Expose white balance coefficients applied during postprocessing

[Copilot]

LibRaw stores the actual white balance multipliers applied during processing in imgdata.color.pre_mul[], but rawpy only exposed pre-processing coefficients (camera_whitebalance, daylight_whitebalance). This makes it impossible to retrieve the coefficients used when use_auto_wb=True or verify what was actually applied.

Changes

• New property auto_whitebalance: Returns the 4-channel WB coefficients from imgdata.color.pre_mul[] after postprocessing

  • Returns None before postprocess() or dcraw_process() is called
  • Works with all WB modes: auto, camera, user-specified, and daylight

• Added dcraw_process_called flag: Tracks processing state to ensure property only returns meaningful values

Usage

import rawpy

with rawpy.imread('image.nef') as raw:
    print(raw.auto_whitebalance)  # None - no processing yet
    
    rgb = raw.postprocess(use_auto_wb=True)
    print(raw.auto_whitebalance)  # [2.34, 1.00, 1.82, 1.0] - actual coefficients used

The property name follows the issue request for "auto white balance coefficients", though it returns the applied coefficients regardless of WB mode.

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Get auto white balance coefficients</issue_title>
<issue_description>Any chance you could add a way to get the auto white balance coefficients after postprocessing? Based on this forum post, it looks like LibRaw supports this with "imgdata.color.pre_mul[] coeffs."

Note that the related issues #12 and #221 are about generating wb coefficients from a gray box (i.e. the -A flag in dcraw). I'm asking about entire-image wb (-a). Also, the above forum post came a year after this was mentioned in #12, so the LibRaw function might be newer.

Also, if it's possible to calculate the wb coefficients without fully postprocessing (e.g. via a separate function), that'd be even better for my use case.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Get auto white balance coefficients #250

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