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context). This attribute can be set by using the SelectObject API function.
To create a memory device context we use the API function CreateCompatibleDC, which returns a memory DC (a long value). In order to select the bitmap into the device context we need a handle to the specific bitmap. This handle can be obtained by using the LoadImage function. This function can load a bitmap from file, and return a handle to the loaded bitmap. The last thing to do is to select the handle of the loaded bitmap into our newly created memory DC, and we now have a useable device context for blitting.
The MEMORYDC sample, in the MEMORYDC sub-directory of the Chap1 directory, demonstrates the steps we have just outlined and will be exploring next, in creating a compatible device context and selecting a bitmap.
Let''''s put all of the required code into a reusable function which will take care of the creation of a memory device context for us and just requires a filename to the actual bitmap. The function will either return a device context (long value) or 0 if something went wrong.
The code for our function looks like this:
Public Function GenerateDC(FileName As String, ByRef MemDC As Long, ByRef hBitmap As Long) As Long
''''Create a Device Context, compatible with the screen
MemDC = CreateCompatibleDC(0)
If MemDC = 0 Then
GenerateDC = 0
Exit Function
End If
''''Load the image
hBitmap = LoadImage(0, FileName, IMAGE_BITMAP, 0, 0, LR_DEFAULTSIZE Or LR_LOADFROMFILE Or _ LR_CREATEDIBSECTION)
If hBitmap = 0 Then ''''Failure in loading bitmap
DeleteDC MemDC
Exit Function
End If
''''Throw the Bitmap into the Device Context
SelectObject MemDC, hBitmap
''''Return OK
GenerateDC = 1
End Function
A device context created with the CreateCompatibleDC must be deleted by calling the DeleteDC API function and the Bitmap handle returned from LoadImage much be deleted with the DeleteObject API. So let''''s create a reusable function for this called DeleteGeneratedDC. This function takes two arguments, a DC to be deleted and the Bitmap Handle for the corresponding DC.
Private Function DeleteGeneratedDC(hBitmap As Long, MemDC As Long) As Long
DeleteGeneratedDC = DeleteDC(MemDC)
DeleteObject hBitmap
End Function
Run the sample project. Press the Load bitmap button. The bitmaps are now loaded and ready for use. We use the usual BltBit function to Blit them from the memory context and into the device context of the form. Press the Draw the sprite button, and observe how the sprite is blitted transparently onto the form.
One word of advice when using this scheme to create a memory device context: Be observant of the scope of the variables which you store the device contexts in. If the variable goes out of scope, and you have not deleted it with the DeleteDC function, you抣l start losing resources. So always delete the created device contexts to be on the safe side.
End of Part II of the three part Drawing and Animation Tutorial by Burt Abreu & S鴕en Skov. Download all Samples]
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