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WinPE Screen Resolution and DPI scaling on UEFI firmware

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#1 misty


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Posted 16 March 2023 - 12:56 PM

This topic is based on observations on screen resolution and DPI scaling in WinPE. The test system including a Lenovo Thinkpad T470s with FHD resolution, with some tests also completed in VMWare Workstation Player. This system has UEFI firmware with CSM disabled. WinPE was based on boot.wim from Windows 21H2 installation media - modified using the Mini-WinFE project. A more detailed guide with screenshots is available - http://mistyprojects...npe_dpi/dpi.htm

Screen resolution can be changed from a running WinPE booted on BIOS firmware. Changing the screen resolution on UEFI firmware is not so simple. Settings in unattend.xml appear to be ignored and the screen resolution is fixed. Based on available information screen resolution appears to be set based on Graphics Output Protocal (GOP) mode. Once WinPE is booted, the resolution cannot be changed.

Investigating further, the screen resolution in WinPE is set to the resolution defined/set by the Windows Boot Manager. bcdedit.exe can be used to modify the BCD store and set the menu resolution that will subsequently by used in WinPE, however the graphicsresolution element does not support many options. Examples of resolutions that can be set using graphicsresolution include -
bcdedit.exe /set {globalsettings} graphicsresolution 800x600
bcdedit.exe /set {globalsettings} graphicsresolution 1024x600
bcdedit.exe /set {globalsettings} graphicsresolution 1024x768
Attempting to use an unsupported resolution (unsupported by bcdedit - not the hardware) resulted in the following output -
C:\windows\system32>bcdedit.exe /set {globalsettings} graphicsresolution 1920x1080
The integer data is not valid as specified.
Run "bcdedit /?" for command line assistance.
The parameter is incorrect.
The highestmode option is also supported in bcdedit.exe. This option will enable boot applications to use the highest graphical mode detected by the firmware -
bcdedit.exe /set {globalsettings} highestmode on
Exercise caution when using higestmode, as depending on the scale used on WinPE (see below) WinPE may be unusable on high resolution displays. As an example, running WinPE with 100% scaling on a Surface Pro with a screen size of 12.3" and maximum resolution of 2736 x 1824 was unreadable due to the content being hard to see and UI targets being difficult to select.

The GRUB4EFI boot manager (aka Grub4dos-for_UEFI) can be used to set a wider range of screen resolutions before control is passed to the Windows Boot Manager. Setting the screen resolution in a GRUB4EFI menu or at the commandline and then chainloading the Windows Boot Manager should keep the resolution set in GRUB4EFI. NOTE that this may not work if the screen resolution is configured in the BCD store (e.g. graphicsresolution or highestmode is set) as these settings will override GRUB4EFI.

To find supported resolutions, use the graphicsmode --info command on the GRUB4EFI commandline. On the test system, the following resolutions are supported -
MODE  Format   Resolution      BitPixel
260     1      1280 x 1024       32
259     1      1024 x 768        32
258     1      800 x 600         32
257     1      640 x 480         32
256     1      1920 x 1080       32
WinPE does not support resolutions lower than 768 vertical lines. This limits supported resolutions on the T470s test system to the following -
1024 x 768 
1280 x 1024
1920 x 1080
GRUB4EFI configuration file (menu.lst) menu entries. Each entry uses the graphicsmode command to set a supported screen resolution prior to chainloading the Windows boot manager (/efi/microsoft/boot/bootmgfw.efi) -
color white/black black/white light-blue/black light-red/black
default 4

title Windows 1024x768
find --set-root /efi/microsoft/boot/bootmgfw.efi
graphicsmode -1 1024 768
chainloader /efi/microsoft/boot/bootmgfw.efi

title Windows 1280x1024
find --set-root /efi/microsoft/boot/bootmgfw.efi
graphicsmode -1 1280 1024
chainloader /efi/microsoft/boot/bootmgfw.efi

title Windows 1920x1080
find --set-root /efi/microsoft/boot/bootmgfw.efi
graphicsmode -1 1920 1080
chainloader /efi/microsoft/boot/bootmgfw.efi

title commandline

title reboot


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#2 misty


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Posted 16 March 2023 - 12:57 PM

The previous post covered setting the screen resolution. This section covers changing the DPI scale used in WinPE. By default WinPE versions 4/5.x/10.x/11.x will automatically set a scaling level based on a combination of the physical panel size and screen resolution. The ThinkPad T470s test system has a physical panel size of 14". Running in FHD on this display panel, WinPE and Windows set a scaling level of 150%

The information in this section has been adapted from an article by Michael Albert - Windows PE: Disable DPI autoscaling. Based on the information provided by Michael, adding three registry settings will disable auto scaling and will set a fixed custom scaling level. The following registry settings need to be applied to an offline WinPE image (with keyname edited to refect the mount point used for the offline registry hive) -
reg add "HKCU\Control Panel\Desktop" /t REG_DWORD /v LogPixels /d 96 /f
reg add "HKCU\Control Panel\Desktop" /v Win8DpiScaling /t REG_DWORD /d 0x00000001 /f
reg add "HKCU\Control Panel\Desktop" /v DpiScalingVer /t REG_DWORD /d 0x00001018 /f
The LogPixels value is used to set the custom scaling, with scaling between 100%-500% supported. Value starts at 96 with incremental steps of 24 adding a further 25% scaling.

LogPixels value of 96 = 100%
LogPixels value of 120 = 125%
LogPixels value of 144 = 150%
LogPixels value of 168 = 175%
LogPixels value of 192 = 200%

Using wimlib to show an example, with following paths/settings -
- d:\boot.wim = path to .wim file containing WinPE
- 2 = index/image number (in d:\boot.wim) containing WinPE
- D:\wimlib\temp = Work directory - the WinPE default registry hive will be extracted here.
- D:\wimlib\wimlib-imagex.exe = Path to wimlib-imagex.exe.

Extract the default registry hive from image 2 in d:\boot.wim to the D:\wimlib\temp directory -
D:\wimlib\wimlib-imagex.exe extract D:\boot.wim 2 /Windows/System32/config/default --dest-dir=D:\wimlib\temp

Use reg.exe to mount the extracted default hive (as HKLM\_WinPE_Default) -
reg.exe load HKLM\_WinPE_Default D:\wimlib\temp\default

Apply the registry settings, editing the LogPixels value as required. In the example below a LogPixels value of 96 is set (100% scale) -
reg add "HKLM\_WinPE_Default\Control Panel\Desktop" /v LogPixels /t REG_DWORD /d 96 /f
reg add "HKLM\_WinPE_Default\Control Panel\Desktop" /v Win8DpiScaling /t REG_DWORD /d 0x00000001 /f
reg add "HKLM\_WinPE_Default\Control Panel\Desktop" /v DpiScalingVer /t REG_DWORD /d 0x00001018 /f

Unload the default registry hive -
reg.exe unload HKLM\_WinPE_Default

Inject the updated default registry hive back to boot.wim -
D:\wimlib\wimlib-imagex.exe  update D:\boot.wim 2 --command="add 'D:\wimlib\temp\default' '/Windows/System32/config/default'"

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#3 Wonko the Sane

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Posted 17 March 2023 - 09:58 AM

Very interesting.


This is what the good MS guys (and all the other peeps that brought us UEFI) like to call "simplifying" (as opposed to the bad, bad, BIOS).


Good work, thanks.  :thumbsup:



Small glitch: your last code needs to be corrected ( it is a repetition of the "Unload the default registry hive -" instead of the "Inject the updated default registry hive back to boot.wim -")




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#4 misty


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Posted 17 March 2023 - 06:04 PM

Thanks for the feedback - and good spot regarding the missing/incorrect code in the "Inject the updated default registry hive back to boot.wim" section of post #2. Post #2 edited and corrected.
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