The local panel sits above the adjust panel because it decides what those
sliders act on; below it, a photographer would set an exposure and only
then discover which scope it landed in. Selecting a layer re-scopes the
existing controls to that layer's chain — there is no second set of
sliders, and there must not be, or every operation added to `ops/` would
need a local twin.
The overlay is drawn over the canvas rather than blended into the render,
because it is a diagnostic and not an edit: it must not reach the
histogram, an export, or the texture handed to the compositor. Nearest-
neighbour always — the map's values are *names*, so smoothing between
region 4 and region 9 invents a colour belonging to neither and softens
exactly the edge the overlay exists to show.
Picking gets its own touch area above the pan handler. Panning wants
press-drag-release and picking wants a click; interleaving them in one
handler is how a drag ends up selecting a region the user was scrolling
past. Shift is tracked as window state because a TouchArea's click carries
no modifiers.
Three states a layer can be in are worth distinguishing, and each has a
different remedy: stale needs re-segmenting, "no adjustment yet" needs a
slider moved, and the ordinary case needs nothing said. A bare selection
renders nothing and looks identical to a broken mask, which is the first
thing a new user will hit.
Known rough edge, commented where it happens: segmentation blocks the UI
thread for about half a second. Moving it to a worker needs the develop
session — GPU resources behind a RefCell shared with every callback — to
be reachable from another thread, which is a restructuring rather than a
change to the call. The button says "Finding regions…" first so the stall
is announced rather than looking like a hang.
dtourolle
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