Using the AlphaBlend Function

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Note: The code in this article is restricted to win98 and win2000.

Do you want a cool splash window? Do you think the general splash window is not good enough for your app? Then this sample source is for you.

My semi-transparent splash window uses the AlphaBlend function. When I found this function, I was so surprised! The function works just like BitBlt for bitmaps that have transparent or semitransparent pixels. Here is the AlphaBlend function's syntax.

BOOL AlphaBlend( HDC hdcDest,
  int nXOriginDest,
  int nYOriginDest,
  int nWidthDest,
  int nHeightDest,
  HDC hdcSrc,
  int nXOriginSrc,
  int nYOriginSrc,
  int nWidthSrc,
  int nHeightSrc,
  BLENDFUNCTION blendFunction
);

The important parameter is the last one, BLENDFUNCTION.

typedef struct _BLENDFUNCTION {
    BYTE     BlendOp;
    BYTE     BlendFlags;
    BYTE     SourceConstantAlpha;
    BYTE     AlphaFormat;
}BLENDFUNCTION;

• BlendOp must be AC_SRC_OVER
• BlendFlags must be 0
• SourceConstantAlpha must be between 0 (transparent) and 255 (semi-transparent)
• AlphaFormat must be AC_SRC_ALPHA

MSDN states: The SourceConstantaAlpha member of BLENDFUNCTION specifies an alpha transparency value to be used on the entire source bitmap. The SourceConstantAlpha value is combined with any per-pixel alpha values. If you set SourceConstantAlpha to 0, it is assumed that your image is transparent. Set the SourceConstantAlpha value to 255 (indicates that the image is opaque) when you only want to use per-pixel alpha values.

See the sample application for a demonstration.

Just enjoy!!

Ajou University C.C. 4^th member.

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