Qt Quick has no partial redraws: any dirty item presents the WHOLE
window, and on the owner's hybrid laptop each full-window present costs
~1.2% GPU flat (KWin composite + dGPU scanout; area-independent, while
the app's own render is 0.08 ms/frame). The bill was presents/s, and
the shell ran unsynchronised ~30 Hz clocks plus two leaks:
- QbzShell.pulseMs: ONE Rust-driven 33 ms edge that every continuous
animator ticks on (knob QBZ_PULSE_MS). VizSettle now stashes on
publish and applies on the pulse (its 16 ms interpolation timer is
gone); AmbientField and ImmersiveAtmosphere ride the same edge.
- VizSettle writes NOTHING while not live: the always-mounted but
invisible immersive panels were dirtying 28 bar rects per FFT
publish (named via QT_LOGGING_RULES=qt.quick.dirty=true).
- AmbientField renders inline again: ShaderEffectSource
scheduleUpdate() double-presents (minimal repro fbo_repro.qml in the
investigation folder).
- Background mode 2 (ImmersiveAtmosphere) composites its four drifting
layers + base + dim + scrim in ONE opaque ShaderEffect
(atmosphere.frag, baked by build.rs); numeric parity vs the Image
stack verified (atmo-shader-parity-check.py, delta 0.0). The
four-Image stack stays as the software/failure fallback.
Measured on the owner's machine: 93-97% -> 25-26% GPU, ~34 W -> 12-13 W,
~33 presents/s in both background modes. Full story:
qbz-nix-docs/qt-frontend/2026-08-11-scenegraph-batches/GPU-COST-INVESTIGATION.md
km
vicrodh
f46c5c95d4