Sampling the overcast sky on a Canon 6D frame set tint to -100 and
temperature to -15 for a patch the canvas showed as pure white. A clipped
photosite is sensor white, not a colour: every channel stopped counting,
so what the tap hands back is the as-shot multipliers themselves, which
are strongly magenta, and the solver dutifully drove green to its stop.
The display shader already fades such a pixel to a neutral of the same
brightness before any operation runs, so the picker was balancing against
something the photographer could not see.
The probe now refuses a sample with any channel at or above the onset the
shader fades from, the way the solver already refuses black. The threshold
is one constant, CLIP_ONSET, formatted into the shader and read by the
probe, so the two cannot drift apart.
Pressing "pick" and clicking a near-neutral wall on a Canon 6D frame set
tint to -77 and turned the whole photograph green. The white balance
operation runs first in the chain, on camera RGB, before the body's base
curve and colour matrix; the probe was read off a display render after
all three, and the solve treated that sRGB triple as if the gains
multiplied it directly. On a JPEG the two spaces coincide, which is why
the existing tests passed while the picker was broken on every raw file.
The probe now reads the camera-space tap a merge stitches from, composed
under the edit's own framing so a fraction of the canvas is a fraction of
the probe, and puts the as-shot balance on itself - exactly the value the
operation's gains are about to multiply. No operations run in the tap, so
nothing has to be stripped and restored, and the display target is left
alone, so a sample that found nothing usable no longer needs a redraw.
A raw-frame test with the 6D's matrix and a typical as-shot balance
samples a warm grey and asserts the rendered pixel comes back neutral; it
fails on the previous probe.
FR-DEV-3 has asked for "white balance (temperature/tint, and picker)" since
it was written, and only the first half existed. `WidgetKind::WhitePoint` was
in the vocabulary and `develop::supported` answered false for it, so the node
degraded to two sliders — correct behaviour that had quietly become the only
behaviour. Sampling a neutral is the first move of the global tonal pass and
every colour judgement afterwards is measured against where the grey was put,
so guessing at two sliders until a wall stops looking green is the wrong way
round.
The awkward part is that a picker genuinely needs to know how far a hundred
units of temperature move red against blue, and that number is declared in
the node's own file. So the inversion lives in `dr_pipeline::neutral` rather
than in the interface: the canvas hands over a colour, the core finds the
operation that asked to be driven by a pixel and bisects its declared
response until the sample comes back grey. Nothing in `ui/` names white
balance, and nothing holds a second copy of a response that would be wrong
the first time somebody adjusted the range. A bisection rather than a
closed-form inverse because only monotonicity is part of the bargain — the
expression is free to become a table tomorrow.
The result is rounded to the precision the control is drawn at, which is not
cosmetic: unrounded, sampling something already neutral lands a
ten-thousandth off zero, and the photograph comes back modified with an undo
step for a correction of nothing.
On the panel side this needed one distinction the generated path was
missing. `is_on_canvas` was being read as "and so the panel draws nothing for
it", which is right for a crop — four edge fractions are not controls anyone
drags in a list — and wrong for an eyedropper, which *writes* temperature and
tint and leaves them exactly the controls a photographer reaches for next.
So a sampling widget keeps its sliders and puts the affordance that arms the
canvas in the group's heading, built like the reset beside it. One click, one
sample, one history step: `Edit::Action` never coalesces, and there is no
hover preview to fill the stack with temperatures nobody chose.
Declaring the presentation also groups temperature and tint under one undo
step, where they were two. That follows from what `Presentation` means and
reads correctly — white balance is one decision — but it is a change, and
worth saying so.
dtourolle