Create a quick monitor input switcher with Hammerspoon

I have a Dell 3419UW monitor, with my work Macbook plugged into the USB-C input, and my personal desktop Mac plugged into DisplayPort 1. Until now, I’ve been manually switching between computers dozens of times a day by menu diving on the actual monitor.

I’m glad that’s all over. I’ll show you how to set up a single hot key with Hammerspoon to seamlessly switch between computers.

Install ddcctl on both computers #

ddcctl is a simple command line program that lets you talk to your monitor using the Display Data Channel protocol. Among other things, it lets you switch inputs!

First, clone ddcctl. I’ve chosen to clone it at ~/.ddcctl:

$ git clone https://github.com/kfix/ddcctl.git ~/.ddcctl

Use system_profiler to figure out what graphics card you are using. Note whether you’re using an amd, nvidia, or intel card.

$ system_profiler SPDisplaysDataType

Graphics/Displays:

    Intel UHD Graphics 630:

      Chipset Model: Intel UHD Graphics 630
      Type: GPU
      Bus: Built-In
      VRAM (Dynamic, Max): 1536 MB
      Vendor: Intel
      Device ID: 0x3e92
      Revision ID: 0x0002
      Metal: Supported, feature set macOS GPUFamily2 v1

    Radeon RX 590:

      Chipset Model: Radeon RX 590
      Type: GPU
      Bus: PCIe
      PCIe Lane Width: x8
      VRAM (Total): 8 GB
      Vendor: AMD (0x1002)
      Device ID: 0x67df
      Revision ID: 0x00e1
      Metal: Supported, feature set macOS GPUFamily2 v1
      Displays:
        DELL U3419W:
          Resolution: 3440 x 1440 (UWQHD - Ultra-Wide Quad HD)
          UI Looks like: 3440 x 1440 @ 60 Hz
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Display Serial Number: 75066T2
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: No
          Connection Type: DisplayPort

As you can see from the output above, I have both Intel and AMD (Radeon) cards, but I have my computer set to always use the Radeon no matter what.

Finally, compile ddcctl:

$ cd ~/.ddcctl

# If you have an intel card
$ make intel

# If you have an AMD (Radeon) card
$ make amd

# If you have an Nvidia card
$ make nvidia

You should now have a compiled binary. Check it real quick; you should see output like this:

$ ~/.ddcctl/ddcctl

D: NSScreen #724072469 (3440x1440 0°) 109.00 DPI
I: found 1 external display
2020-09-03 00:10:24.436 ddcctl[33078:42148351] Usage:
ddcctl  -d <1-..>  [display#]
        -w 100000  [delay usecs between settings]

----- Basic settings -----
        -b <1-..>  [brightness]
        -c <1-..>  [contrast]
        -rbc       [reset brightness and contrast]

----- Settings that don't always work -----
        -m <1|2>   [mute speaker OFF/ON]
        -v <1-254> [speaker volume]
        -i <1-18>  [select input source]
        -p <1|2-5> [power on | standby/off]
        -o         [read-only orientation]

----- Settings (testing) -----
        -rg <1-..>  [red gain]
        -gg <1-..>  [green gain]
        -bg <1-..>  [blue gain]
        -rrgb       [reset color]

----- Setting grammar -----
        -X ?       (query value of setting X)
        -X NN      (put setting X to NN)
        -X <NN>-   (decrease setting X by NN)
        -X <NN>+   (increase setting X by NN)

Repeat this process on your second computer.

Figure out which input each computer is on #

There is a table of input sources on ddcctl’s README. However, I found it to be incomplete–for example, my monitor’s USB-C input source is not in the table.

Turns out you can just ask the monitor which input source is active using ddcctl, which will save some guess and check. Just use the ddcctl -d 1 -i "?" command on each of your computers.

When I ran it on my desktop, I got a current value of 15 (DisplayPort 1):

$ ~/.ddcctl/ddcctl -d 1 -i ?

D: NSScreen #724072469 (3440x1440 0°) 109.00 DPI
I: found 1 external display
I: polling display 1's EDID
I: got edid.serial: 75066T2
I: got edid.name: DELL U3419W
D: action: i: ?
D: querying VCP control: #96 =?
I: VCP control #96 (0x60) = current: 15, max: 18

When I ran it on my laptop, I got a current value of 27 (USB-C):

$ ~/.ddcctl/ddcctl -d 1 -i ?

D: NSScreen #724072469 (3440x1440 0°) 109.00 DPI
I: found 1 external display
I: polling display 1's EDID
I: got edid.serial: 75066T2
I: got edid.name: DELL U3419W
D: action: i: ?
D: querying VCP control: #96 =?
I: VCP control #96 (0x60) = current: 27, max: 18

Create a Hammerspoon hotkey to switch the display #

Now that you have the right input values for the monitor, it’s fast to throw together a hotkey. My approach was to check the current computer’s hostname to figure out which computer I want to switch to.

-- i have a hyper key for global binds like this one, but you can use
-- whatever modifiers you like
local hyper = {'cmd', 'shift', 'alt', 'ctrl'}

hs.hotkey.bind(hyper, 'm', function()
  -- path to your `ddcctl` binary
  local binary = os.getenv("HOME") .. "/.ddcctl/ddcctl"

  -- figure out your hostname by running `hostname` from your shell
  local desktopHostname = "sorny"
  local desktopInput = 15
  local laptopInput = 27

  local inputNumber = nil

  if hs.host.localizedName() == desktopHostname then
    -- we're on the desktop, we want to switch to the laptop
    inputNumber = laptopInput
  else
    inputNumber = desktopInput
  end

  hs.execute(binary .. " -d 1 -i " .. inputNumber)
end)

Once you’ve added this snippet, reload your Hammerspoon config and give it a try!