Monitors are not perfectly calibrated, so if you send one a raw sRBG image, the result will look subtly wrong. To fix this we have monitor profiles (mappings from sRGB colors to the values needed to accurately display said colors on a particular monitor) and CLUTs (color look-up table, a table that the graphics chipset uses to perform said transformation on the fly as it synthesizes the video signal. It was implemented in analog video signals with RAMDACs, now with digital signals we don't need the "DAC" part, but you get the idea).What f.lux does is (ab)use the CLUT to change the color temperature of the video signal sent to your monitor, cheaply and without requiring any modification to existing software. This is great if you trust f.lux, however clearly Apple cannot trust random apps with persistent access to the CLUT, because people could use it to do things like, say, completely blacken the screen.
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