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1) LibRaw uses Olympus

1) LibRaw uses Olympus SensorCalibration tag (1st value) as linear_max. It looks like this is not valid for specifically XZ-1 (XZ-2 and XZ-10 are ok). We'll issue some fix for that, thank you for the problem report.

2) For very dark (near-black, or just black w/ lens caps on) areas it is expected that some values are below black+cblack. For completely black shots (lens caps on), if no processing was performed (in camera) about half of pixels will be below that threshold.

3) linear_max is vendor specified (if any) 'specular white'

maximum is either guessed from format bit count (or bit count + linearization curve), or hardcoded (and may be adjusted with data_maximum is not turned off via params.adjust_maximum_thr).
maximum also may be adjusted on processing stage if LibRaw's exposure correction is used.

data_maximum is real data maximum calculated on current frame data.

There is no universal answer on 'what maximum should I use', it very depends on application targets.

-- Alex Tutubalin