IPC clients can capture their frame: MCP capture_frame, per-exe trigger, service + shell triggers (#1810)

[dfattal]

Fixes #1810: an IPC client (forced IPC, or an app under the shell) had no working capture route. All four routes now work; hardware-verified on the win box, 2026-10-03.

Root causes and fixes

1. App MCP capture_frame reported "not wired".
   • Cause: the IPC client compositor never registered a capture handler.
   • Fix: it now installs one (Windows). It requests this client's own per-client atlas from the service over the existing workspace_capture_frame IPC call, which xrCaptureAtlasDXR's IPC branch already used. The service gains comp_d3d11_service_capture_frame_for_client; the old entry point forwards with xc=NULL, so existing callers are unchanged.
2. The per-exe trigger displayxr_atlas_trigger.<exe> was never consumed.
   • Fix: the IPC client now polls it after each commit (the same u_capture_intent semantics as in-process) and writes the same output path the in-process compositors use.
3. The service workspace_screenshot_trigger never fired for a client-present (ADR-029 client-texture) presenter.
   • Cause: pipeline_default_policy_render returns early for that presenter kind, and the client's weave runs in pipeline_client_texture_weave, which had no trigger check.
   • Fix: the check was added there, at most 4 times a second, before render_mutex, consuming the trigger only after a woven frame. No new waits.
4. The shell MCP capture_frame returned ok:false.
   • Cause: the shell sends flags=0 ("runtime picks"), which capture_frame rejected.
   • Fix: flags==0 now defaults to the full atlas, and a failed capture logs a WARN.

• Also: a request-only IPC_CAPTURE_FLAG_RAW_ALPHA bit, so DXR_ATLAS_CAPTURE_RAW_ALPHA=1 in the client env works over IPC. The wire structs are unchanged.

In-process capture paths are untouched.

Verified (dev service from this branch, cube_zones_d3d11_win, XRT_FORCE_MODE=ipc DISPLAYXR_ZONES=1 DISPLAYXR_MCP=1)

route	result
per-exe trigger	displayxr_atlas.cube_zones_d3d11_win.png written (144 KB)
app MCP capture_frame	{"path": "...displayxr-mcp-capture-<pid>-<frame>.png", "size_bytes": 145155}
service workspace_screenshot_trigger	workspace_screenshot.png + workspace_screenshot_atlas_2_2x1.png
shell MCP capture_frame (installed shell v2.3.2 on this dev service)	ok:true, displayxr_capture_atlas_2_2x1.png

Used straight away: these captures numerically verified #1809's service pure-zones path. The UNORM-strip leg reads exactly one encode above the _SRGB leg in the per-client atlas: greys 60→133, 210→234, red (38,8,8)→(108,50,50).

Not covered

• Under a workspace, an app still can't capture itself (ipc: explicit client classes (CONTROLLER / APP / PRESENT_OWNER / RELAY / PROVIDER_HOST / DIAG) assigned and verified at connect; per-class quotas and priority #960, denied by design). Use the shell's route.
• Windows only: only the D3D11 service answers the call.
• The browser-style present-owner presenter still has no screenshot-trigger site.

🤖 Generated with Claude Code