Fixing the External Monitor Color Problem with My 2018 MacBook Pro

I not too long ago upgraded to a 2018 MacBook Professional. I’m having fun with the extra cores and reminiscence (😀) and coming to phrases with the touchbar (🤨). A low level of my first week got here after I plugged into my exterior show, and the picture seemed actually dangerous. On this put up, I’ll describe what I realized and the way I managed to repair it.

The Symptom

I’ve been utilizing a 27″ Dell at work for a pair years, linked to my Mac by way of DisplayPort. It’s a fairly good show with a shiny IPS panel, good colours, and huge viewing angles. Once I plugged it into my new Mac, the textual content seemed horrible. It’s tough to {photograph}, however it seemed one thing like this:

There have been unusual coloration fringes on the perimeters of characters. Some slim strokes appeared as blue or pink blurs.

The impact diversified with typeface and utility, however it was universally distracting and tough to learn.

What’s Going On?

I’m conscious of contemporary working methods’ use of subpixel rendering, individually manipulating the pink, inexperienced, and blue elements of a pixel to enhance textual content rasterization. My first hunch was that one thing was going incorrect with this. Disabling it by way of LCD Font Smoothing helped considerably, however the general picture was nonetheless noticeably worse than I had skilled earlier than.

To verify I wasn’t loopy, I plugged in my previous pc, and positive sufficient, it seemed nice! Evaluating additional, I couldn’t discover any significant variations reported in software program. Ultimately, I seen one thing within the monitor’s settings: My new Mac was connecting with an Enter Coloration Format of YPbPr, whereas my previous Mac used RGB.

I wasn’t positive whether or not this mattered till I began googling and located a number of different cases–going again a number of years–of customers reporting poor picture high quality related to this coloration format throughout numerous Macs and exterior shows.

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YPWhat?

When you’ve labored with colours on computer systems earlier than, you’re in all probability accustomed to representing colours as pink, inexperienced, and blue elements (“RGB”). When you’ve refilled ink on a printer, you will have encountered one other decomposition: cyan/magenta/yellow/black (“CMYK”).

YCbCr/YPbPr is one other coloration area, which represents colours with a black channel (“luminance”) and two totally different coloration channels (“chrominance”).

The one place I’ve encountered this one earlier than is in element video, which is (confusingly) a trio of pink, inexperienced, and blue RCA-style connectors that was used for high-end residence theater earlier than HDMI was a factor.

How Do I Repair It?

Sadly, I can’t simply change the setting within the monitor. The menu possibility solely appears to have an effect on how the monitor interprets the sign it’s already receiving, quite than how the sign is negotiated within the first place.

It’s additionally not attainable to decide on the colour area in software program on Mac OS.

…Or is it?

EDIDs

When a show is first linked, it identifies itself and describes its capabilities by offering a blob of bits known as the Prolonged Show Identification Knowledge (EDID). The Wikipedia web page helpfully supplies the total spec.

There are a few bits in the course of byte 24 that specify the accepted coloration codecs:

00 = RGB 4:4:4 01 = RGB 4:4:4 + YCrCb 4:4:4 10 = RGB 4:4:4 + YCrCb 4:2:2 11 = RGB 4:4:4 + YCrCb 4:4:4 + YCrCb 4:2:2

My Dell emits 11, that means that it claims to assist all three coloration modes. Excited to have met a language associate, my new Mac insists on conversing in YCrCb, as an alternative of the the show’s native tongue of RGB. (I’m unsure whether or not it’s utilizing 4:4:Four or the chroma subsampled 4:2:2)

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I haven’t discovered a solution to alter this determination. Given my Dell’s EDID, macOS will at all times select YCrCb.

Happily, macOS has a built-in mechanism to override the EDID knowledge offered by a show. When you take a look at /System/Library/Shows/Contents/Sources/Overrides/, you’ll discover a bunch of override recordsdata for explicit shows, going manner again to previous beige CRTs just like the AppleVision 1710 (from 1995!).

So, the answer we’re approaching is to override the Dell’s configuration, to deceive the working system and declare it’s solely capable of converse RGB. To try this, we’ll have to flip a number of bits!

Patching the EDID

Happily, anyone else has already solved this. We’ll have to learn the show’s EDID, patch it, write a brand new override file, after which put that file in a protected system listing.

Right here’s how I did it (primarily based on this weblog remark):

  1. Join the problematic show.
  2. Obtain and run this script (./patch-edid.rb).
  3. Check out the generated file and keep in mind its path (mine was ~/Desktop/DisplayVendorID-10ac/DisplayProductID-4080 ).
  4. Reboot into restoration mode (⌘-R at boot).
  5. Mount your principal disk (enter Disk Utility, choose the grey Macintosh HD, click on Mount, present your password, shut Disk Utility).
  6. Open up a terminal (Utilities -> Terminal), backup the overrides listing, copy in your new override file, and reboot:
    cd /Volumes/Macintosh HD/System/Library/Shows/Contents/Sources cp -R Overrides Overrides.bak cp -R /Volumes/Macintosh HD/Customers/jrr/Desktop/DisplayVendorID-10ac Overrides/ reboot

Voila! Our override seems in System Profiler, the Dell connects with RGB, and the picture high quality is dramatically improved.

Closing Ideas

I realized far more about how this works than I'd have ever anticipated, however I nonetheless don’t absolutely perceive precisely what’s happening. Maybe Dell’s implementation of YCbCr is poor, and it shouldn’t be promoting compatibility within the first place. Perhaps I shouldn’t count on to simply use a monitor that doesn’t particularly assist macOS with a vendor-provided driver.

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Regardless, this expertise uncovered a gap in macOS’s assist for exterior shows:

  • Why does macOS favor YCbCr over RGB?
  • Why isn’t management over this uncovered to the person? It shouldn’t be essential to edit protected system recordsdata to manage this habits!

For now, this challenge appears fairly uncommon (I used to be unable to breed it with colleagues’ 2017 MBPs), however it does appear prone to have an effect on many upgrading customers over the following couple of years. Hopefully, this hack helps a number of of them preserve utilizing their previous displays!

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