External monitor brightness driver

lilgrassin · January 28, 2025, 1:25am

I'm interested in writing a driver capable of adjusting the brightness of external monitors using the built-in Windows brightness controls over DDC/CI. It looks like it should be possible to effectively trick the OS using "Supporting Brightness Controls for External Display Connectors" (I don't seem to be able to include links in my post), but it's not quite clear to me how INF files work in relation to drivers. Would I need to write a whole new monitor driver or is an INF file on its own be sufficient? Supposedly there's a default INF somewhere which would be useful to look at, but I couldn't figure out where it's located. It's also not clear what protocol is in use (maybe it's determined by the driver?). I have experience writing kernel drivers, but minimal experience with device drivers and definitely not with display/monitor drivers.

Tim_Roberts · January 28, 2025, 2:04am

Well, hang on. You can't force a driver to support a capability it doesn't already have. Adjusting the monitor requires writing to the graphics chip registers. You don't know which registers to write, and you don't have access to the register space if you did.

The article you cited is here:

But that just tells you how to inform the system that your driver has this ability. If the driver doesn't already expose this, you can't force it!

lilgrassin · January 28, 2025, 2:28am

I'm aware that I wouldn't be able to create a protocol that isn't there, but most external monitors these days support DDC/CI over HDMI/Displayport and all I would need to do is write a driver that can "talk" DDC/CI if the built-in driver can't. My understanding was that either the built-in monitor.sys can, in which case I just need to inform it that the monitor supports it via the INF/registry, or I can write a similar driver to monitor.sys which can.

Tim_Roberts · January 28, 2025, 7:26am

Yes, but those commands are transmitted over the HDMI cable by the graphics card. Monitor.sys can expose that the monitor supports them, but the wires that carry those commands connect to the graphics chip. Monitor.sys doesn't know how to write to registers on the graphics chip.

Pavel_A1 · January 28, 2025, 3:10pm

Here is example how to control brightness of monitor connected thru HDMI

gist.github.com

https://gist.github.com/pavel-a/dd3a4320176e69a0f6c4b4871e69e56b

brightness_ctl_DDC.cpp

// Phys monitor control via DDC/CI or VESA commands
// (David Lowndes)
// From https://social.msdn.microsoft.com/Forums/en-US/5437ddb8-af86-4cf0-8061-798e6601e08e/how-to-control-system-brightness-using-windows-api-?forum=windowsgeneraldevelopmentissues#554b7b7f-0aab-429d-9c36-fd39608979b0

#include <Windows.h>
#include <physicalmonitorenumerationapi.h>
#include <highlevelmonitorconfigurationapi.h>
#pragma comment( lib, "Dxva2.lib" )

void

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test-monitors.cpp

// test-monitors.cpp
// https://social.msdn.microsoft.com/Forums/en-US/668e3cf9-4e00-4b40-a6f8-c7d2fc1afd39/how-can-i-retrieve-monitor-information?forum=windowsgeneraldevelopmentissues#be6e67f3-c2a7-4647-bb39-ed4fbec5dce4

#include <Windows.h>
#include <iostream>
#include <vector>
#include <cstdio>

std::wstring getMonitorName(HMONITOR monitor) {
    MONITORINFOEXW info;

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(original code by David Lowndes)