Microsoft KB Archive/92533

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Implementing Display Driver Device Bitmaps

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Q92533

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The information in this article applies to:


 * Microsoft Windows Device Development Kit (DDK) for Windows, versions 3.1, 3.11

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SUMMARY
The &quot;Microsoft Windows 3.1 Device Driver Kit: Device Driver Adaptation Guide&quot; (DDAG) documentation on implementing display driver Device Bitmaps is inconsistent between chapters 2 and 10.

Chapter 10 information for RealizeObject states that you can fail the call to realize a device bitmap (i.e. when the wStyle parameter is OBJ_PBITMAP). This information appears to conflict with that found in chapter 2, section 2.1.14. The information in chapter 2 is correct, and is more specific to display drivers.

MORE INFORMATION
If you implement Device Bitmaps, you can not fail any calls to SelectBitmap, BitmapBits, or RealizeObject. If you can't find room in off-screen video memory, you must allocate main system memory with GlobalAlloc.

You can use SelectBitmap as the place to lock-down your huge bitmaps that won't fit off-screen. However, these bitmaps won't always be selected prior to MemoryDIBBits or BitmapBits (these are the places where the bitmap is passed through GDI directly rather than indirectly via DCs). Furthermore, bitmaps may be selected multiple times without being deselected. Additionally, a bitmap may be freed while it is still selected.

According to the DDK documentation, GDI will leave you to manage your PBITMAP and its header if the first word is non-zero. (The PBITMAP must still be implemented as documented on page 504 of the 3.1 DDK DDAG.) There are at least two cases where this is not true: GDI will periodically overwrite the dword at the bmBits offset (with seg:0020) and it may also overwrite the bmlpPDevice field. Some driver developers have used the &quot;reserved&quot; structure members of the PBITMAP to store the information that should be in the overwritten fields, thus avoiding the overwrite problem (this does however create version dependence.)

There is also a small memory leak which may be specific to device bitmaps: GDI will create a bitmap to hold a copy of the bits passed into CreatePatternBrush, but it will never free the copy. Excel makes a lot of these calls when it draws charts, resulting in a pile of orphaned 8x8 brush copies (which will eventually exhaust GDI's heap!)

Because of these problems, we recommend that Device Bitmaps be implemented as a SYSTEM.INI option, allowing users to turn them off if they determine Device Bitmaps are causing problems.

Additional query words: 3.10 3.11

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