pcbjam/wasm/gl1/README.md

wasm/gl1 — GL 1.x fixed-function pipeline on WebGL2

Emulation layer that lets KiCad's 3D viewer renderer (RENDER_3D_OPENGL, pure GL 1.x fixed-function, compiled unmodified) render in the browser. Supersedes the no-op link stubs of wasm/stubs/gl_ffp_stub.c (git history has the old file): same public surface, real implementations.

TDD harness: tests/3d-regression/ — 47 native-golden scenarios; parity via npm run 3d:check:parity (see that README).

How the symbols resolve

In the WASM build, gl* are plain C functions split between two providers:

• Emscripten's WebGL library (-sMAX_WEBGL_VERSION=2) owns every modern/WebGL2 name (glClear, glDrawArrays, glTexImage2D, buffers, shaders, stencil...).
• This shim owns the FFP-only names WebGL2 lacks (glBegin, display lists, matrix stack, glLight*/glMaterial*, client-array pointers, glTexEnv*, glAlphaFunc, GLU quadrics) — see src/gl1_entry_ffp.cpp.

The emulator must also observe a few Emscripten-owned calls (FFP glEnable caps, draws over client arrays, matrix readback, state recorded inside display lists). Those are intercepted with wasm-ld --wrap: every name in wrapped_symbols.txt gets a -Wl,--wrap=<sym> flag at both link sites, the interceptors live in src/gl1_entry_wrapped.cpp, and __real_* forwards to the WebGL library. No kicad/wxwidgets sources are touched.

Link sites (both read sources.txt + wrapped_symbols.txt):

• tests/3d-regression/wasm/Makefile (the TDD harness)
• scripts/kicad/build-kicad-target.sh (GL3D_LINK_FLAGS, production kicad_editor)

Draw routing

A glDrawArrays/glDrawElements call is FFP traffic iff BOTH hold:

• the CURRENT WebGL context is the shim's owner context (contextIsOwner(), adopted at the first FFP client-state mutation or programSync() in a context — a draw under any other context is a modern-GL consumer by definition and always passes through, even mid-display-list-recording);
• GL_VERTEX_ARRAY client state is enabled (only GL1 code calls glEnableClientState; the raytracer blit and the 2D WebGL GAL drive their own GLSL programs).

The owner gate exists because the client-state flag is process-global and an interrupted FFP window (e.g. MODEL_3D::BeginDrawMulti loops crossing a JSPI suspension) can leave it set — without the gate, the 2D GAL's draws got routed through the FFP pipeline (the engine-toggle blank-viewer bug). Two further hardenings from the same bug:

• VAO isolation (ScopedDefaultVAO, gl1_draw.cpp): draw executors bind VAO 0 for their attribute setup and restore the caller's binding — a routed draw arriving with the caller's VAO bound (the blit's attribute 0 collides with ATTR_POSITION) must never scribble it.
• contextSync() resets the client-array mirror entirely on a context change: enables, pointers and captured VBO names all described the dead context.

Invariants that are easy to break (learned from the native goldens)

• Display lists snapshot client arrays at record time. The renderer sets gl*Pointer, records glDrawArrays inside glNewList, then delete[]s the arrays right after glEndList (layer_triangles.cpp) — a recorder that stores pointers reads freed memory. Copy eagerly at record.
• Lights live in eye space, captured at glLightfv time. init_lights() runs under an identity modelview → the directional lights are anchored to the camera. Re-deriving light directions at draw time breaks every lit scenario.
• Do not "fix" mismatched normal counts. generate_middle_triangles rejecting countersink walls (normals ≠ vertices) is upstream KiCad behavior the goldens document; tolerating it would diverge from native.
• Lighting is per-vertex (Gouraud) to match fixed-function output; per-fragment lighting visibly mismatches speculars on coarse meshes.
• GL object caches are per-context, and the context is mortal. Closing the 3D viewer destroys its wxGLCanvas's WebGL context; reopening mints a new one in which the cached names (FFP program, stream/scratch VBOs) are invalid — every draw then dies with INVALID_OPERATION and the viewer is blank. contextSync() (gl1_state.cpp) detects the change in programSync() — the one choke point every shim draw crosses and a path the 2D GAL never reaches (a check in any __wrap_* would see the GAL's context and ping-pong the owner on 2D↔3D paint alternation) — and drops the caches so they rebuild lazily. Identity comes from a monotonic id stamped on Emscripten's per-context record — NOT the EMSCRIPTEN_WEBGL_CONTEXT_HANDLE, which Emscripten recycles (a destroy-then-create can return the same number). Display-list/immediate state is deliberately untouched: it is CPU-only, and the change can be detected mid-scene-rebuild (even inside glNewList). Known limit (pre-existing): two simultaneously live FFP contexts would thrash the caches on every alternation — the shim still assumes one live 3D-viewer context at a time.

Layout

sources.txt / wrapped_symbols.txt   single source of truth for both link sites
include/gl1_shim.h                  internal API + the state mirror
src/gl1_entry_ffp.cpp               the 52 FFP-only public entry points
src/gl1_entry_wrapped.cpp           __wrap_* interceptors (mechanism-aware TU)
src/gl1_state.cpp                   state singleton, capability routing
src/gl1_matrix.cpp                  MODELVIEW/PROJECTION stacks (+readback)
src/gl1_immediate.cpp               glBegin/glEnd + primitive conversion
src/gl1_dlist.cpp                   display-list recorder/replayer
src/gl1_draw.cpp                    draw execution (stream VBO, attrib setup)
src/gl1_shaders.cpp                 the FFP uber-program (ES 3.00) + uniforms
src/gl1_glu.cpp                     GLU quadrics (SGI tessellation) + gluPerspective