Screen Resolutions

November 19, 2002
By Steve Anderson

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First, I must emphasise that some of the constants required to do this are undocumented in the VB API viewer, but are listed below and in the sample projects.

To change the screen mode, you must enumerate the possible screen modes, as described earlier, then select the closest match. You then can use the ChangeDisplaySettings API function to actually change the screen resolution:

ChangeDisplaySettings

This API function changes the display settings on the current machine. There are two parameters that are passed which are:

lpDevMode – This acts as a pointer to the DevMode structure that contains information that you want to change. You can set the following parameters of the DevMode structure that will be changed

Meaning	Description
dmBitsPerPel	This is the number of colours that you want to use or the bits per pixel as I explained earlier on in this article, which can either be 4, 6, 8, 16, 24 or 32 depending on the modes supported.
dmPelsWidth	This is the width of the screen in pixels or otherwise known as the x-axis of the screen.
dmPelsHeight	This is the height of the screen in pixels or otherwise known as the y-axis of the screen.

This function demonstrates how to change the screen resolution. It enumerates through the available modes, checking each one. When the correct one is found, it is selected. Don’t forget to add the earlier declarations to the ones here:

Private Declare Function ChangeDisplaySettings _
    Lib "user32" _
Alias "ChangeDisplaySettingsA" (lpDevMode As Any, _
ByVal dwflags As Long) As Long

Public Sub ChangeScreenSettings(lWidth As Integer, _
lHeight As Integer, lColors As Integer)

Dim tDevMode As DEVMODE, lTemp As Long, lIndex As Long
' Declare variables
lIndex = 0
Do

lTemp = EnumDisplaySettings(0&, lIndex, tDevMode)
' Call the API function
If lTemp = 0 Then Exit Do
' If there is no more data or an error occurs
' then return 0 and exit do
lIndex = lIndex + 1
' Increase the index to be enumerated
With tDevMode
If .dmPelsWidth = lWidth And .dmPelsHeight = lHeight _
And .dmBitsPerPel = lColors Then
lTemp = ChangeDisplaySettings(tDevMode, CDS_UPDATEREGISTRY)
Exit Do
End If

End With
Loop
' Set the new data
' Change the display type. This depends on the paramter used
' It can either be:
' 0 - Dynamic change if possible
' CDS_UPDATEREGISTRY - Dynamically change if possible
' and if not possible then update registry for change
' on the next boot-up
' CDS_TEST - Test the new settings

Select Case lTemp ' Check for errors
Case DISP_CHANGE_SUCCESSFUL
MsgBox "The display settings change was successful", _
vbInformation

Case DISP_CHANGE_RESTART
MsgBox "The computer must be restarted in order for _
the graphics mode to work", vbQuestion

Case DISP_CHANGE_FAILED
MsgBox "The display driver failed the specified graphics _
mode", vbCritical

Case DISP_CHANGE_BADMODE
MsgBox "The graphics mode is not supported", vbCritical

Case DISP_CHANGE_NOTUPDATED
MsgBox "Unable to write settings to the registry", vbCritical
' NB. Windows NT Only

Case DISP_CHANGE_BADFLAGS
MsgBox "An invalid set of flags was passed in", vbCritical

End Select

End Sub

The functions here are demonstrated in the ChangeDispMode demo project.

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