Epic: full 2D/3D display-zones paradigm parity on D3D12, VK, and GL (transparency, layers, sub-rects) + demo apps

[dfattal]

Goal

Bring the full XR_EXT_display_zones paradigm — N 3D zones + Local2D zones + per-frame wish mask + transparency (see-through) + layer composition + canvas sub-rects — to the D3D12, VK, and GL Leia DPs, at D3D11 parity, and ship demo/parity apps for each.

Today the paradigm is D3D11-only on the DP side.

Current state (per Leia DP)

API	Transparency	Canvas sub-rect	Zones DP path (publish/clear/get_local_zone*)	#68 bg-UV remap-skip
D3D11 (leia_display_processor_d3d11.cpp)	✅ compose-under-bg + alpha-gate	✅	✅	✅ active
D3D12 (leia_display_processor_d3d12.cpp)	✅	✅ (remap code)	❌ none	plumbed, dormant
GL (leia_display_processor_gl.cpp)	✅	✅ (remap code)	❌ none	plumbed, dormant
VK (leia_display_processor.cpp)	✅ full-window only	❌ (void)canvas_* — TODO(#85)	❌ none	n/a (no canvas remap)

Notably the demos are VK, where canvas sub-rects are ignored (#85) and there is no zones path — so windowed/zoned VK demos are unsupported today (full-window VK works fine).

Scope (per non-D3D11 API)

• Implement the zones DP vtable: get_local_zone_caps, publish_local_zone_mask, clear_local_zone_mask (mirror D3D11 — drives the per-region 2D/3D lens via request_display_mode; mask is a physical lens signal only, never content — ADR-028).
• Honor canvas_offset/width/height in process_atlas (VK: resolve the #85 TODO — sub-rect viewport/scissor + weaver phase offset).
• Activate the #68 set_shared_texture_present bg-UV remap-skip gate (shared_texture_present && zone_active) once a zones path exists.
• Alpha-gate / compose zone-awareness for multi-zone super-atlas + Local2D bands + layer composition (premultiplied), at D3D11 parity.
• Transparent self-present support for texture-zones apps (DComp on D3D12/GL per ADR-029; the VK equivalent).

Demo / parity apps

• cube_zones_texture_d3d12_win (mirror cube_zones_texture_d3d11_win)
• cube_zones_texture_vk_win (the demos' API — highest priority)
• cube_zones_texture_gl_win (defensive; no GL texture app exists yet)
• Port the real demos (modelviewer / mediaplayer / gauss / earthview / avatar) to zoned/windowed where applicable.

References

• ADR-027 display zones; docs/specs/extensions/XR_EXT_display_zones.md; docs/roadmap/display-zones.md
• ADR-028 (mask = hw lens, not content), ADR-029 (DP reconstructs alpha, never bakes bg; transparent present = DComp + premultiplied)
• Auto-start displayxr-service via Windows startup task #68 (D3D11 texture-zones transparency: set_shared_texture_present bg-UV 1:1 + DComp present) — the D3D11 reference implementation
• Leia SR: support canvas sub-rect for _shared app weaving #85 (VK canvas sub-rect offset TODO)
• Epic: Unified 2D/3D compositing — present-ownership × alpha-mask, one mask shared with local-3d-zones #439 (unified 2D/3D compositing model epic — related)