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OSX External Monitor RGB Support
  1. Download the patch-edid.rb script from the forums thread above and put it in your home directory.
  2. Restart your Mac.
  3. Before OS X starts up, hold down Command-R and keep it held down until you see an Apple icon and a progress bar. Release. This boots you into Recovery.
  4. From the Utilities menu, select Terminal.
  5. At the prompt type exactly the following and then press Return: csrutil disable
  6. Terminal should display a message that SIP was disabled.
  7. From the  menu, select Restart.
  8. Connect only the external monitor(s) in question (I closed my MacBook lid, for example). The script will make override files for any connected monitor.
  9. Type “ruby patch-edid.rb” in Terminal.
  10. A new folder will be created in your home directory.
  11. If You're using a OSX version older than El Capitan, move it into the “/System/Library/Displays/Overrides” folder. If Finder tells you that you are overwriting an existing folder, consider backing it up first.
  12. Mac OS 10.11 El Capitan does things a little differently. You should use a different overrides folder: /System/Library/Displays/Contents/Resources/Overrides.
  13. Restart your computer, enjoy your monitor.
#!/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
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