It seems that this problem has been reported by a lot of people for a long time, and I expect it would require a fairly simple software update. My last word is to wonder what Apple is playing at.

I’ve summarised the required steps below.

It was written to fix problems connecting an external monitor via DisplayPort, but it fixed my HDMI issue just the same. Your thread and script was incredibly helpful and informative. It didn’t appear to be available out-of-the-box, but I have had some experience in overriding EDID settings for similar purposes so I searched instead for that. I was sure that it was using RGB when hooked up to my PC, so I started searching instead for forcing RGB mode in OS X. My lucky break was reading that the current colour space was “YCbCr” when I was browsing the monitor’s settings menu. (I realise that owning a Mac makes me an Apple person, too. “You just haven’t calibrated it!”, “You are just too used to Retina now!”, “You just need to buy a Thunderbolt display!” Apple people also like to solve problems by throwing more money at it. The most common response was to blame the monitor, despite assurances from the stricken users that the monitor worked beautifully in Linux and Windows, even on the same machine under Boot Camp. Googling for problems with external monitors on MacBooks found dozens of threads on official and unofficial Apple forums, all full of people with the same problem. Long answer: I haven’t owned a Mac for a while and had forgotten how difficult much of the “Apple community” can be when it comes to anything that can’t be adjusted in System Preferences. I had to override an EDID setting to force the RGB colour space, and it is now working correctly. Short answer: OS X thinks my monitor is a TV, and is using the YCbCr colour space rather than RGB. No amount of calibration in the monitor or software would fix it. The contrast was all wrong and text was misshapen. I recently bought a MacBook Pro (with ‘Retina’ screen), but when I hooked it up to my Dell U2410 monitor via HDMI cable I was shocked by the poor picture quality. Force RGB Mode in Mac OS X to Fix the Picture Quality of an External Monitor

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