| 8. Rendering | The backend vtable, the frame contract, cameras, and embedding a host's own SDL_Renderer |
| 9. Drawing | Points, lines, rectangles, circles, flood fill, and saving a region |
| 10. Spritesheets and sprites | One texture shared by path, the sprite JSON format, frame animation |
| 11. Characters | State-to-sprite mappings, movement constants, the character JSON format |
| 12. Actors | The state bitmask, the seven behaviour hooks, parents and children, layers |
| 13. Tilemaps | Tiled TMJ, libakgl's three extensions, and the limits that bind a map |
| 14. Physics | Thrust, environment and velocity; the null and arcade backends; what is not implemented |
| 15. Collision | Shapes, masks, tiles as geometry, the response hook, and the two partitioners |
| 16. Input | Control maps, push-not-poll dispatch, the keystroke ring, gamepads |
| 17. Text and fonts | Loading, drawing, measuring, and the teardown order that matters |
| 18. Audio | The three-voice synthesizer, and the separate background-music path |
| 19. Utilities | Rectangle overlap, path resolution, the JSON accessors, static strings |
| 20. Tutorial: a 2D sidescroller | Gravity, a jump, collision, coins and hazards |
| 21. Tutorial: a top-down JRPG | A town map, NPCs spawned from map objects, four-way animation, a text box, a follower |
| 22. Appendix | Every limit, every status, every configuration property |
Both tutorials are complete programs under examples/. They build with the
library and run headless in CI, and the chapters quote them rather than restating them — so
a tutorial cannot drift from a program that compiles.
Every example here is checked
The examples in these chapters are compiled, linked, run, and cross-checked against the
source tree by the test suite. A snippet that stops compiling, an excerpt that no longer
matches the header it quotes, or a JSON document the loader would reject turns
ctest -R docs_examples red.
This exists because the documentation it replaces had drifted badly: the previous FAQ's
examples did not compile — two unbalanced PASS() calls, a sprite->frameids = [0, 1, 2, 3];
that is not C in any dialect, and, in the first snippet a reader ever saw, the exact
strncpy call AGENTS.md forbids. The prose had drifted with it, and writing these
chapters turned up twenty-seven header claims that were false against src/.
Where a chapter documents behaviour that is a known defect rather than a design decision,
it says so and points at TODO.md. See MAINTENANCE.md if you are editing an example.
Assets
The tutorial art, tiles and music are CC0, vendored under
tutorials/assets/ with provenance recorded per file in
PROVENANCE.md and geometry documented in
README.md. CC0 specifically, not merely free: a reader who
copies a tutorial into their own game inherits no obligation.