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+# Campaign V — OpenGL → Vulkan migration, results so far
+
+**Date:** 2026-07-28
+**Plan:** [`docs/plans/2026-07-27-vulkan-campaign.md`](../plans/2026-07-27-vulkan-campaign.md)
+**Issues filed:** [`docs/ISSUES.md`](../ISSUES.md) #248–#253
+
+acdream's mandatory modern renderer was built on GL 4.3 core +
+`ARB_bindless_texture` + `ARB_shader_draw_parameters`, an API that is no longer
+evolving and whose hardware floor is narrow — bindless textures are absent on
+Intel integrated GPUs and on every Mesa software/D3D12 stack, which is exactly
+what parked the Linux graphical slice (Slice L) at its L1 checkpoint. Campaign
+V replaces it with a single Vulkan 1.3 backend, keeping the pinned Vulkan-shaped
+RHI contract that a GL implementation satisfied first. It is **a compatibility
+and efficiency campaign, not a rescue** — the GL path worked and was fast;
+nothing about what the game looks like was supposed to change, and the campaign
+spent most of its effort proving that it didn't. As of this report: every
+production renderer draws on both backends, the GL-versus-Vulkan differential
+is inside its acceptance threshold on deterministic content, the user has run
+one full connected session on Vulkan and signed off on it, and the performance
+gate (V8) is running.
+
+---
+
+## The screenshots
+
+**V6c — first content on Vulkan.** The synthetic bring-up scene: colour
+quadrants, a chequerboard, a gradient circle. This is the first frame the
+Vulkan RHI ever produced — proof that the SPIR-V toolchain, pipelines, passes,
+push constants and swapchain present all work together, before any production
+renderer touched the backend.
+
+![V6c: first Vulkan content](assets/campaign-v/v6c-vulkan-first-scene.png)
+
+**V6d — the retained UI on Vulkan.** Same synthetic scene, now composited
+underneath acdream's real `TextRenderer`/`DebugLineRenderer` output ("acdream —
+retained UI on Vulkan / top-left origin — slice V6d"). This is the first
+acdream-authored draw call on the new backend, proving the UI's fill+border+
+glyph path was already backend-neutral.
+
+![V6d: retained UI on Vulkan](assets/campaign-v/v6d-vulkan-retained-ui.png)
+
+**V6j — Dereth draws on Vulkan (the centerpiece).** `ACDREAM_RENDER_BACKEND=vulkan`
+rendering the full offline scene: blended terrain with road overlays, the water
+edge and its shoreline, static world meshes with correct silhouettes and closed
+shells, procedural scenery, per-instance lighting, and the complete retained UI
+including the radar. This is the slice where the world arm landed — three
+renderers' RHI submission arms, a backend-neutral pass-executor pair, and a
+winding-inversion bug (below) fixed on the same commit. Sky is still flat fog
+here; `SkyRenderer` doesn't cross until V6k.
+
+![V6j: Dereth on Vulkan](assets/campaign-v/v6j-dereth-vulkan.png)
+
+**V6k — the sky lands.** Same view, now with the dome, the horizon band, the
+cloud sheet and the fog gradient rendered by the Vulkan arm — the same place
+and colours as the GL capture of the same scene. This slice also discharged
+§5.4's `BeginPass`-on-a-null-target divergence and moved the paperdoll/appraisal
+viewports onto `IGpuRenderTarget`.
+
+![V6k: sky rendered on Vulkan](assets/campaign-v/v6k-sky-vulkan.png)
+
+**V6m — the retail portal tunnel on Vulkan.** One frame from a ten-frame transit
+capture: the same tube, the same blue-grey shading and drifting purple motes,
+the retained UI composited above it, with an opaque black surround and no world
+bleeding through (the pass clears colour rather than loading it — a Vulkan-forced
+decision documented in §5.5.18). `PortalTunnelPresentation` was the last raw-GL
+world-adjacent renderer; this frame is its Vulkan arm.
+
+![V6m: portal tunnel on Vulkan](assets/campaign-v/v6m-portal-tunnel-vulkan.png)
+
+**V6l — the paperdoll on Vulkan.** The inventory/paperdoll panel open on the
+Vulkan arm, rendered through the private entity viewport with correct pose,
+orientation and alpha. This exercises the offscreen-render-target path the
+backbuffer-only captures above don't reach.
+
+![V6l: paperdoll on Vulkan](assets/campaign-v/v6l-paperdoll-vulkan.png)
+
+**The GL-versus-Vulkan difference map.** Red channel encodes per-pixel delta
+between a GL and a Vulkan capture of the same Holtburg scene (log scale, mostly
+black). The visible structure is exactly the two populations the differential
+work isolated: a thin band across the treeline/roofline near the top, and a
+scatter of isolated dots at building corners and cloud edges — not a diffuse,
+whole-frame shift. That shape is what let the investigation rule out a global
+cause (gamma, half-pixel offset, depth precision) and converge on anisotropic
+filtering as the remaining explanation (see the verdict tables below).
+
+![GL vs Vulkan difference map](assets/campaign-v/v6k-diff-gl-vs-vulkan.png)
+
+---
+
+## The blank-world investigation
+
+V4c and V4d — the slices that were meant to move the world draw path onto the
+RHI on GL first — shipped, passed their offline pixel gate, and then produced a
+**blank world** in the first connected sessions: UI rendered, sounds played, the
+log was clean, `world-reveal` reported `visible=True`, and the user's own screen
+surfaced one AMD driver-level timeout during testing. Both slices were reverted.
+What followed was six investigation cycles (§5.5.1–§5.5.6 of the plan), each one
+correcting the cycle before it.
+
+**Cycle 1** measured the defect honestly for the first time: interleaved A/B
+runs of the same binary (to share whatever machine-state drift had been
+confounding earlier attributions) found **4/5 blank in the V4c arm versus 0/5 in
+the parent arm, p ≈ 0.024** — real, but intermittent. It also found the first
+broken instrument: the client's own screenshot read a 4×-multisampled default
+framebuffer with `glReadPixels`, which is undefined behavior per the GL spec.
+
+**Cycle 2** tested every "shared 3-D state" theory that could explain "world
+dies, UI survives" — depth plane, camera constants, clip distances, GPU context
+reset — and found all four **bit-identical** between blank and rendered frames.
+It also found a second broken instrument: the in-process capture of framebuffer
+0 returned all-zero pixels on a blank run, even over screen regions the desktop
+compositor plainly showed the UI drawing into. Screenshot bytes were reporting
+the readback, not the renderer.
+
+**Cycle 3** built an occlusion-query instrument to settle whether draws were
+executing and being discarded, or never executing at all — and found a third and
+fourth broken instrument in the process: a CPU-side query read
+(`glGetQueryObject`) **deadlocked the render thread inside the driver** on every
+attempt, and routing the same query through a GPU-timeline `GL_QUERY_BUFFER`
+write showed the write itself **never landing** on a blank run. The verdict: the
+GPU renders the first world frame identically to the working build — the same
+1,692,830 terrain samples and 317,561 entity samples — and then every GPU→CPU
+reporting channel dies at once, permanently, while presentation and fence
+signalling keep running normally.
+
+**Cycles 4–5** laid out three ways forward and picked the cheapest discriminator:
+run the identical ported world path on Vulkan, on the same GPU. If it renders
+clean there, the defect is the AMD GL driver's, not the ported code's — settled
+in one measurement instead of a multi-hour bisect that would keep pinning the
+user's machine.
+
+**Cycle 6** ran that measurement — first as a cross-vendor check while Vulkan
+bring-up was still in flight: the exact V4c binary, ten repeat-connected runs,
+on a separate NVIDIA machine against the same server, same account, same
+worst-case cell. **10/10 rendered**, bounding the NVIDIA failure rate below ~4%
+against a measured 30%+ on the AMD box. Same binary, same server, same scene,
+two drivers — only AMD's failed, and it failed below the API in four
+independent instruments. The driver-defect conclusion was adopted: the GL
+backend keeps the legacy raw-GL world path as a scoped, documented exception to
+the no-fork rule, and V4c/V4d's content returns as the Vulkan world path
+instead (landed at V6j — see the screenshots above).
+
+**Falsified along the way:** the frame ring's `glBufferSubData` hazard, a
+capture-FBO binding leak, reveal ordering, shared 3-D state, and CPU-side
+visibility — five mechanisms, none of them the cause. **Broken along the way:**
+the multisampled `glReadPixels`, the in-process backbuffer capture, the
+CPU-blocking query read, and the GPU-timeline query write — four instruments
+that were measuring the wrong thing, all on the same driver.
+
+---
+
+## The verdict tables
+
+**V7's per-stop differential**, full connected route, both backends, tolerance
+2 / fraction 0.001, MSAA off, day group 0, world time pinned to 0.5, sky phase
+pinned to 0 (`artifacts/v7-diff-c2`):
+
+| Stop | Differing px | Fraction | Max Δ | Verdict |
+|---|---|---|---|---|
+| `holtburg_town` | 26,330 | 2.86e-02 | 255 | **EXCEPTION** — phase + AD-46 |
+| `facility_hub_interior` | 7,176 | 7.79e-03 | 255 | **EXCEPTION** — phase |
+| `aerlinthe_island` | 62,892 | 6.82e-02 | 255 | **EXCEPTION** — AD-46 + dark-scene floor |
+
+No connected stop passes yet, and none of the three exceptions is a renderer
+defect: `holtburg_town`'s residual is an animated portal, two wandering NPCs, a
+smoke plume, and vitals that genuinely regenerated between two logins minutes
+apart, plus the treeline; `facility_hub_interior`'s entire residual is the local
+player's idle pose and its lighting — walls, floor and doorway are pixel-black
+in the difference map; `aerlinthe_island`'s residual is half AD-46 shrubbery and
+half an artifact of an *absolute* tolerance against a scene averaging 28/255
+luminance, where half the differing pixels are exactly one step over threshold.
+
+**The offline GL-versus-Vulkan pair**, both clocks pinned, MSAA off — the
+instrument the investigation converged on, because the offline scene has no
+session, no server, no entities, no camera settle and no wandering NPCs, so a
+difference is the renderer or it is nothing:
+
+| Pair | Differing px | Fraction |
+|---|---|---|
+| GL vs GL, same commit (control) | 1,966 | 2.13e-03 |
+| Vulkan vs Vulkan, same commit (control) | 1,039 | 1.13e-03 |
+| **GL vs Vulkan, whole frame** | 28,807 | 3.13e-02 |
+| **GL vs Vulkan, below the tree band (rows 280–720)** | **497** | **8.82e-04 — inside the 0.001 threshold** |
+
+Terrain, terrain blending, roads, the water edge, fog, statics, scenery below
+the horizon, and the entire retained UI are at parity. Everything outside that
+threshold lives in one population: the treeline. Four hypotheses were tested —
+sub-pixel offset (refuted: best alignment is (0,0)), sharpness/LOD mismatch
+(refuted: high-frequency energy matches within 2–5%), depth precision (refuted:
+forcing GL's depth-range mapping onto Vulkan moved the number by only 3%), and
+anisotropic filtering (confirmed: going from anisotropy 1 to 16 — GL's and
+retail's own value — nearly halved the band, and no knob is left once both arms
+request the same setting). What remains is the anisotropic tap pattern itself,
+which both GL and Vulkan specs leave implementation-defined and AMD's own two
+drivers don't agree about. Filed as **`AD-46`** in the
+[divergence register](../architecture/retail-divergence-register.md), dormant
+until V10.
+
+---
+
+## The defects the migration found in its own code
+
+Each of these compiled clean, passed validation, and had a test — and each was
+still wrong, caught only when a slice's pixel gate finally exercised the path
+that mattered. That's the campaign's per-slice discipline working as intended:
+
+- **Winding inversion (V6j).** `VulkanViewportMapping` had inverted the front
+  face since V6c on the standard negative-viewport argument, but nothing before
+  the world arm actually culled anything to test it. The world arm was its
+  first culling consumer, and it was wrong twice on one frame: terrain vanished
+  outright, and every closed building shell rendered inside-out with its front
+  wall culled and its interior beams visible through the gap.
+- **Stride mismatch (V6k).** The Vulkan sky's vertex layout declared a 32-byte
+  stride (position, normal, texcoord) while the actual CPU vertex record is
+  36 bytes — it carries a fourth field the sky shader never reads but the GL
+  arm's `sizeof(Vertex)` always counted. The dome came up as a field of
+  blue-white noise until the layout stated the record's real footprint.
+- **Sampler overrides (V7).** The Vulkan world texture arrays asked for
+  `MaxAnisotropy: 1` where the GL arm reads the driver's actual capability
+  twice over — and retail itself sets every sampler stage to the device's
+  reported maximum. Fixing it to request the device ceiling halved the
+  treeline difference band and dropped everything below it by two-thirds.
+- **A one-binding, two-renderer hazard (V6i-1).** A single descriptor binding
+  would have let the GL particle draw clobber `WbDrawDispatcher`'s instance
+  data mid-frame the moment both renderers were active in the same pass. Closed
+  before the world arm could ever exercise it, with one descriptor-set pair per
+  renderer scope derived from the descriptor state itself rather than declared
+  by hand.
+- **MSAA readback undefined behavior (the blank-world investigation).** Every
+  pixel gate and every blank/rendered verdict in the campaign, until this was
+  found, read a 4×-multisampled default framebuffer directly with
+  `glReadPixels` — undefined per the GL spec. The capture now resolves to a
+  single-sampled framebuffer first; re-measuring the noise band afterward
+  showed the fix itself moved the offline gate by 23 pixels out of 563,200,
+  indistinguishable from its own same-commit control.
+
+---
+
+## Current status
+
+- **V8 (performance gate)** is **running**. Its first commit landed the
+  missing instrument: the Vulkan arm had been silently wired to
+  `NullRenderFrameGpuMeasurement` and emitted no `[frame-prof]` line at all, so
+  the campaign's own performance tooling had nothing to point at it. GPU
+  timestamp measurement now brackets identically on both arms. The acceptance
+  table (CPU frame p50 ≤ 1.60 ms, GPU p50 ≤ 1.00 ms, working set ≤ 600 MiB,
+  private set ≤ 860 MiB, 0 B/frame managed allocation, CPU/GPU p99 ≤ GL) is the
+  next thing this slice measures against the RX 9070 XT.
+- **V9 (Linux CI)** is **implemented, first CI run pending**. A `linux-vulkan`
+  job now runs the probe on lavapipe under Xvfb, asserts a clean accepting
+  verdict, checks the forced-unsupported exit-4 path, and verifies committed
+  `.spv` binaries are byte-identical to a fresh compile. lavapipe satisfies
+  every one of the seventeen gated features, including `samplerAnisotropy`,
+  which V7 made load-bearing. The physical Linux GPU row and Wayland stay
+  deferred on the same precedent as Slice L.
+- **V10 (cutover)** is **ahead**: Vulkan becomes the default backend, GL stays
+  reachable by environment variable for one further slice, and **user visual
+  sign-off** is the gate — the same kind of connected session the user already
+  ran once mid-campaign (six targeted checks, all passed with merits: the AD-46
+  treeline judged invisible in practice, dyed-gear composites, doorway
+  look-out/threshold crossing, stacked translucency, icon/text crispness, and
+  motion feel — the one open finding from that session is #253 below).
+- **V11 (closeout)** deletes `Gpu/Gl`, `OpenGLGraphicsDevice`, the
+  `ManagedGL*` stack, the ImGui developer stack, and UI Studio, and re-measures
+  working set one final time.
+
+---
+
+## Filed issues (#248–#253)
+
+- **[#248](../ISSUES.md)** — `FrustumCuller` extracts its near plane with the
+  GL `[-1,1]`-NDC formula against a `[0,1]`-NDC projection; harmless today
+  (makes culling strictly more permissive near the eye) but wrong hygiene.
+  OPEN.
+- **#249** — Released bindless texture slots never call `MakeNonResident`
+  before the texture is deleted, which `GL_ARB_bindless_texture` leaves
+  undefined, and leaks a resident handle for the process lifetime per release.
+  OPEN.
+- **#250** — Four "zero managed allocation" tests fail intermittently, roughly
+  one run in three, from tiered-JIT/GC measurement noise rather than a real
+  regression — but an acceptance-gating test that flakes trains everyone to
+  re-run until green. OPEN.
+- **#251** — `glClientWaitSync` returned a fence status of `0` (not a value
+  the API is specified to return) and crashed the render loop once in nine
+  connected runs; possibly the same below-the-API driver family the
+  blank-world investigation documented four other instances of. OPEN.
+- **#252** — Ten App test classes raced on process-global `CameraDiagnostics`/
+  `RenderingDiagnostics`/`Console.Out` statics under xUnit's parallel-by-default
+  class execution. **DONE** — serialized via a `CameraDiagnosticsCollection`
+  marker.
+- **#253** — Attribute/skill icons in the character sheet sit off-center in
+  their cells and render fully opaque, where retail uses vitae-window-like
+  translucency; found during the user's mid-campaign Vulkan visual session,
+  suspected pre-existing (the retained UI compares identical between backends
+  at 3.94e-04) rather than a Vulkan regression. OPEN.
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