Attaching third monitor - always make clone of second - how?

Question

_{Tuesday, August 21, 2018 4:31 PM}

Hello,

We are working on a Windows 10 based device. The user will have two built-in screens (one touch, one regular) to work with. In addition, the device features a HDMI out connector. The idea is to be able to display content from the secondary monitor on a connected, external HDMI display / duplicate the secondary monitor.

Problem:
Connecting the HDMI display, Windows' default behavior is to make it an extended display and cloning both 
monitors on the device. Users will not have access to the settings / should not have to adjust this.
In other words, I get config [1/2] & [3] but I want [1] & [2/3].

Yes, Windows will remember per display-device the settings once configured but what is connected via HDMI will almost always be a different device.
Any idea where to flip a switch to make it work?
I was unable to find the right setting in the registry.

Many thanks.

All replies (6)

_{Wednesday, August 22, 2018 7:41 AM}

Hi, 

I searched the Internet and found the registry key "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\GUID number" only apply to specify graphic model, but not available on nVidia, MatroX.  

So we could change the multiple display setting through "Display" configration and capture the changed registry key through tool named as "regshot".  After you got the corresponding registry key, try to change those keys through registry and check if the UI setting changed correspondingly at the same time.

Here is a capture which I searched on the Internet to change Display configuration. As the limited of our environment, we can't test it on our side. 

Here is a case might be useful for you.

How to identify the current display configuration from registry?

Bests,

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_{Wednesday, August 22, 2018 6:01 PM}

Joy-Qiao,

Thank you. I had pretty much the same thought yesterday.
The settings changed track back to:

HKEY_LOCAL_MACHINE\System\ControlSet001\Control\GraphicsDrivers\Connectivity\

Each display config the system has seen and has been configured for is stored here.
Good info but sadly it does not help setup a default behavior what to do with a third unknown display when connected to the system.

_{Thursday, September 6, 2018 6:11 AM}

Hi,

Do you mean the high light registry key is the captured one?

"sadly it does not help setup a default behavior what to do with a third unknown display when connected to the system."

Are there any changed value inside the registry key?

If we can't specify the third monitor model, try to change with another monitor as the third one and check if it changed a same registry value in different registry key. 

Bests,

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_{Tuesday, September 11, 2018 4:39 PM}

Sorry, no, the highlighted row happened by accident.

All entries under C*onnectivity *are display configurations the system has encountered and the config is saved within each entry.

An entry is added every time one of the systems encounters a new display configuration and every time users have to deal with the default settings in Windows.

To reiterate, I can set a default behavior for two monitors (expand, or duplicate) but I cannot set a default behavior for a third screen added when I want a mixed mode of expanded and duplicated displays.
From what I have discovered so far, this is not possible to accomplish if the next display to be connected is unknown to the system / has not been configured and saved in Registry prior.

_{Thursday, September 13, 2018 8:42 PM}

This is not the issue.

Problem #1
The third monitor we attach to the systems is almost always unknown / has never been seen by a system,
so there is no entry in the Registry for the default configuration with that monitor.
This is not a static setup on a desk.

Problem #2
The default behavior what to do with an additional display introduced to a system can be switched between Duplicate and Extend. This does not work in my case as I have to work in a mixed mode. 
The built-in displays are extended, the additional third monitor has to be a duplicate of the second built-in display (by default it is always a duplicate of the first/primary display). 

_{Friday, September 14, 2018 10:27 AM}

Hi,

I have same thoughts with you. It would be impossible to achieve your desire.

As the third monitor is not a specify monitor, if it switch with another monitor as the third one, the new monitor will log a new registry key with a different GUID in registry. So it would be impossible to apply your configuration on a random display as the third one. 

Bests,

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