Questions regarding DDC brightness control

[weiweihuanghuang]

Summary

After the display wakes from sleep, set -brightness=… from the CLI has no effect on the physical monitor and returns no error. In the same state, get -hardwareBrightness returns the last value BetterDisplay wrote rather than the panel's actual value, so the failure is invisible to a script that verifies its own writes.

The state persists across repeated CLI invocations. Dragging the brightness slider in the menubar menu clears it, after which identical CLI commands work again.

Environment

• BetterDisplay 4.3.6 (Build 50119)
• macOS 26.6.2 (25G83)
• Mac: MacBook Pro, Apple M4 Pro
• Display: DELL S2721QS
• Connection:

Steps to reproduce

1. Sleep the Mac with the external display connected, then wake it.
2. Run:

   /Applications/BetterDisplay.app/Contents/MacOS/BetterDisplay set -name="DELL S2721QS" -brightness=0.1
   

3. Check actual brightness via the monitor's hardware OSD menu.

Expected: panel changes to 10%, or the command reports an error.

Actual: panel unchanged, command exits silently. Repeating it has no effect.

get returns a stale cached value

Reproduced directly:

1. Panel at 75%, set by an earlier successful CLI call.

2. Changed brightness to 50% using the monitor's hardware OSD buttons.

3. Queried BetterDisplay:

   $ /Applications/BetterDisplay.app/Contents/MacOS/BetterDisplay get -name="DELL S2721QS" -hardwareBrightness
   0.75
   

The panel was at 50% and the OSD confirmed it. get -hardwareBrightness returned the last written value, not the display's current state.

Consequence: a script that reads the current value and skips a redundant write will skip precisely when the panel is wrong; a script that writes and verifies via get sees its own cached write echoed back and concludes success.

Context

Driven by a shell script run on wake via sleepwatcher. It waits 10 seconds after wake and retries the display-presence check three times before proceeding, so this is not the script running before the display enumerates — the display is present and get responds, the writes just do not reach it. The same commands typed manually in Terminal well after a wake work reliably.

Workaround

Open the menubar menu and drag the brightness slider.

Questions

1. Is get -hardwareBrightness intended to perform a real DDC read, or to return the cached last-written value? If the latter, is there a way to force a read from the display?
2. Is there a CLI equivalent of whatever the menubar slider does, to re-establish the connection without user interaction?

[waydabber]

Hi there,

0. In your initial "steps to reproduce" you are not interacting with hardwareBrightness, but brightness which by default is set to be a combined brightness control combinedBrightness (this uses software dimming below 0.5, hardware control above 0.5).
1. get -hardwareBrightness is not intended to perform DDC read, it returns the managed, normalized value (0.0-1.0 range value the app thinks best describes the display state currently). You can use get -ddc -vcp=luminance to do an actual DDC read or set -ddc=50 -vcp=luminance to write directly.
2. The slider does not establish connection, it just modifies the value and sends it to the display, but if you change the OSD, it won't be automatically updated again. The reason is that DDC read does not work reliably on all displays. But, you can configure the app to periodically update the brightness value (if reading works reliably with the display).

[weiweihuanghuang]

Thanks for answering that. I can run set -name="DELL S2721QS" -ddc=75 -vcp=luminance but when I try get -name="DELL S2721QS" -ddc -vcp=luminance I get a Failed.