dtourolleandClaude Opus 5
c396a22dfd
The last line of FR-DEV-3, and the mask ARCH §5.4 was written for. darktable rasterises drawn masks on the CPU and users call the result unworkable; the architecture's answer is that a stroke arrives as *parameters* and the device draws it. This is that, from the model through the sidecar to the pixels — but not the finger: the canvas is somebody else's change, and this leaves it a seam rather than reaching into it. **A stroke is a swept disc along a polyline**, plus erase, radius, hardness and flow. `MaskSource::Brush` holds an ordered list of them, and the order is the mask: an erase after an add takes it away and the same pair reversed does not. Nothing about it is pixels, which is what makes a mask that costs a line of text, diffs by the gesture, and survives a crop, a straighten and an export at any size — the properties a stored raster has none of, and the same argument the region ids were chosen for. Two things keep the point count honest. While the finger is down, a position closer to the last than an eighth of the radius is dropped: a touch screen reports 120 a second, so a finger held still for five seconds is six hundred points in the same place, and simplification would only remove them once the gesture had ended — after every frame in between had drawn all of them. When it ends, Douglas–Peucker at an eighth of the radius removes what a disc that wide cannot express: a swept circle moved by r/8 moves its own edge by r/8, which is inside the soft part of any brush. Coordinates snap to a ten-thousandth of the frame on the way in *and* are written at that precision, so a round trip is exact rather than nearly exact — a file that drifts in the sixth decimal every save is a per-field merge conflict a day, over nothing. **Cost is why the strokes are not drawn by the full-screen triangle the other masks use.** A swept disc is the minimum distance to any of its segments, so a stroke over the whole frame costs `pixels × segments` and both terms grow together — the quadratic that is darktable's problem moved onto the GPU rather than solved. Each stroke is instead drawn over its own bounding box, grown by the radius, so the rasteriser never invokes the shader for a pixel the stroke cannot reach: `area(box) × segments`, which for a dab or a swipe is a small fraction of the frame. A gesture past 256 points continues as a second stroke for the same reason, since a shorter stroke has a smaller box. Add and erase are `dst + a(1 - dst)` and `dst(1 - a)`, which are exactly a source-over and a one-minus-source blend — so they are blend state, not arithmetic, and no pass ever reads the slice it is writing. That is what permits one draw per stroke at all. Within a stroke the coverage is the *minimum* distance over its segments rather than a sum: a path that crosses itself must not build up where it did, or every circle and every scribble would be blotchy wherever consecutive dabs overlap, which is everywhere. Not a distance field, deliberately. `dr-segment`'s transform documents the two conditions that make CPU work right there — once per mask edit, over input already CPU-side — and a stroke fails both: it changes while the finger moves, and its input is a handful of coordinates that never needed to be pixels. It also needs no transform, because the distance to a swept disc is closed form. A stroke is the one mask whose distance field is known without computing one. An unpainted brush layer is inactive rather than empty, which is not an optimisation: `invert` turns empty into everything, so a layer created with invert already set would apply its adjustment to the whole photograph before a single stroke was made. That is the loud, confident kind of wrong this codebase refuses everywhere else a mask can go missing, and there is a rendered test for it. The tests read pixels back off a device rather than checking that the two halves agree with each other. What they pin down is what is silent when wrong: the y flip between mask space and clip space, which a centred stroke would not notice; a bounding box not grown by the radius, which makes a tap draw nothing at all; an aspect ratio ignored, which makes a dab an ellipse on any frame that is not square; a stroke doubling back and building up; and an erase that lost its place in the order and put back paint the user had taken off. Not done here: the interaction. The canvas needs to begin, extend and end a stroke on the active layer, and `DevelopSession::rasterise_masks` still returns early without a segmentation — it takes the proxy size from one, and a brush needs no model to have run over the photograph first. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
DarkRoom
A cross-platform, non-destructive RAW photo editor for Linux and Android.
Status: early. v0.1 is a remote library viewer — see docs/milestone-v0.1.md.
Documentation
| Document | Contents |
|---|---|
| requirements.md | What the software must do — 122 numbered requirements |
| architecture.md | How it is built — crates, GPU pipeline, data model, sync |
| milestone-v0.1.md | The first buildable milestone |
Building
Desktop:
cargo run -p darkroom-desktop
Android (containerised toolchain, see docker/android):
./docker/android/build.sh cargo ndk -t arm64-v8a build --release
Current state
Working: workspace, GPU context and compute pass, adaptive Slint shell, Android cross-compilation of the core crates.
Not yet working: the zero-copy display path. The build currently uploads frames through the CPU, which is exactly what ARCH §6.1 forbids — measured at 96% of frame time at 4K. Replacing it is spike S1, the project's highest priority.
cargo run -p dr-gpu --example bench --features readback
reproduces that measurement.
Licence
GPL-3.0-or-later.