One-tap Auto WB grab for Kelvin mode

[erik-sutton95]

Goal

Add a one-tap Auto WB Grab inside OpenZCine's Kelvin white-balance controls. The button should evaluate the current scene, capture a useful automatic white-balance approximation, and lock the camera back into explicit Kelvin mode for a stable take.

Why it matters

Operators often want the speed of Auto WB for finding a starting point but do not want white balance drifting during a shot. Today that means switching modes, judging the result, and manually recreating it in Kelvin mode. A grab button would make Auto WB useful as a meter while preserving the predictability of manual Kelvin white balance.

Proposed experience

• Place a clearly labelled Grab Auto WB action within the Kelvin tab or picker.
• Let the operator evaluate the full scene or an optional selected sample region.
• Prefer a camera-authoritative Auto WB temperature and tint estimate when the connected Nikon body exposes one.
• If the camera does not expose a live estimate, investigate an explicitly labelled on-device approximation from the clean live-view image.
• Show the proposed Kelvin value, tint where available, and an honest confidence or limitation message before or as it is applied.
• Write white-balance mode, Kelvin, and tint through the camera's advertised controls in the required safe order, then confirm the result through authoritative readback.
• Keep a one-step undo to restore the previous Kelvin and tint values.
• Never leave the camera in Auto WB unexpectedly, and never claim that an estimate was accepted without camera confirmation.
• Disable or constrain the action during recording if the camera cannot perform it safely.

Success looks like

While setting up a Nikon Z shot, an operator can open Kelvin mode, tap Grab Auto WB, and quickly receive a sensible scene-based starting value that remains locked and stable for recording.

Open questions

• Does the Nikon PTP/MTP interface expose the camera's current Auto WB result, or only the selected Auto mode?
• Can a camera-authoritative value be obtained without taking a still or visibly disrupting live view?
• Should the first version capture Kelvin only, or Kelvin plus green/magenta tint?
• How should an on-device estimate account for N-Log, HLG, Picture Controls, LUT preview, mixed lighting, and scenes without a neutral reference?
• Should an optional gray-card mode invoke Nikon preset-manual measurement as a more accurate alternative?

Nikon reference

• Nikon white balance, Auto reference values, Kelvin selection, and preset-manual measurement