Microsoft KB Archive/139165

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PRB: Memory DC Produces Monochrome Images

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Q139165

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.1, used with:
 * Microsoft Windows 3.11
 * Microsoft Win32 Application Programming Interface (API)

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SYMPTOMS
When you create and use a memory Device Context (DC) to draw and store GDI images, the bitmap displayed when transferred from the memory device context to the physical display is monochrome. All colors are converted to either black or white.

CAUSE
When creating a compatible bitmap for the memory DC, the handle to the memory DC is used as the first parameter to the call to CreateCompatibleBitmap. This create a monochrome bitmap because a memory DC created with CreateCompatibleDC is given a 1x1 monochrome bitmap as its default bitmap.

For example, the following code creates a monochrome bitmap and selects it into a memory DC:

  HDC hdc, hdcMem; HBITMAP hBitmap;

hdc = GetDC(hWnd); hdcMem = CreateCompatibleDC(hdc); hBitmap = CreateCompatibleBitmap(hdcMem, 400, 400); SelectObject(hdcMem, hBitmap); SetTextColor(hdcMem, RGB(0, 0, 255));

RESOLUTION
Send a physical screen DC to the CreateCompatibleBitmap function rather than the memory DC that you plan on selecting the bitmap into.

The following example code properly sets up a color memory DC with a color bitmap with the same bit depth as the physical display:

  HDC hdc, hdcMem; HBITMAP hBitmap;

hdc = GetDC(hWnd); hdcMem = CreateCompatibleDC(hdc); hBitmap = CreateCompatibleBitmap(hdc, 400, 400); SelectObject(hdcMem, hBitmap); SetTextColor(hdcMem, RGB(0, 0, 255));

STATUS
This behavior is by design.

Additional query words: 3.10 4.00

Keywords : kbcode kbOSWinNT351 kbOSWin2000 kbOSWin310 kbOSWin95 kbDSupport kbSDKWin16

Issue type : kbprb

Technology : kbAudDeveloper kbSDKSearch kbWinSDKSearch