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GNOME Camera (gnome-snapshot) with a document scanning mode

patches/0001-document-scan-mode.patch adds a fourth capture mode next to picture, video and QR codes:

  • the camera streams at full resolution in it (3200x2400 on the Surface Pro 7 rear camera instead of 1080p)
  • while framing, the page is outlined live over the preview (aperture hands the app a 320 px copy of the feed 5 times a second, read in passing so the preview stays zero-copy and sharp; the same detection runs on it in a worker thread)
  • after each shot the page is found (src/document.rs): grown out from the brightest paper across gradual changes, so a shadow over the page (often the tablet's own) is crossed while the page's edge stops it, and the corners moved to where the outline covers the most page; failing that, the brightest area at several thresholds, the paper's colour at any brightness, or colours rare along the border
  • a dialog (src/scan_adjust.rs) shows the outline with draggable corners; a page lying on other white paper has no edge to find, so this is where it gets fixed
  • Save warps the page upright and, with the filter on (Off / Filter / B&W in the dialog, remembered), evens the paper out to neutral white, lifting shadows (and cleaning harder where it had to lift); the scan lands next to the photo as … (scan).jpeg
  • the focus button sits next to the shutter (it and a tap on the picture make the surface-kit libcamera autofocus sweep again)

Plain Rust and cairo, no new crates; builds offline from Ubuntu's vendored crates.

Build and install

sudo apt build-dep gnome-snapshot     # once
snapshot/build-deb.sh                 # prints the apt install command

An Ubuntu update to gnome-snapshot replaces it; re-run the script.

Working on the page detection

doctest/ builds just document.rs (from the unpacked tree in vendor/snapshot-src/gnome-snapshot-50.0, see apt-get source gnome-snapshot) into a command-line tool, in seconds instead of minutes:

cd snapshot/doctest && cargo build --release --offline
DOC_DEBUG=1 target/release/doctest photo.jpg ... > run.log   # writes "(scan)" files
python3 sheet.py run.log sheet.jpg     # photos with the outline, next to the scans
DOC_NOSAVE=1 target/release/doctest photo.jpg ... > run.log   # outlines only
python3 outlines.py run.log out.jpg    # just the outlines, 4 a row
target/release/doctest frame_320x240.rgb   # time the live detection on a raw frame

The patch is regenerated from the working tree with snapshot/make-patch.sh (changed files against vendor/snapshot-src/orig).

Sample photos used for tuning: camera/captures/doctest*/ (not in git; doctest3 has the shadowed binder pages).