I'm trying to process a sequence of images using the same white balance correction. I need daylight white balance so I thought that I just read out pre_mul (docs say "White balance coefficients for daylight (daylight balance). Either read from file, or calculated on the basis of file data, or taken from hardcoded constants.") and then use that for all images. However I found an odd behaviour. When I do open_file and unpack, then pre_mul is [1.912400484085083, 0.9313587546348572, 1.0938434600830078, 0.0]. When I then call dcraw_process, pre_mul changed to [2.0533447265625, 1.0, 1.1744598150253296, 1.0] which seems more like the correct values. What is happening here?