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A script to fix EDID problems on external monitors in macOS

patch-edid.rb

A script to fix EDID problems on external monitors in macOS.

Instructions

Connect only the problem display.
Create this directory structure (if it doesn't already exist):
```
sudo mkdir -p /Library/Displays/Contents/Resources/Overrides
```

Download this ruby script in that directory:

cd /Library/Displays/Contents/Resources/Overrides
sudo curl -O https://gist.githubusercontent.com/ejdyksen/8302862/raw/patch-edid.rb

Note: You may want to use adaugherity's version of the script instead.

Run the script we just downloaded (as root again). This creates a new display override plist file.
```
cd /Library/Displays/Contents/Resources/Overrides
sudo ruby patch-edid.rb
```
Unplug and replug in the problem display.

Additional reading/acknowledgements

The original forum thread
An improved version of the script by adaugherity
An explaination of the problem from Atomic Object's blog
Thanks so much to @stackrainbow for pointing out that this can be done without disabling SIP.
This version appears to work in Catalina and Big Sur. See earlier revisions for what worked (with disabling SIP) in earlier versions of macOS, which require the override plist to be in a different directory.

	#!/usr/bin/ruby
	# Create display override file to force Mac OS X to use RGB mode for Display
	# see http://embdev.net/topic/284710
	#
	# Update 2013-06-24: added -w0 option to prevent truncated lines

	require 'base64'

	data=`ioreg -l -w0 -d0 -r -c AppleDisplay`

	edid_hex=data.match(/IODisplayEDID.*?<([a-z0-9]+)>/i)[1]
	vendorid=data.match(/DisplayVendorID.*?([0-9]+)/i)[1].to_i
	productid=data.match(/DisplayProductID.*?([0-9]+)/i)[1].to_i

	puts "found display: vendorid #{vendorid}, productid #{productid}, EDID:\n#{edid_hex}"

	bytes=edid_hex.scan(/../).map{\|x\|Integer("0x#{x}")}.flatten

	puts "Setting color support to RGB 4:4:4 only"
	bytes[24] &= ~(0b11000)

	puts "Number of extension blocks: #{bytes[126]}"
	puts "removing extension block"
	bytes = bytes[0..127]
	bytes[126] = 0

	bytes[127] = (0x100-(bytes[0..126].reduce(:+) % 256)) % 256
	puts
	puts "Recalculated checksum: 0x%x" % bytes[127]
	puts "new EDID:\n#{bytes.map{\|b\|"%02X"%b}.join}"

	Dir.mkdir("DisplayVendorID-%x" % vendorid) rescue nil
	f = File.open("DisplayVendorID-%x/DisplayProductID-%x" % [vendorid, productid], 'w')
	f.write '<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
	<plist version="1.0">'
	f.write "
	<dict>
	<key>DisplayProductName</key>
	<string>Display with forced RGB mode (EDID override)</string>
	<key>IODisplayEDID</key>
	<data>#{Base64.encode64(bytes.pack('C*'))}</data>
	<key>DisplayVendorID</key>
	<integer>#{vendorid}</integer>
	<key>DisplayProductID</key>
	<integer>#{productid}</integer>
	</dict>
	</plist>"
	f.close