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HOWTO: How to Mix GDI and WinG

Article ID: 125928

Article Last Modified on 7/7/2005

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This article was previously published under Q125928
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SUMMARY

BitBlt(), WinGBitBlt(), StretchBlt(), and WinGStretchBlt(), can all be used to move bits in an image from one DC to another. However, there are different restrictions on what the source and destination DC can be for each of the functions. In addition, the platform that the application is running on also affects what the source and destination DCs can be.

Possible Source and Destination DC Combinations

The following two tables list some of the possible source and destination DC combinations for each function:

The following two tables apply to:
  • 16-bit Windows-based application running under Windows version 3.1.

    -or-
  • 32-bit Windows-based application running under Windows version 3.1 with Win32s. 
       BitBlt(), StretchBlt()

      From                 To: WinGDC    To: Screen/Memory DC
      -------------------------------------------------------
      WinGDC               Yes           *No
      Screen/Memory DC     *No           Yes

     *On a memory DC, the blt's return value will indicate success
      but the function will not perform as expected.

   WinGBitBlt(), WinGStretchBlt()

      From                 To: WinGDC    To: Screen/Memory DC
      -------------------------------------------------------
      WinGDC               No            Yes (screen only)
      Screen/Memory DC     No            No
The following two tables apply to:
  • 16-bit or 32-bit Windows-based application running under Windows NT or Windows 95: 
       BitBlt(), StretchBlt()

      From                 To: WinGDC    To: Screen/Memory DC
      -------------------------------------------------------
      WinGDC               Yes           Yes
      Screen/Memory DC     Yes           Yes

   WinGBitBlt(), WinGStretchBlt()

      From                 To: WinGDC    To: Screen/Memory DC
      ----------------------------------------------------
      WinGDC               Yes           Yes
      Screen/Memory DC     Yes           Yes

MORE INFORMATION

Mixing GDI and WinG

There are two ways to use GDI and WinG together. You can mix GDI and custom drawing operations into WinGBitmaps, and you can mix GDI operations and WinG blts to the screen. Both have their caveats.

Drawing into WinGBitmaps

WinG allows drawing onto the DIB surface of a WinGDC with GDI, but there are some anomalies to keep in mind.
  • Most importantly, GDI does NOT regard WinGDCs as palette devices. WinGDCs are actually 256-color RGB devices. You can modify the device color table using the WinGSetDIBColorTable API.
  • Drawing with GDI on a WinGDC surface does not always produce a pixel-perfect duplicate of the image you would see using GDI on a display device. The images will be acceptably similar, but some stray pixels will remain if you XOR the two images together.
  • Brushes realized in a WinGDC will be aligned to the upper-left corner of the WinGDC whereas brushes used in screen DCs are aligned to the upper-left corner of the screen. This means that when you blt a WinGDC that has been filled with a pattern into a screen DC that has been filled with the same pattern, the patterns will not necessarily align correctly.

    If you have this problem, you can either change the brush origins and re-realize the brushes in either DC (see the section "1.6.8 Brush Alignment" in the Windows SDK Programmer's Reference Volume 1, also available on the Microsoft Developer Network CD) or you can make off-screen brushes align correctly with on-screen brushes by blting the WinGDC to a brush-aligned position on the screen. For example, an 8x8 brush pattern can be correctly aligned to the screen by blting the WinGDC to an x, y position when x and y are both multiples of 8.
  • BitBlt does not blt between DCs owned by different devices. You can't blt from a Printer DC to a Display DC, for example. A WinGDC is a new type of device context, and just as with other DCs, you can't use BitBlt to blt between a WinGDC and any other type of DC (such as a WinGDC to a Display DC or a Memory DC to a WinGDC). To blt from a WinGDC to a DisplayDC, use WinGBitBlt or WinGStretchBlt. Again, you can't use BitBlt to blt from a Display DC to a WinGDC on Windows version 3.x or Win32s, and you can only use WinGBitBlt or WinGStretchBlt to blt from a WinGDC to the screen.

    To blt from the screen into a WinGDC on Windows version 3.x, you will need to create a compatible bitmap, BitBlt the screen rectangle into this bitmap, and then GetDIBits from the bitmap into the WinGDC's DIB surface. On Windows 95 and Windows NT version 3.5, BitBlt into a WinGDC will work properly.

    Win32-based applications that use GDI and custom drawing routines to draw on the surface of a WinGBitmap should call GDIFlush after calling GDI functions and before calling custom functions. Win32 GDI batches drawing commands, including WinGBitBlt and WinGStretchBlt. This guarantees that all GDI drawing is completed before custom drawing begins. Without this call, drawing may be done in an incorrect order.

Drawing to the Screen

Win32-based applications should call GDIFlush after blting to the screen from a WinGDC.

REFERENCES

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