DPI-related APIs and registry settings

Applies To: Windows 8.1

If you need to perform deployment customizations, the following sections explain the registry keys and system parameters that your post-installation scripts might need to access.

In this topic:

• Primary display native resolution

• Primary display DPI scale factor

• Scaling mode

• Scaling override in Windows 8.1 scaling mode

• System-wide scale factor in Windows 8 scaling mode

• Simplest mitigation for 125% displays

• Mitigation for 150% displays

Primary display native resolution

Table 1 Windows 8.1 Scaling Levels, while by no means exhaustive, provides information on the Windows 8.1 scaling level for a number of common displays. Panel DPI indicates the physical pixel density of the panel, and Scaling level indicates the scale factor that will be used for this display.

Table 1 Windows 8.1 Scaling Levels

To programmatically find this information for any device, you can write a utility program that reports back data. The native primary resolution is retrieved by calling the API GetDeviceCaps() function, using the hdc for the desktop and the HORZRES and VERZRES indices:

// Get desktop dc
desktopDc = GetDC(NULL);
// Get native resolution
horizontalResolution = GetDeviceCaps(desktopDc,HORZRES);
verticalResolution = GetDeviceCaps(desktopDc,VERZRES);

For more information about GetDC, see GetDC() function.

Primary display DPI scale factor

Similarly, you can get the pixel density by using the LOGPIXELSX and LOGPIXELSY indices:

// Get desktop dc
desktopDc = GetDC(NULL);
// Get native resolution
horizontalDPI = GetDeviceCaps(desktopDc,LOGPIXELSX);
verticalDPI = GetDeviceCaps(desktopDc,LOGPIXELSY);

These results are returned in a coordinate system in which 96 corresponds to 100%, as shown in Table 2 DPI Scale Factors.

Table 2 DPI Scale Factors

<asmv3:application>
    <asmv3:windowsSettings xmlns="https://schemas.microsoft.com/SMI/2005/WindowsSettings">
      <dpiAware>Per monitor</dpiAware>
    </asmv3:windowsSettings>
  </asmv3:application>

For more information about this manifest setting, see SetProcessDPIAware function.

Scaling mode

The Control Panel\ Appearance and Personalization\Display user interface (UI) includes a checkbox: Let me choose one scaling level for all my displays, which controls whether the system applies a single scale factor to all displays (as in Windows® 8 and earlier versions of Windows), or different scale factors that take into account the pixel density of each display (the Windows 8.1 default). This checkbox configures the HKCU\Control Panel\Desktop\Win8DpiScaling registry key in Windows 8.1.

Table 3 HKCU\Control Panel\Desktop\Win8DpiScaling Values

Scaling override in Windows 8.1 scaling mode

When the Let me choose one scaling level for all my displays checkbox is cleared and the system is running in the Windows 8.1 scaling mode, the user is provided with a slider that lets them override the current scale factors, from Smaller, to Medium, to Larger. This setting is configured in the HKCU\Control Panel\Desktop\DesktopDPIOverride registry key.

Table 4 HKCU\Control Panel\Desktop\DesktopDPIOverride Values

All display scale factors in this mode are constrained to be one of these four values: 100%, 125%, 150%, 200%. In addition, after scaling is applied, applications expect to have at least 720 effective lines of resolution (that is, the physical vertical resolution of the display divided by the scale factor); this can further limit the range of allowed display scale factors. Table 5 Display Values shows which values are allowed for different sized displays:

Table 5 Display Values

System-wide scale factor in Windows 8 scaling mode

When the Let me choose one scaling level for all my displays checkbox is checked, the user can specify a scale factor that applies to all displays, regardless of each display’s pixel density. By using the custom setting, the user can select values other than 100%, 125%, 150%, 200%, although they are limited to the range (100%-500%). This setting is configured in the HKCU\ControlPanel\Desktop\LogPixels registry key.

Table 6 HKCU\ControlPanel\Desktop\LogPixels Values

Simplest mitigation for 125% displays

The simplest generic fix is to place the system in single-scale-factor mode with a scale factor of 125%. This is not recommended unless the enterprise environment includes a large number of apps that aren’t DPI-aware and that scale poorly at 125%. In pseudo-code this would look something like this:

If (verticalResolution < 1080) and (DPI == 125%)
Set HKCU\...\Win8DPIScaling to 1
Set HKCU\...\LogPixels to 120
Require logoff/logon

Mitigation for 150% displays

For more pixel-dense displays, the same approach can be used as described in the previous section. This is not recommended as a general practice; in addition to disabling the Windows 8.1 DPI feature benefits, it also results in making display content significantly smaller than optimal. As a result, we recommend that only users customize their DPI scaling on these devices.

See Also

Concepts

High DPI Support for IT Professionals