The grid tore while scrolling on the tablet, in portrait only, and was
flawless in landscape. It was not vsync and it was not the grid.
Measured on the device, same build, only the tablet rotated:
landscape bufferTransform=ROT_180 composition=DEVICE (2) clean
portrait bufferTransform=ROT_270 composition=CLIENT (1) torn
The panel is mounted landscape — 1920x3000 at installOrientation 3 — so a
portrait window needs a 90 degree rotation before scanout. wgpu-hal hardcodes
the swapchain's `preTransform` to `IDENTITY` and says so in a comment beside
the line:
// On Android 10+, libvulkan's `vkQueuePresentKHR` returns
// `VK_SUBOPTIMAL_KHR` if not doing pre-rotation ... This is always the
// case when the device orientation is anything other than the identity
// one, as we unconditionally use `VK_SURFACE_TRANSFORM_IDENTITY_BIT_KHR`.
That is gfx-rs/wgpu#3345, and it cannot be fixed by setting the field:
`preTransform` is a *promise* that the content is already rotated, so keeping
it needs the renderer to rotate what it draws, which wgpu cannot do on Skia's
behalf.
We do not have to be on that swapchain. `AndroidWindowAdapter` chooses
`SkiaRenderer::default_wgpu_29` only because this crate enables
`unstable-wgpu-29`; without it `SkiaRenderer::default` resolves — through
i-slint-renderer-skia's build script, which selects OpenGL on anything that is
not Apple, Windows or wasm — to Skia over OpenGL, where the driver owns the
rotation and there is no transform to get wrong. So both renderer features
move to the desktop-only dependency, and desktop is untouched.
# The cost, stated rather than hidden
Skia over OpenGL cannot sample a `wgpu::Texture`, so the develop view's frame
comes back through memory: `AdjustPass::export_pixels`, already ungated and
already used by the export path, into a `SharedPixelBuffer`. That is the
round-trip ARCH §6.1 and AC-8 exist to forbid, and it is the right trade only
because of what the alternative actually is — not a faster develop view, but a
grid that tears in the orientation a tablet is mostly held in.
Two things keep it small. The device is still opened on Android, so demosaic
and the adjust pass are untouched on the GPU; only the last hop changes. And
`render` fits the pass to the canvas before it runs, so the readback is at
viewport resolution, a fraction of the ~7 ms at 4K the original measurement
was taken against.
Four other explanations died on the way here, each by measurement rather than
argument: the present mode (a patch confirmed in the installed binary reached
`AutoVsync`, and the rows still duplicated), our shared wgpu device (Slint
opened its own, unchanged), Skia's partial rendering (off for GPU surfaces),
and client composition itself (unavoidable in portrait on this panel, so it
cannot be what distinguishes a torn frame from a clean one).
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
dtourolleandClaude Opus 5
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