Fix external display picture quality issue on Mac OS X

Recently I bought an external display the LG 24MP88HV-S, hoping that it would be a good companion for my macbook pro. But to my surprise the font rendering and the contrast was horrible. It was like I needed a pair of corrective lens. After digging a couple of hours on the internet for I seemed to have identified what was going wrong.

Problem

The display port uses YCbCr colors instead of RGB to drive the display, which limits the range of colors and apparently causes the display to apply some undesired post processing.

Solution

Manually override the the Display Profile to use RGB mode.

1. Boot the mac into recovery mode (hold down ⌘ + R when the computer is booting)
2. Open the Terminal window by going to Utilities -> Terminal

3. Disable the System Integrity Protection.

   It is a security technology to prevent modifying protected files (read more). Run csrutil disable; reboot and press Enter

4. Let the mac boot normally (not in recovery mode)

5. Navigate to display overrides

   For OS X Catalina / Mojave / High Sierra run:

   cd /System/Library/Displays/Contents/Resources/Overrides

   For OS X Sierra and older run:

   cd /System/Library/Displays/Overrides

6. Place the patch file in the overrides directory

   To directly download patch-edid.rb run:

   sudo curl -O https://gist.githubusercontent.com/Joel-Raju/0e8d1f1891175f544d75b4ba4dda1926/raw/d350425daf4bbf7ec953ed70ac77e9e6399d633b/patch-edid.rb

   Alternately you create a file called patch-edid.rb. And copy the following code.

   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

7. Execute the script

   sudo ruby patch-edid.rb

8. Disconnect and reconnet the monitor.

9. Dont forget to re-enable System Integrity Protection.

   Reboot into Recovery Mode, by holding ⌘ + R when the mac is booting.

   Open the Terminal window by going to Utilities -> Terminal

   run csrutil enable; reboot and press Enter

10. Let the mac boot up normally. Done !

The display should look much better now.

Final thoughts

This hack might not be for everyone. If you dont mind spending a couple of hundred dollars more, you can get a much nicer display recommended by Apple and avoid this hassle.

References

• http://embdev.net/topic/284710
• https://gist.github.com/ejdyksen/8302862