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

require 'base64'

data=`ioreg -l -d0 -w 0 -r -c AppleDisplay`

edids=data.scan(/IODisplayEDID.*?<([a-z0-9]+)>/i).flatten
vendorids=data.scan(/DisplayVendorID.*?([0-9]+)/i).flatten
productids=data.scan(/DisplayProductID.*?([0-9]+)/i).flatten

displays = []
edids.each_with_index do |edid, i|
    disp = { "edid_hex"=>edid, "vendorid"=>vendorids[i].to_i, "productid"=>productids[i].to_i }
    displays.push(disp)
end

# Process all displays
if displays.length > 1
    puts "Found %d displays!  You should only install the override file for the one which" % displays.length
    puts "is giving you problems.","\n"
elsif displays.length == 0
    puts "No display data found!  Are any external displays connected?"
end
displays.each do |disp|
    # Retrieve monitor model from EDID display descriptor
    i = disp["edid_hex"].index('000000fc00')
    if i.nil?
        monitor_name = "Display"
    else
        # The monitor name is stored in (up to) 13 bytes of text following 00 00 00 fc 00.
        # If the name is shorter than 13 bytes, it is terminated with a newline (0a) and then padded with spaces.
        monitor_name = [disp["edid_hex"][i + 10, 26].to_s].pack("H*")
        monitor_name.rstrip!            # remove trailing newline/spaces
    end

    puts "Found display '#{monitor_name}': vendor ID=#{disp["vendorid"]} (0x%x), product ID=#{disp["productid"]} (0x%x)" %
            [disp["vendorid"], disp["productid"]]
    puts "Raw EDID data:\n#{disp["edid_hex"]}"

    bytes=disp["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" % disp["vendorid"]) rescue nil
    filename = "DisplayVendorID-%x/DisplayProductID-%x" % [disp["vendorid"], disp["productid"]]
    puts "Output file: #{Dir.pwd}/#{filename}"
    f = File.open(filename, '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>#{monitor_name} - forced RGB mode (EDID override)</string>
  <key>IODisplayEDID</key>
  <data>#{Base64.encode64(bytes.pack('C*'))}</data>
  <key>DisplayVendorID</key>
  <integer>#{disp["vendorid"]}</integer>
  <key>DisplayProductID</key>
  <integer>#{disp["productid"]}</integer>
</dict>
</plist>"
    f.close
    puts "\n"
end             # displays.each

A fix for weird behaviour between MacOS and some monitors. If a picture is produced, and resolutions etc all appear correct, yet text is fuzzy - this might solve the problem.

Worked for me on OSX 10.15.1.

This fix will essentially force the Mac to output an RGB colourspace to the monitor instead of the YPbPr one, which is generally used for TVs.

1. Run the above script.

• If the above script is saved as patch-edid.rb, then: ruby patch-edid.rb.
• It is advisable to disconnect any other monitors than the offending one, and to run the terminal on the display in question.

2. A folder with a name of the form DisplayVendorID-xxxx will be created in your home directory.

3. Reboot into recovery mode.

• With the machine powered down, hold Cmd+R while powering it on, until the Apple logo and a progress bar appears.

4. Inside recovery mode, open a terminal window (Utilities > Terminal on the top menu bar).

5. Copy the directory to the relevant path (change username appropriately):

• cp -r /Volumes/Macintosh\ HD/Users/username/DisplayVendorID-xxxx /Volumes/Macintosh\ HD/System/Library/Displays/Contents/Resources/Overrides/

• If /Volumes/Macintosh\ HD/System/Library/Displays/Contents/Resources/Overrides/ doesn't exist (/Resources/Overrides may not) you can make them without issue.

• If a folder matching the DisplayVendorID-xxxx already exists, consider copying it/backing it up somewhere first.

6. Reboot

• This should have rectified the problem by now (if it hasn't worked, double check the output of the script relates to the right monitor, the folder has been copied and put in the right place). If it still hasn't worked then this isn't the fix for you.

---

UPDATE 08.12.2020

It appears that on Big Sur and Catalina, it is no longer necessary to use the above approach and try to edit system partitions. The OS protections will stop this. Instead, the process can all be done within OSX, as follows:

1. Run the script as before

2. Make the filestructure if it doesn't already exist

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

3. Copy the directory produced by the ruby script as before, to this newly created destination:

• sudo cp -r ~/DisplayVendorID-xxxx/ /Library/Displays/Contents/Resources/Overrides/

• This also means you do not need to reboot in to recovery mode or attempt to bypass the SIP (csrutil).

4. Unplug and replug the offending monitor, and the changes should apply

Useful further reading about the latter approach here: https://gist.github.com/ejdyksen/8302862#gistcomment-3460110