The AUTO chip measured the photo and wrote one knob, EV. It now writes the four
the measurement actually names, off the same binned ramp (ui/Histogram.tsx),
which is why it is one chip and not four: the means it needs are all in the
histogram the exposure answer already reads.
EV the mean luma, unchanged
HIGHLIGHT the top 1% (p99 > 0.9 pulls back), to -5 of the ruler at most
SHADOW the bottom 1% (p01 < 0.02 opens up), to +5
TEMPERATURE/TINT the gain that puts the three channel means on each other,
green as the anchor: gray-world on linearised means, then the
closest of 76 temperatures x 21 tints under the renderer's own
kelvinToRGB, so the pair cannot drift from what the ruler applies.
The ends rather than the average is what keeps a small blown window from
dragging the whole frame: a specular in the corner wants HIGHLIGHT, not a
flatter picture everywhere. Both ends stop at half the ruler, so the frame is
corrected and a hand can still finish the move; the knobs then report the
numbers AUTO chose, the way the EV knob does.
Each reading is a pure function of the ramp, so pressing AUTO twice lands on the
same recipe by construction, and a frame with a dead channel leaves the WB ruler
where it is rather than inventing a cast. ponytail: one linear ramp per end and
no scene analysis; add a curve, or weight by how much of the frame is clipped,
when AUTO starts overshooting a scene with a genuine specular in it.
scripts/auto-tone-check.mjs holds the three readings: the percentile walk, the
thresholds that leave a knob alone, and the scan landing back on the gain it was
asked for.
