CODEWITHSUNDEEP
pythonwinapipywin32
Zack Yoshyaro
Zack Yoshyaro

Reputation: 2124

Can you get a Device Context from a Bitmap?

I'm trying to workout how to resize an image after I've converted it to a bitmap. From the MSDN examples, I was able to figure out how to resize from a DC (before it's converted to a bitmap) 

# Big snapshot of the desktop
hwnd = win32gui.GetDesktopWindow()

zhwndDevice = win32gui.GetWindowDC(hwnd)
zmfcDC  = win32ui.CreateDCFromHandle(zhwndDevice)
zsaveDC = zmfcDC.CreateCompatibleDC()
zsaveBitMap = win32ui.CreateBitmap()
zsaveBitMap.CreateCompatibleBitmap(zmfcDC, width, height)
zsaveDC.SelectObject(zsaveBitMap)
zsaveDC.BitBlt((0, 0), (width, height), zmfcDC, (left, top), win32con.SRCCOPY)

# Creates a smaller bitmap and resizes the first image to fit it 
hwnd = win32gui.GetDesktopWindow()
hwndDevice = win32gui.GetWindowDC(hwnd)
mfcDC   = win32ui.CreateDCFromHandle(hwndDevice)
saveDC = mfcDC.CreateCompatibleDC()
saveBitMap = win32ui.CreateBitmap()
saveBitMap.CreateCompatibleBitmap(mfcDC, target_size[0], target_size[1])
saveDC.SelectObject(saveBitMap)
win32gui.StretchBlt(pywintypes.HANDLE(saveDC.GetHandleAttrib()), 0,0,target_size[0], target_size[1], zsaveDC.GetHandleAttrib(), 0,0,width, height, win32con.SRCCOPY)

My question is, if I already have a Bitmap object, how would I go about creating a DC from it so that I can pass it to the StretchBlt method?

Upvotes: 0

Views: 629

Answers (1)

abarnert
abarnert

Reputation: 366093

The whole point of a Device Context is that it's a context suitable for some kind of device—your monitor, a printer, whatever. So, creating a DC for a bitmap doesn't make any sense. Read Device Contexts for more detail.

If you want to create an in-memory device context that's compatible with the current screen, that's exactly what you get from calling CreateCompatibleDC with NULL (from Python, None or 0, depending on which wrapper you're using) as the hdc parameter:

A handle to an existing DC. If this handle is NULL, the function creates a memory DC compatible with the application's current screen.

However, in your case, you're ultimately trying to blit this into a target window (or some other target) for which you already have a DC, right? So I think what you really want is to create a memory DC compatible with the target DC, then create a bitmap compatible with that target DC, then select that bitmap into the memory DC.

Upvotes: 3

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