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Show the develop frame itself, instead of a photocopy of it

Browse Source 
The oldest open item in the project (ARCH §6.1, spike S1, AC-8). Every frame
in develop was read off the GPU into a `SharedPixelBuffer` and handed back to
Slint to upload again: ~7 ms at 4K against a 0.28 ms compute pass, 96% of the
frame spent carrying pixels to the CPU and back so they could be drawn where
they already were.

Slint 1.17 will adopt a `wgpu::Texture` directly, and the whole of what that
needs is arrangement rather than code.

**One device, made before the window.** A texture belongs to the device that
allocated it, so the compute passes and the compositor cannot each open their
own. `GpuContext::new_shared` opens one and hands back the instance and
adapter alongside it; `dr_ui::shared_gpu` gives all four to
`BackendSelector::require_wgpu_29(WGPUConfiguration::Manual { .. })`. That
call has to come before the first window, because creating one selects a
backend for you — which is why the GPU is now opened at the top of `run`
rather than two hundred lines down beside the other controllers.

dr-gpu still names no UI type. It hands out raw wgpu and does not ask who is
compositing (ARCH §6.5a).

**Vulkan only on the shared path**, where headless keeps its GL fallback.
wgpu's GL backend reaches its display through EGL at instance creation, and
before a window exists there is no display handle to give it — so a GL
instance cannot later produce the window surface Slint needs from it. A
machine with no Vulkan gets no shared device and browses without develop,
which is the same degradation as no adapter at all.

**`renderer-femtovg` becomes `renderer-femtovg-wgpu`.** The old one is FemtoVG
over OpenGL and cannot be handed a wgpu texture at all. It is not kept
alongside as a fallback: FemtoVG-over-GL has no branch for an imported
texture, falls through to "render this image into a buffer", gets nothing, and
draws nothing — a blank canvas with no error, which is worse than the failure
it would be papering over. The consequence is stated plainly in the manifest:
the desktop app now needs a working wgpu adapter to open a window.

**Two output textures, not one, and this is the part that is not obvious.**
Slint repaints when the image property *changes*, and it decides that with
`PartialEq` — which for two images over the same `wgpu::Texture` says
"unchanged". A pass that reused a single target would have rendered every
slider move correctly on the GPU and shown none of them: right, and invisible.
`AdjustPass` alternates between two targets, so consecutive frames are
genuinely different values. It also settles the read-while-write question that
one queue was already answering.

`RENDER_ATTACHMENT` is added to both render targets. Neither pass uses it;
Slint rejects an imported texture without it, on the reasoning that a
compositor handed a texture may need to draw into it.

**`AdjustPass::read_output` is deleted rather than gated.** It and
`export_pixels` were the same transfer under two names, and the comments
explaining why they were separate are the point of the whole criterion:
reading pixels back to *display* them is the defect, reading them back to
*encode a file* is the only way a file is made. The display twin is now gone
outright, which is stronger than a feature flag — it cannot be turned back on.
`export_pixels` is untouched and still ungated. The `readback` feature comes
off dr-ui, darkroom-desktop and darkroom-android; it stays in dr-gpu, where it
still gates `RenderTarget::read_pixels` and the segmentation field readback.
`examples/develop` moves to `export_pixels`, which is honest — it writes a
PPM — and so no longer needs the feature.

Four tests, each named for what it protects and each of which fails without a
screen if the property it guards breaks:

- the adjust target satisfies every condition Slint's import checks, asserted
  in the crate that owns the descriptor, because a descriptor that drifts
  fails at runtime on a real display and nothing else would notice;
- consecutive renders are different textures, and the third is the first
  again, so the alternation is a rotation and not an allocation per frame;
- the develop canvas has no CPU pixel buffer and does have a wgpu texture —
  AC-8 itself, in the terms Slint uses;
- consecutive frames compare unequal as `slint::Image`, which is the property
  the repaint actually depends on.

The zoom test's readback moves into the test module. It has to: there is no
library function that copies a displayed frame to the CPU any more, and that
is the point — the round-trip now exists in the test binary and nowhere a
shipping build can reach.

**What is not proven.** No GUI was run. What is verified is that the texture
satisfies the import contract, that the import succeeds, that the canvas is a
texture rather than a buffer, and that consecutive frames are distinguishable.
What is unverified is everything that needs a display: that Slint's FemtoVG
wgpu renderer adopts the Manual configuration on a real surface, that the
picture appears the right way up and the right colour, and the frame timing
that motivated the whole exercise. Android is untouched by testing — the
android backend routes a WGPU29 request to Skia, whose wgpu surface does
handle imported textures, but that is read from the source, not observed.

56 dr-gpu tests and 255 dr-ui tests pass, clippy clean under `-D warnings`,
fmt clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
dtourolle
2026-08-17 09:54:00 +02:00
co-authored by Claude Opus 5
parent 2330ed25e9
commit cf8f5b632f
11 changed files with 555 additions and 142 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ui/dr-ui/Cargo.toml
+33 -7
View File
@@ -7,10 +7,11 @@ license.workspace = true
[dependencies]
dr-types.workspace = true
# The readback feature is on because Slint's texture-import path is unwired
# until spike S1; the develop view has no other way to reach the screen. It
# must come off when S1 lands (ARCH §6.1, AC-8).
dr-gpu = { workspace = true, features = ["readback"] }
# No `readback`. S1 wired Slint's texture import, so the develop view hands
# the compositor the texture itself and there is no display round-trip left to
# gate (ARCH §6.1, AC-8). The export path reads pixels back through
# `export_pixels`, which is ungated and always was.
dr-gpu.workspace = true
dr-decode.workspace = true
serde_json.workspace = true
tokio.workspace = true
@@ -29,7 +30,34 @@ rusqlite.workspace = true
# android-activity on Android, and enabling both makes the backend selector
# pick at random. So the backend features live on the target-specific
# dependencies below rather than here.
slint = { workspace = true, features = ["compat-1-2", "renderer-femtovg"] }
#
# `renderer-femtovg-wgpu` rather than `renderer-femtovg`: the latter is
# FemtoVG over OpenGL, and a compositor drawing through GL cannot be handed a
# `wgpu::Texture`. Importing one requires Slint itself to be rendering with
# wgpu, and this is the FemtoVG backend that does (ARCH §6.1, spike S1).
#
# `unstable-wgpu-29` is the other half: the renderer feature makes Slint draw
# with wgpu, and this one exposes the API to say so — `BackendSelector::
# require_wgpu_29` and `Image::try_from(wgpu::Texture)`. Unstable is Slint's
# word for it; the surface is small and the alternative is the 7 ms round-trip.
#
# `renderer-femtovg` is *not* kept alongside as a fallback, though it would
# still compile. Slint's winit backend prefers the wgpu FemtoVG renderer
# whenever both are built, so the GL one would only ever be reached by someone
# setting `SLINT_BACKEND=winit-femtovg` — and on that path an imported texture
# is not drawn at all. FemtoVG-over-GL has no branch for a `wgpu::Texture`, so
# it falls through to "render this image to a buffer", gets nothing back, and
# draws nothing. A blank canvas with no error is a far worse failure than the
# one below, so the fallback is removed rather than left as a trap.
#
# Consequence worth stating plainly: the desktop app now needs a working wgpu
# adapter to open a window at all. Slint refuses a CPU adapter for this
# renderer unless `SLINT_WGPU_CPU` is set in the environment.
slint = { workspace = true, features = [
"compat-1-2",
"renderer-femtovg-wgpu",
"unstable-wgpu-29",
] }
wgpu.workspace = true
anyhow.workspace = true
# `SettingsError` distinguishes an io failure from a malformed file, which the
@@ -63,8 +91,6 @@ serde_norway.workspace = true
[features]
default = []
# Temporary CPU readback path; see dr-ui docs and spike S1.
readback = ["dr-gpu/readback"]
# Debug convenience: re-read style.yaml at startup so a palette can be tuned
# without rebuilding. Costs the constant-folding of every token, so it stays
# off by default and has no business in a release build.
ui/dr-ui/src/develop.rs
+176 -18
View File
@@ -502,15 +502,25 @@ impl DevelopSession {
Some((op.id, param.id))
}
/// TRACES: FR-DSP-1 | AC-8
/// Render at the requested display size and hand back a Slint image.
///
/// Renders at *viewport* resolution rather than sensor resolution, which
/// is what keeps slider interaction inside the frame budget on a 24 MP
/// file (FR-DSP-1).
///
/// The readback at the end is the temporary bridge documented on
/// `AdjustPass::read_output`: ARCH §6.1 forbids it, and spike S1 removes
/// it by importing the texture into Slint directly.
/// **The image is the texture, not a copy of it.** This used to end in a
/// `read_output` into a `SharedPixelBuffer` — the GPU→CPU→GPU round-trip
/// ARCH §6.1 forbids and AC-8 asserts against, measured at ~7 ms at 4K
/// against a 0.28 ms compute pass. Spike S1 replaced it with
/// `slint::Image::try_from`, which wraps the texture where it already is.
/// The `clone` below is a refcount on the wgpu handle, not on the pixels.
///
/// This only works because the compositor is drawing with the same device
/// the pass wrote with; see `shared_gpu` in the crate root for how that is
/// arranged, and note that nothing here can detect it having gone wrong —
/// a texture from a foreign device is a runtime fault on a real screen,
/// which is why the arrangement is made once at startup and never again.
pub fn render(&mut self, width: u32, height: u32) -> Result<slint::Image, String> {
// Fit the render to the viewport while preserving aspect, so the
// pass does no work on pixels the view will letterbox away.
@@ -523,14 +533,17 @@ impl DevelopSession {
let (w, h) = fit(fw, fh, width.max(1), height.max(1));
let shader = self.graph.compose();
self.adjust
let texture = self
.adjust
.render(&self.demosaiced, &shader, w, h)
.map_err(|e| e.to_string())?;
let (pixels, rw, rh) = self.adjust.read_output().map_err(|e| e.to_string())?;
let buffer =
slint::SharedPixelBuffer::<slint::Rgba8Pixel>::clone_from_slice(&pixels, rw, rh);
Ok(slint::Image::from_rgba8(buffer))
// The import is fallible on format and usage only, and both are fixed
// in `AdjustPass`'s texture descriptor — so a failure here is a
// descriptor that drifted, not anything the caller did. Say that,
// rather than surfacing "InvalidUsage" to a photographer.
slint::Image::try_from(texture.clone())
.map_err(|e| format!("the render target is not importable by the compositor: {e}"))
}
/// Render the *whole* frame for the crop overlay to be drawn over.
@@ -592,13 +605,17 @@ impl DevelopSession {
/// screen-sized file. This renders the framed output size instead, so the
/// export is the full-quality path FR-EXP-9 requires.
///
/// The readback here is `export_pixels`, not the display bridge: a file
/// is made of bytes on the CPU and there is no path to one that avoids
/// the transfer. See the note on that method for why the two are separate.
/// This reads pixels back and [`Self::render`] does not, and that is the
/// whole distinction AC-8 draws: a file is made of bytes on the CPU and
/// there is no path to one that avoids the transfer, whereas a frame on
/// screen had no business making the trip. See `AdjustPass::export_pixels`
/// for the longer version.
///
/// Leaves the pass holding a full-resolution target, so the caller should
/// expect the next display render to reallocate. Cheaper than keeping a
/// second pass alive for the exports a session rarely performs.
/// Leaves one of the pass's two targets at full resolution; it is dropped
/// and reallocated on the second display render after this, since the
/// other target still holds a viewport-sized texture and comes up first.
/// Cheaper than keeping a second pass alive for the exports a session
/// rarely performs.
pub fn render_for_export(&mut self) -> Result<dr_export::Frame, String> {
let (sw, sh) = self.demosaiced.size();
let (w, h) = self.graph.output_size(sw, sh);
@@ -996,6 +1013,145 @@ mod tests {
use super::*;
use dr_pipeline::EditGraph;
/// TRACES: FR-DSP-1 | AC-8
/// Copy a displayed frame back to the CPU, for assertions and nothing else.
///
/// The library has no such function on purpose: S1 removed the display
/// readback, and AC-8 is the assertion that it stayed removed. A test that
/// wants to look at the pixels therefore has to do the copy itself, which
/// is exactly the right shape — the round-trip lives in the test binary
/// and cannot be reached from a shipping one.
///
/// Doubles as the proof: this only compiles because the image *is* a wgpu
/// texture. Hand it a `SharedPixelBuffer`-backed image and it panics.
fn read_back(ctx: &GpuContext, image: &slint::Image) -> Vec<u8> {
let texture = image
.to_wgpu_29_texture()
.expect("the develop canvas must be a GPU texture, not a pixel buffer");
let (w, h) = (texture.width(), texture.height());
// Buffer rows must be aligned to COPY_BYTES_PER_ROW_ALIGNMENT.
let unpadded = w * 4;
let align = wgpu::COPY_BYTES_PER_ROW_ALIGNMENT;
let padded = unpadded.div_ceil(align) * align;
let buf = ctx.device.create_buffer(&wgpu::BufferDescriptor {
label: Some("test-readback"),
size: u64::from(padded * h),
usage: wgpu::BufferUsages::COPY_DST | wgpu::BufferUsages::MAP_READ,
mapped_at_creation: false,
});
let mut enc = ctx.device.create_command_encoder(&Default::default());
enc.copy_texture_to_buffer(
wgpu::TexelCopyTextureInfo {
texture: &texture,
mip_level: 0,
origin: wgpu::Origin3d::ZERO,
aspect: wgpu::TextureAspect::All,
},
wgpu::TexelCopyBufferInfo {
buffer: &buf,
layout: wgpu::TexelCopyBufferLayout {
offset: 0,
bytes_per_row: Some(padded),
rows_per_image: Some(h),
},
},
wgpu::Extent3d {
width: w,
height: h,
depth_or_array_layers: 1,
},
);
ctx.queue.submit(Some(enc.finish()));
let slice = buf.slice(..);
let (tx, rx) = std::sync::mpsc::channel();
slice.map_async(wgpu::MapMode::Read, move |r| {
let _ = tx.send(r);
});
ctx.device
.poll(wgpu::PollType::wait_indefinitely())
.expect("poll");
rx.recv().expect("map").expect("map");
let data = slice.get_mapped_range();
let mut out = Vec::with_capacity((unpadded * h) as usize);
for row in 0..h {
let start = (row * padded) as usize;
out.extend_from_slice(&data[start..start + unpadded as usize]);
}
drop(data);
buf.unmap();
out
}
/// TRACES: FR-DSP-1 | AC-8
#[test]
fn the_displayed_frame_is_a_texture_and_not_a_pixel_buffer() {
// The acceptance criterion itself, asserted from the side that would
// notice it regressing. `to_rgba8` returning `Some` would mean the
// frame had come back through system memory to be looked at, which is
// the ~7 ms per frame at 4K that ARCH §6.1 forbids; `to_wgpu_29_texture`
// returning `Some` means the compositor got the texture where it lay.
//
// Note this passes without a display: the import is a wrapper, and it
// is the *compositor* adopting the device that needs a screen. What
// cannot be proved here is that the picture arrives; what can be
// proved is that no copy was made on the way.
let Ok(ctx) = pollster::block_on(dr_gpu::GpuContext::new_headless()) else {
log::warn!("no GPU adapter; skipping");
return;
};
let rgba = vec![128u8; 32 * 32 * 4];
let mut session =
DevelopSession::open_rgb(&ctx, &rgba, 32, 32, dr_types::Orientation::NORMAL)
.expect("session");
let frame = session.render(32, 32).expect("render");
assert!(
frame.to_rgba8().is_none(),
"the canvas has CPU pixels, so something copied them there"
);
let texture = frame
.to_wgpu_29_texture()
.expect("the canvas is neither a texture nor a pixel buffer");
assert_eq!((texture.width(), texture.height()), (32, 32));
}
/// TRACES: FR-DSP-1 | AC-8
#[test]
fn consecutive_frames_look_different_to_the_property_system() {
// The catch that comes free with handing over a texture instead of a
// buffer. Slint repaints when the image property *changes*, and it
// decides that with `PartialEq` — which for two images over one
// `wgpu::Texture` says "unchanged". A pass that reused a single target
// would therefore render every slider move correctly and show none of
// them.
//
// `AdjustPass` alternates between two targets to prevent it. This
// asserts the consequence in the terms Slint actually uses, so it
// would still catch the regression if the mechanism were replaced.
let Ok(ctx) = pollster::block_on(dr_gpu::GpuContext::new_headless()) else {
log::warn!("no GPU adapter; skipping");
return;
};
let rgba = vec![128u8; 32 * 32 * 4];
let mut session =
DevelopSession::open_rgb(&ctx, &rgba, 32, 32, dr_types::Orientation::NORMAL)
.expect("session");
let first = session.render(32, 32).expect("first render");
let second = session.render(32, 32).expect("second render");
assert_ne!(
first, second,
"the canvas property would not change, so the frame would never be shown"
);
}
/// The whole scroll-to-zoom path, end to end, in the order the user drives
/// it: show the image fitted, *then* turn the wheel.
///
@@ -1032,12 +1188,14 @@ mod tests {
assert!(session.is_zoomed(), "the session did not register the zoom");
let zoomed = session.render(64, 64).expect("zoomed render");
let before = fitted.to_rgba8().expect("fitted pixels");
let after = zoomed.to_rgba8().expect("zoomed pixels");
// Both images are still readable here because consecutive frames go to
// alternating textures; see `AdjustPass::targets`. Holding two frames
// at once would be meaningless against a single reused target.
let before = read_back(&ctx, &fitted);
let after = read_back(&ctx, &zoomed);
let differing = before
.as_bytes()
.iter()
.zip(after.as_bytes().iter())
.zip(after.iter())
.filter(|(a, b)| a != b)
.count();
ui/dr-ui/src/lib.rs
+90 -26
View File
@@ -3,12 +3,17 @@
//! A viewer with a develop panel: open a folder of RAW files, decode and
//! demosaic on the GPU, and adjust.
//!
//! **Read before assuming A1 is proven.** Slint's public API for adopting an
//! externally created wgpu texture is not wired up here; this build uploads
//! through `SharedPixelBuffer`, which *is* a CPU round-trip — explicitly the
//! thing ARCH §6.1 forbids in production. Spike S1 replaces it. Until then A1
//! is unvalidated, and the develop path pays a readback per frame that the
//! finished one will not.
//! **The develop frame never leaves the GPU** (ARCH §6.1, AC-8). Spike S1
//! wired Slint's texture import: [`shared_gpu`] opens one wgpu device and
//! gives it to *both* the compute passes and Slint's renderer, and
//! `DevelopSession::render` then hands the compositor the very texture the
//! adjust pass wrote. What used to be a readback and an upload per frame is
//! now a refcount.
//!
//! `SharedPixelBuffer` still appears in this file and in the library grid, and
//! that is not a relapse: an embedded JPEG preview and a thumbnail are decoded
//! on the CPU and have no texture to hand over. AC-8 is about pixels that were
//! *computed on the GPU* travelling to the CPU and back to be looked at.
//!
//! **The develop panel is generated, not written.** [`develop`] asks the
//! pipeline what parameters it has and builds a control per answer; no code
@@ -576,6 +581,64 @@ enum PointsUpdate {
Unchanged,
}
/// TRACES: FR-DSP-1 | AC-8
/// Open the one wgpu device the compute passes and the compositor share.
///
/// **This is the whole of the zero-copy display path, and it is four lines of
/// configuration.** A `wgpu::Texture` belongs to the device that allocated it;
/// handing one to a compositor drawing on a *different* device is meaningless,
/// and the two would have to meet through system memory — which is the round
/// trip ARCH §6.1 forbids. So there is exactly one device, made here, before
/// anything else needs it.
///
/// **Called before the window exists, and it must be.** `BackendSelector`
/// installs the Slint platform, and Slint installs a default one the first
/// time a window is created; selecting afterwards is too late. That is why the
/// GPU is opened at the top of [`run`] rather than beside the other
/// controllers, where it used to sit.
///
/// `None` means develop is unavailable and the viewer falls back to embedded
/// previews — the same degradation as a machine with no adapter at all.
fn shared_gpu() -> Option<dr_gpu::GpuContext> {
let shared = match pollster::block_on(dr_gpu::GpuContext::new_shared()) {
Ok(shared) => shared,
Err(e) => {
log::warn!("no shareable GPU: {e}");
return None;
}
};
let dr_gpu::SharedGpu {
ctx,
instance,
adapter,
} = shared;
// `Manual` is the variant that means "render with these, do not open your
// own". The two clones are of wgpu handles, which are refcounts over the
// one device and the one queue — not copies of either.
let configuration = slint::wgpu_29::WGPUConfiguration::Manual {
instance,
adapter,
device: (*ctx.device).clone(),
queue: (*ctx.queue).clone(),
};
if let Err(e) = slint::BackendSelector::new()
.require_wgpu_29(configuration)
.select()
{
// Dropping the context rather than keeping it: Slint has fallen back
// to a renderer that did not adopt our device, so every texture this
// context produces is one the compositor cannot sample. A disabled
// develop panel is a visible, explicable failure; a texture handed
// across devices is undefined behaviour on a good day.
log::warn!("Slint would not adopt the GPU device, develop disabled: {e}");
return None;
}
Some(ctx)
}
/// TRACES: M-13 | M-14
/// Build and run the viewer.
pub fn run(paths: Vec<PathBuf>) -> Result<()> {
@@ -587,7 +650,21 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
let entries = Rc::new(RefCell::new(collect(&paths)));
log::info!("{} image(s) to browse", entries.borrow().len());
// Before the window, and it has to be: this selects the Slint backend, and
// creating a window selects one for us. See `shared_gpu`. The device is
// shared by demosaic, the adjust pass and the compositor; without one the
// app still browses through the preview path, just without develop.
let gpu = shared_gpu();
let window = AppWindow::new()?;
match &gpu {
Some(ctx) => {
log::info!("adapter: {} ({:?})", ctx.adapter_name(), ctx.backend());
window.set_adapter(ctx.adapter_name().into());
window.set_backend(format!("{:?}", ctx.backend()).to_uppercase().into());
}
None => window.set_backend("NO GPU".into()),
}
// Every background job reports here, and this draws the bar across the top
// of the shell and fills the settings page's list. Built before the
@@ -913,22 +990,6 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
});
}
// The device is shared by demosaic and the adjust pass. Without one the
// app still browses through the preview path, just without develop.
let gpu = match pollster::block_on(dr_gpu::GpuContext::new_headless()) {
Ok(ctx) => {
log::info!("adapter: {} ({:?})", ctx.adapter_name(), ctx.backend());
window.set_adapter(ctx.adapter_name().into());
window.set_backend(format!("{:?}", ctx.backend()).to_uppercase().into());
Some(ctx)
}
Err(e) => {
log::warn!("no GPU adapter: {e}");
window.set_backend("NO GPU".into());
None
}
};
window.set_total(entries.borrow().len() as i32);
let index = Rc::new(RefCell::new(0usize));
// The current develop session, if the file yielded sensor data.
@@ -965,9 +1026,11 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
// **Half resolution while the gesture is still moving.**
//
// The adjust pass and the readback both scale with pixel count, so
// halving each edge is roughly a quarter of the work — the
// difference between keeping up with a drag and lagging behind it.
// The adjust pass scales with pixel count, so halving each edge is
// roughly a quarter of the work — the difference between keeping
// up with a drag and lagging behind it. Less dramatic since S1
// removed the readback that scaled the same way and cost far more,
// but a dispatch is still not free at 4K.
// A draft frame is visible for one gesture and is replaced by a
// full-resolution one the moment motion stops, so the cost is a
// little softness exactly while the image is moving too fast to
@@ -1013,7 +1076,8 @@ pub fn run(paths: Vec<PathBuf>) -> Result<()> {
// **Rendering is decoupled from input, and this is why.**
//
// A render is a blocking GPU round-trip (see `AdjustPass::read_output`).
// A render used to be a blocking GPU round-trip — S1 removed the block,
// but not the reason for this, so read it as history that still applies.
// Running one straight from a `moved` handler put that stall *inside* the
// gesture: touch events arrive far faster than a render completes, so the
// input queue backed up, positions arrived stale, and Android — seeing the