Using the -b option to adjust the brightness of the output image increases the color saturation for any brightness > 1. This is particularly noticeable for large brightness values (e.g. 16) for which the colors start looking entirely unrealistic. A benchmark comparison can be done by doing the exposure adjustment in Photoshop (Camera Raw) or Lightroom instead. Is the underlying algorithm flawed or does the -b option need to get used in conjunction with some other setting ?
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imgdata.params.bright options
imgdata.params.bright options affects only output (gamma) curve that used on (internal) linear data -> gamma corrected output. Is is very similar to Photoshop curve tool w/ linear curve that starts at 0,0 and ends, for example, at 100,255 (source, dest)
To adjust exposure one may use exp_correc/exp_shift/exp_preserve parameters.
-- Alex Tutubalin @LibRaw LLC