Screen Resolutions
Now that you have know how to retrieve the screen resolution you may want to know how you can retrieve the number of colours that is available to Windows. The options are: 16 colours, 4 bit; 256 colours,8 bit; 65536 colours, 16 bit; 16.7 million colours, either 24 bit or 32 bit.
GetDeviceCaps
This API function is important in retrieving the number of colours that is available to windows. It returns the information on a given index. The paramters are:
Hdc – This is the handle of the device context that you find by using CreateIC.
nIndex – This is the information that you want to retrieve from the display driver. The constants that are important in this section of the article are
| Constant | Description |
| BITSPIXEL | This is the number of bits that are available per pixel. This will return 4, 8, 16, 24 or 32. |
| PLANES | This returns the number of colour planes |
CreateIC
This API function is used to create a device context, which allows easy access to device information. This CreateIC API function has these parameters:
LpDriverName – This specifies the name of the driver that you want to retrieve information on. This is usually ‘Display’ for information on the display driver.
LpDeviceName – This should be left as null
LpOutput – This should be left as null
LpInitData – This should be left as null
DeleteDC
This simply deletes the device context that was created by using CreateIC. You must do this for every device context that you create in order to free up Windows resources.
To retrieve the number of colours that are available to Windows we first need to find out how many planes are available to windows, which will be either 1 or 4. Most machines will support 1 plane.
This function returns the number of colours and colour depth as a string:
Private Const BITSPIXEL = 12
Private Const PLANES = 14
Private Declare Function CreateIC Lib "gdi32" _
Alias "CreateICA" (ByVal lpDriverName As String, _
ByVal lpDeviceName As Any, ByVal lpOutput As Any, _
ByVal lpInitData As Any) As Long
Private Declare Function GetDeviceCaps Lib "gdi32" _
(ByVal hdc As Long, ByVal nIndex As Long) As Long
Private Declare Function DeleteDC Lib "gdi32" _
(ByVal hdc As Long) As Long
Public Function GetAvailableColours() As String
Dim lHdc As Long, lPlanes As Long, lBitsPerPixel As Integer
' Declare variables
lHdc = CreateIC("DISPLAY", 0&, 0&, 0&)
' Create the device context for the display
If lHdc = 0 Then
' An error has occurred and the function will exit
GetAvailableColours = "Error"
Exit Function
End If
lPlanes = GetDeviceCaps(lHdc, PLANES)
' Return info on number of planes
lBitsPerPixel = GetDeviceCaps(lHdc, BITSPIXEL)
' Use display device
' context to return info on the
' number of pixels
lHdc = DeleteDC(lHdc)
' Delete the device context
Select Case lPlanes
Case 1
' 1 plane is available. This
' will be the same for most
' computer systems
Select Case lBitsPerPixel
' Select the number of colours available
Case 4: GetAvailableColours = _
"4 Bit, 16 Colours"
Case 8: GetAvailableColours = _
"8 Bit, 256 Colours"
Case 16: GetAvailableColours = _
"16 Bit, 65536 Colours"
Case 24: GetAvailableColours = _
"24 Bit True Colour, 16.7 Million Colours"
Case 32: GetAvailableColours = _
"32 Bit True Colour, 16.7 Million Colours"
End Select
Case 4
GetAvailableColours = "16 Bit, 65536 Colours"
' If there are 4 planes then the availible
' colours will be 16-bit
Case Else
GetAvailableColours = "Undetermined"
' The number of colours could not bee determined
End Select
End Function
The functions here are demonstrated in the GetColourDepth demo project.
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