A script to fix EDID problems on external monitors in macOS

patch-edid.md

patch-edid.rb

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

Instructions

1. Connect only the problem display.

2. Create this directory structure (if it doesn't already exist):

   sudo mkdir -p /Library/Displays/Contents/Resources/Overrides

3. 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.

4. 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

5. 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.

patch-edid.rb

#!/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

@xavierchen0 yes, you were trying to get (pseudo) HiDPI on standard monitors in order for the macOS interface to look a bit better. They don't seem to optimize the UI for the 1080p monitors anymore. Even the cursor line when typing can cover come characters since it's too thick/bold.

Anyway, the resolution should still show double in the system information and the scale should be set to 2x.

Here is a detailed guide how to enable HiDPI mode on standard monitors: https://gist.github.com/GetVladimir/c89a26df1806001543bef4c8d90cc2f8?permalink_comment_id=4207112#gistcomment-4207112

Please note that it won't look nearly as good as it would look on a real 4K display with 2x scaling